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diff --git a/Documentation/virtual/00-INDEX b/Documentation/virtual/00-INDEX
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+Virtualization support in the Linux kernel.
+
+00-INDEX
+        - this file.
+kvm/
+        - Kernel Virtual Machine.  See also http://linux-kvm.org
+lguest/
+        - Extremely simple hypervisor for experimental/educational use.
+uml/
+        - User Mode Linux, builds/runs Linux kernel as a userspace program.
diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
new file mode 100644
index 000000000000..9bef4e4cec50
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+The Definitive KVM (Kernel-based Virtual Machine) API Documentation
+===================================================================
+
+1. General description
+
+The kvm API is a set of ioctls that are issued to control various aspects
+of a virtual machine.  The ioctls belong to three classes
+
+ - System ioctls: These query and set global attributes which affect the
+   whole kvm subsystem.  In addition a system ioctl is used to create
+   virtual machines
+
+ - VM ioctls: These query and set attributes that affect an entire virtual
+   machine, for example memory layout.  In addition a VM ioctl is used to
+   create virtual cpus (vcpus).
+
+   Only run VM ioctls from the same process (address space) that was used
+   to create the VM.
+
+ - vcpu ioctls: These query and set attributes that control the operation
+   of a single virtual cpu.
+
+   Only run vcpu ioctls from the same thread that was used to create the
+   vcpu.
+
+2. File descriptors
+
+The kvm API is centered around file descriptors.  An initial
+open("/dev/kvm") obtains a handle to the kvm subsystem; this handle
+can be used to issue system ioctls.  A KVM_CREATE_VM ioctl on this
+handle will create a VM file descriptor which can be used to issue VM
+ioctls.  A KVM_CREATE_VCPU ioctl on a VM fd will create a virtual cpu
+and return a file descriptor pointing to it.  Finally, ioctls on a vcpu
+fd can be used to control the vcpu, including the important task of
+actually running guest code.
+
+In general file descriptors can be migrated among processes by means
+of fork() and the SCM_RIGHTS facility of unix domain socket.  These
+kinds of tricks are explicitly not supported by kvm.  While they will
+not cause harm to the host, their actual behavior is not guaranteed by
+the API.  The only supported use is one virtual machine per process,
+and one vcpu per thread.
+
+3. Extensions
+
+As of Linux 2.6.22, the KVM ABI has been stabilized: no backward
+incompatible change are allowed.  However, there is an extension
+facility that allows backward-compatible extensions to the API to be
+queried and used.
+
+The extension mechanism is not based on on the Linux version number.
+Instead, kvm defines extension identifiers and a facility to query
+whether a particular extension identifier is available.  If it is, a
+set of ioctls is available for application use.
+
+4. API description
+
+This section describes ioctls that can be used to control kvm guests.
+For each ioctl, the following information is provided along with a
+description:
+
+  Capability: which KVM extension provides this ioctl.  Can be 'basic',
+      which means that is will be provided by any kernel that supports
+      API version 12 (see section 4.1), or a KVM_CAP_xyz constant, which
+      means availability needs to be checked with KVM_CHECK_EXTENSION
+      (see section 4.4).
+
+  Architectures: which instruction set architectures provide this ioctl.
+      x86 includes both i386 and x86_64.
+
+  Type: system, vm, or vcpu.
+
+  Parameters: what parameters are accepted by the ioctl.
+
+  Returns: the return value.  General error numbers (EBADF, ENOMEM, EINVAL)
+      are not detailed, but errors with specific meanings are.
+
+4.1 KVM_GET_API_VERSION
+
+Capability: basic
+Architectures: all
+Type: system ioctl
+Parameters: none
+Returns: the constant KVM_API_VERSION (=12)
+
+This identifies the API version as the stable kvm API. It is not
+expected that this number will change.  However, Linux 2.6.20 and
+2.6.21 report earlier versions; these are not documented and not
+supported.  Applications should refuse to run if KVM_GET_API_VERSION
+returns a value other than 12.  If this check passes, all ioctls
+described as 'basic' will be available.
+
+4.2 KVM_CREATE_VM
+
+Capability: basic
+Architectures: all
+Type: system ioctl
+Parameters: none
+Returns: a VM fd that can be used to control the new virtual machine.
+
+The new VM has no virtual cpus and no memory.  An mmap() of a VM fd
+will access the virtual machine's physical address space; offset zero
+corresponds to guest physical address zero.  Use of mmap() on a VM fd
+is discouraged if userspace memory allocation (KVM_CAP_USER_MEMORY) is
+available.
+
+4.3 KVM_GET_MSR_INDEX_LIST
+
+Capability: basic
+Architectures: x86
+Type: system
+Parameters: struct kvm_msr_list (in/out)
+Returns: 0 on success; -1 on error
+Errors:
+  E2BIG:     the msr index list is to be to fit in the array specified by
+             the user.
+
+struct kvm_msr_list {
+        __u32 nmsrs; /* number of msrs in entries */
+        __u32 indices[0];
+};
+
+This ioctl returns the guest msrs that are supported.  The list varies
+by kvm version and host processor, but does not change otherwise.  The
+user fills in the size of the indices array in nmsrs, and in return
+kvm adjusts nmsrs to reflect the actual number of msrs and fills in
+the indices array with their numbers.
+
+Note: if kvm indicates supports MCE (KVM_CAP_MCE), then the MCE bank MSRs are
+not returned in the MSR list, as different vcpus can have a different number
+of banks, as set via the KVM_X86_SETUP_MCE ioctl.
+
+4.4 KVM_CHECK_EXTENSION
+
+Capability: basic
+Architectures: all
+Type: system ioctl
+Parameters: extension identifier (KVM_CAP_*)
+Returns: 0 if unsupported; 1 (or some other positive integer) if supported
+
+The API allows the application to query about extensions to the core
+kvm API.  Userspace passes an extension identifier (an integer) and
+receives an integer that describes the extension availability.
+Generally 0 means no and 1 means yes, but some extensions may report
+additional information in the integer return value.
+
+4.5 KVM_GET_VCPU_MMAP_SIZE
+
+Capability: basic
+Architectures: all
+Type: system ioctl
+Parameters: none
+Returns: size of vcpu mmap area, in bytes
+
+The KVM_RUN ioctl (cf.) communicates with userspace via a shared
+memory region.  This ioctl returns the size of that region.  See the
+KVM_RUN documentation for details.
+
+4.6 KVM_SET_MEMORY_REGION
+
+Capability: basic
+Architectures: all
+Type: vm ioctl
+Parameters: struct kvm_memory_region (in)
+Returns: 0 on success, -1 on error
+
+This ioctl is obsolete and has been removed.
+
+4.7 KVM_CREATE_VCPU
+
+Capability: basic
+Architectures: all
+Type: vm ioctl
+Parameters: vcpu id (apic id on x86)
+Returns: vcpu fd on success, -1 on error
+
+This API adds a vcpu to a virtual machine.  The vcpu id is a small integer
+in the range [0, max_vcpus).
+
+4.8 KVM_GET_DIRTY_LOG (vm ioctl)
+
+Capability: basic
+Architectures: x86
+Type: vm ioctl
+Parameters: struct kvm_dirty_log (in/out)
+Returns: 0 on success, -1 on error
+
+/* for KVM_GET_DIRTY_LOG */
+struct kvm_dirty_log {
+        __u32 slot;
+        __u32 padding;
+        union {
+                void __user *dirty_bitmap; /* one bit per page */
+                __u64 padding;
+        };
+};
+
+Given a memory slot, return a bitmap containing any pages dirtied
+since the last call to this ioctl.  Bit 0 is the first page in the
+memory slot.  Ensure the entire structure is cleared to avoid padding
+issues.
+
+4.9 KVM_SET_MEMORY_ALIAS
+
+Capability: basic
+Architectures: x86
+Type: vm ioctl
+Parameters: struct kvm_memory_alias (in)
+Returns: 0 (success), -1 (error)
+
+This ioctl is obsolete and has been removed.
+
+4.10 KVM_RUN
+
+Capability: basic
+Architectures: all
+Type: vcpu ioctl
+Parameters: none
+Returns: 0 on success, -1 on error
+Errors:
+  EINTR:     an unmasked signal is pending
+
+This ioctl is used to run a guest virtual cpu.  While there are no
+explicit parameters, there is an implicit parameter block that can be
+obtained by mmap()ing the vcpu fd at offset 0, with the size given by
+KVM_GET_VCPU_MMAP_SIZE.  The parameter block is formatted as a 'struct
+kvm_run' (see below).
+
+4.11 KVM_GET_REGS
+
+Capability: basic
+Architectures: all
+Type: vcpu ioctl
+Parameters: struct kvm_regs (out)
+Returns: 0 on success, -1 on error
+
+Reads the general purpose registers from the vcpu.
+
+/* x86 */
+struct kvm_regs {
+        /* out (KVM_GET_REGS) / in (KVM_SET_REGS) */
+        __u64 rax, rbx, rcx, rdx;
+        __u64 rsi, rdi, rsp, rbp;
+        __u64 r8,  r9,  r10, r11;
+        __u64 r12, r13, r14, r15;
+        __u64 rip, rflags;
+};
+
+4.12 KVM_SET_REGS
+
+Capability: basic
+Architectures: all
+Type: vcpu ioctl
+Parameters: struct kvm_regs (in)
+Returns: 0 on success, -1 on error
+
+Writes the general purpose registers into the vcpu.
+
+See KVM_GET_REGS for the data structure.
+
+4.13 KVM_GET_SREGS
+
+Capability: basic
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_sregs (out)
+Returns: 0 on success, -1 on error
+
+Reads special registers from the vcpu.
+
+/* x86 */
+struct kvm_sregs {
+        struct kvm_segment cs, ds, es, fs, gs, ss;
+        struct kvm_segment tr, ldt;
+        struct kvm_dtable gdt, idt;
+        __u64 cr0, cr2, cr3, cr4, cr8;
+        __u64 efer;
+        __u64 apic_base;
+        __u64 interrupt_bitmap[(KVM_NR_INTERRUPTS + 63) / 64];
+};
+
+interrupt_bitmap is a bitmap of pending external interrupts.  At most
+one bit may be set.  This interrupt has been acknowledged by the APIC
+but not yet injected into the cpu core.
+
+4.14 KVM_SET_SREGS
+
+Capability: basic
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_sregs (in)
+Returns: 0 on success, -1 on error
+
+Writes special registers into the vcpu.  See KVM_GET_SREGS for the
+data structures.
+
+4.15 KVM_TRANSLATE
+
+Capability: basic
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_translation (in/out)
+Returns: 0 on success, -1 on error
+
+Translates a virtual address according to the vcpu's current address
+translation mode.
+
+struct kvm_translation {
+        /* in */
+        __u64 linear_address;
+
+        /* out */
+        __u64 physical_address;
+        __u8  valid;
+        __u8  writeable;
+        __u8  usermode;
+        __u8  pad[5];
+};
+
+4.16 KVM_INTERRUPT
+
+Capability: basic
+Architectures: x86, ppc
+Type: vcpu ioctl
+Parameters: struct kvm_interrupt (in)
+Returns: 0 on success, -1 on error
+
+Queues a hardware interrupt vector to be injected.  This is only
+useful if in-kernel local APIC or equivalent is not used.
+
+/* for KVM_INTERRUPT */
+struct kvm_interrupt {
+        /* in */
+        __u32 irq;
+};
+
+X86:
+
+Note 'irq' is an interrupt vector, not an interrupt pin or line.
+
+PPC:
+
+Queues an external interrupt to be injected. This ioctl is overleaded
+with 3 different irq values:
+
+a) KVM_INTERRUPT_SET
+
+  This injects an edge type external interrupt into the guest once it's ready
+  to receive interrupts. When injected, the interrupt is done.
+
+b) KVM_INTERRUPT_UNSET
+
+  This unsets any pending interrupt.
+
+  Only available with KVM_CAP_PPC_UNSET_IRQ.
+
+c) KVM_INTERRUPT_SET_LEVEL
+
+  This injects a level type external interrupt into the guest context. The
+  interrupt stays pending until a specific ioctl with KVM_INTERRUPT_UNSET
+  is triggered.
+
+  Only available with KVM_CAP_PPC_IRQ_LEVEL.
+
+Note that any value for 'irq' other than the ones stated above is invalid
+and incurs unexpected behavior.
+
+4.17 KVM_DEBUG_GUEST
+
+Capability: basic
+Architectures: none
+Type: vcpu ioctl
+Parameters: none)
+Returns: -1 on error
+
+Support for this has been removed.  Use KVM_SET_GUEST_DEBUG instead.
+
+4.18 KVM_GET_MSRS
+
+Capability: basic
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_msrs (in/out)
+Returns: 0 on success, -1 on error
+
+Reads model-specific registers from the vcpu.  Supported msr indices can
+be obtained using KVM_GET_MSR_INDEX_LIST.
+
+struct kvm_msrs {
+        __u32 nmsrs; /* number of msrs in entries */
+        __u32 pad;
+
+        struct kvm_msr_entry entries[0];
+};
+
+struct kvm_msr_entry {
+        __u32 index;
+        __u32 reserved;
+        __u64 data;
+};
+
+Application code should set the 'nmsrs' member (which indicates the
+size of the entries array) and the 'index' member of each array entry.
+kvm will fill in the 'data' member.
+
+4.19 KVM_SET_MSRS
+
+Capability: basic
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_msrs (in)
+Returns: 0 on success, -1 on error
+
+Writes model-specific registers to the vcpu.  See KVM_GET_MSRS for the
+data structures.
+
+Application code should set the 'nmsrs' member (which indicates the
+size of the entries array), and the 'index' and 'data' members of each
+array entry.
+
+4.20 KVM_SET_CPUID
+
+Capability: basic
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_cpuid (in)
+Returns: 0 on success, -1 on error
+
+Defines the vcpu responses to the cpuid instruction.  Applications
+should use the KVM_SET_CPUID2 ioctl if available.
+
+
+struct kvm_cpuid_entry {
+        __u32 function;
+        __u32 eax;
+        __u32 ebx;
+        __u32 ecx;
+        __u32 edx;
+        __u32 padding;
+};
+
+/* for KVM_SET_CPUID */
+struct kvm_cpuid {
+        __u32 nent;
+        __u32 padding;
+        struct kvm_cpuid_entry entries[0];
+};
+
+4.21 KVM_SET_SIGNAL_MASK
+
+Capability: basic
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_signal_mask (in)
+Returns: 0 on success, -1 on error
+
+Defines which signals are blocked during execution of KVM_RUN.  This
+signal mask temporarily overrides the threads signal mask.  Any
+unblocked signal received (except SIGKILL and SIGSTOP, which retain
+their traditional behaviour) will cause KVM_RUN to return with -EINTR.
+
+Note the signal will only be delivered if not blocked by the original
+signal mask.
+
+/* for KVM_SET_SIGNAL_MASK */
+struct kvm_signal_mask {
+        __u32 len;
+        __u8  sigset[0];
+};
+
+4.22 KVM_GET_FPU
+
+Capability: basic
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_fpu (out)
+Returns: 0 on success, -1 on error
+
+Reads the floating point state from the vcpu.
+
+/* for KVM_GET_FPU and KVM_SET_FPU */
+struct kvm_fpu {
+        __u8  fpr[8][16];
+        __u16 fcw;
+        __u16 fsw;
+        __u8  ftwx;  /* in fxsave format */
+        __u8  pad1;
+        __u16 last_opcode;
+        __u64 last_ip;
+        __u64 last_dp;
+        __u8  xmm[16][16];
+        __u32 mxcsr;
+        __u32 pad2;
+};
+
+4.23 KVM_SET_FPU
+
+Capability: basic
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_fpu (in)
+Returns: 0 on success, -1 on error
+
+Writes the floating point state to the vcpu.
+
+/* for KVM_GET_FPU and KVM_SET_FPU */
+struct kvm_fpu {
+        __u8  fpr[8][16];
+        __u16 fcw;
+        __u16 fsw;
+        __u8  ftwx;  /* in fxsave format */
+        __u8  pad1;
+        __u16 last_opcode;
+        __u64 last_ip;
+        __u64 last_dp;
+        __u8  xmm[16][16];
+        __u32 mxcsr;
+        __u32 pad2;
+};
+
+4.24 KVM_CREATE_IRQCHIP
+
+Capability: KVM_CAP_IRQCHIP
+Architectures: x86, ia64
+Type: vm ioctl
+Parameters: none
+Returns: 0 on success, -1 on error
+
+Creates an interrupt controller model in the kernel.  On x86, creates a virtual
+ioapic, a virtual PIC (two PICs, nested), and sets up future vcpus to have a
+local APIC.  IRQ routing for GSIs 0-15 is set to both PIC and IOAPIC; GSI 16-23
+only go to the IOAPIC.  On ia64, a IOSAPIC is created.
+
+4.25 KVM_IRQ_LINE
+
+Capability: KVM_CAP_IRQCHIP
+Architectures: x86, ia64
+Type: vm ioctl
+Parameters: struct kvm_irq_level
+Returns: 0 on success, -1 on error
+
+Sets the level of a GSI input to the interrupt controller model in the kernel.
+Requires that an interrupt controller model has been previously created with
+KVM_CREATE_IRQCHIP.  Note that edge-triggered interrupts require the level
+to be set to 1 and then back to 0.
+
+struct kvm_irq_level {
+        union {
+                __u32 irq;     /* GSI */
+                __s32 status;  /* not used for KVM_IRQ_LEVEL */
+        };
+        __u32 level;           /* 0 or 1 */
+};
+
+4.26 KVM_GET_IRQCHIP
+
+Capability: KVM_CAP_IRQCHIP
+Architectures: x86, ia64
+Type: vm ioctl
+Parameters: struct kvm_irqchip (in/out)
+Returns: 0 on success, -1 on error
+
+Reads the state of a kernel interrupt controller created with
+KVM_CREATE_IRQCHIP into a buffer provided by the caller.
+
+struct kvm_irqchip {
+        __u32 chip_id;  /* 0 = PIC1, 1 = PIC2, 2 = IOAPIC */
+        __u32 pad;
+        union {
+                char dummy[512];  /* reserving space */
+                struct kvm_pic_state pic;
+                struct kvm_ioapic_state ioapic;
+        } chip;
+};
+
+4.27 KVM_SET_IRQCHIP
+
+Capability: KVM_CAP_IRQCHIP
+Architectures: x86, ia64
+Type: vm ioctl
+Parameters: struct kvm_irqchip (in)
+Returns: 0 on success, -1 on error
+
+Sets the state of a kernel interrupt controller created with
+KVM_CREATE_IRQCHIP from a buffer provided by the caller.
+
+struct kvm_irqchip {
+        __u32 chip_id;  /* 0 = PIC1, 1 = PIC2, 2 = IOAPIC */
+        __u32 pad;
+        union {
+                char dummy[512];  /* reserving space */
+                struct kvm_pic_state pic;
+                struct kvm_ioapic_state ioapic;
+        } chip;
+};
+
+4.28 KVM_XEN_HVM_CONFIG
+
+Capability: KVM_CAP_XEN_HVM
+Architectures: x86
+Type: vm ioctl
+Parameters: struct kvm_xen_hvm_config (in)
+Returns: 0 on success, -1 on error
+
+Sets the MSR that the Xen HVM guest uses to initialize its hypercall
+page, and provides the starting address and size of the hypercall
+blobs in userspace.  When the guest writes the MSR, kvm copies one
+page of a blob (32- or 64-bit, depending on the vcpu mode) to guest
+memory.
+
+struct kvm_xen_hvm_config {
+        __u32 flags;
+        __u32 msr;
+        __u64 blob_addr_32;
+        __u64 blob_addr_64;
+        __u8 blob_size_32;
+        __u8 blob_size_64;
+        __u8 pad2[30];
+};
+
+4.29 KVM_GET_CLOCK
+
+Capability: KVM_CAP_ADJUST_CLOCK
+Architectures: x86
+Type: vm ioctl
+Parameters: struct kvm_clock_data (out)
+Returns: 0 on success, -1 on error
+
+Gets the current timestamp of kvmclock as seen by the current guest. In
+conjunction with KVM_SET_CLOCK, it is used to ensure monotonicity on scenarios
+such as migration.
+
+struct kvm_clock_data {
+        __u64 clock;  /* kvmclock current value */
+        __u32 flags;
+        __u32 pad[9];
+};
+
+4.30 KVM_SET_CLOCK
+
+Capability: KVM_CAP_ADJUST_CLOCK
+Architectures: x86
+Type: vm ioctl
+Parameters: struct kvm_clock_data (in)
+Returns: 0 on success, -1 on error
+
+Sets the current timestamp of kvmclock to the value specified in its parameter.
+In conjunction with KVM_GET_CLOCK, it is used to ensure monotonicity on scenarios
+such as migration.
+
+struct kvm_clock_data {
+        __u64 clock;  /* kvmclock current value */
+        __u32 flags;
+        __u32 pad[9];
+};
+
+4.31 KVM_GET_VCPU_EVENTS
+
+Capability: KVM_CAP_VCPU_EVENTS
+Extended by: KVM_CAP_INTR_SHADOW
+Architectures: x86
+Type: vm ioctl
+Parameters: struct kvm_vcpu_event (out)
+Returns: 0 on success, -1 on error
+
+Gets currently pending exceptions, interrupts, and NMIs as well as related
+states of the vcpu.
+
+struct kvm_vcpu_events {
+        struct {
+                __u8 injected;
+                __u8 nr;
+                __u8 has_error_code;
+                __u8 pad;
+                __u32 error_code;
+        } exception;
+        struct {
+                __u8 injected;
+                __u8 nr;
+                __u8 soft;
+                __u8 shadow;
+        } interrupt;
+        struct {
+                __u8 injected;
+                __u8 pending;
+                __u8 masked;
+                __u8 pad;
+        } nmi;
+        __u32 sipi_vector;
+        __u32 flags;
+};
+
+KVM_VCPUEVENT_VALID_SHADOW may be set in the flags field to signal that
+interrupt.shadow contains a valid state. Otherwise, this field is undefined.
+
+4.32 KVM_SET_VCPU_EVENTS
+
+Capability: KVM_CAP_VCPU_EVENTS
+Extended by: KVM_CAP_INTR_SHADOW
+Architectures: x86
+Type: vm ioctl
+Parameters: struct kvm_vcpu_event (in)
+Returns: 0 on success, -1 on error
+
+Set pending exceptions, interrupts, and NMIs as well as related states of the
+vcpu.
+
+See KVM_GET_VCPU_EVENTS for the data structure.
+
+Fields that may be modified asynchronously by running VCPUs can be excluded
+from the update. These fields are nmi.pending and sipi_vector. Keep the
+corresponding bits in the flags field cleared to suppress overwriting the
+current in-kernel state. The bits are:
+
+KVM_VCPUEVENT_VALID_NMI_PENDING - transfer nmi.pending to the kernel
+KVM_VCPUEVENT_VALID_SIPI_VECTOR - transfer sipi_vector
+
+If KVM_CAP_INTR_SHADOW is available, KVM_VCPUEVENT_VALID_SHADOW can be set in
+the flags field to signal that interrupt.shadow contains a valid state and
+shall be written into the VCPU.
+
+4.33 KVM_GET_DEBUGREGS
+
+Capability: KVM_CAP_DEBUGREGS
+Architectures: x86
+Type: vm ioctl
+Parameters: struct kvm_debugregs (out)
+Returns: 0 on success, -1 on error
+
+Reads debug registers from the vcpu.
+
+struct kvm_debugregs {
+        __u64 db[4];
+        __u64 dr6;
+        __u64 dr7;
+        __u64 flags;
+        __u64 reserved[9];
+};
+
+4.34 KVM_SET_DEBUGREGS
+
+Capability: KVM_CAP_DEBUGREGS
+Architectures: x86
+Type: vm ioctl
+Parameters: struct kvm_debugregs (in)
+Returns: 0 on success, -1 on error
+
+Writes debug registers into the vcpu.
+
+See KVM_GET_DEBUGREGS for the data structure. The flags field is unused
+yet and must be cleared on entry.
+
+4.35 KVM_SET_USER_MEMORY_REGION
+
+Capability: KVM_CAP_USER_MEM
+Architectures: all
+Type: vm ioctl
+Parameters: struct kvm_userspace_memory_region (in)
+Returns: 0 on success, -1 on error
+
+struct kvm_userspace_memory_region {
+        __u32 slot;
+        __u32 flags;
+        __u64 guest_phys_addr;
+        __u64 memory_size; /* bytes */
+        __u64 userspace_addr; /* start of the userspace allocated memory */
+};
+
+/* for kvm_memory_region::flags */
+#define KVM_MEM_LOG_DIRTY_PAGES  1UL
+
+This ioctl allows the user to create or modify a guest physical memory
+slot.  When changing an existing slot, it may be moved in the guest
+physical memory space, or its flags may be modified.  It may not be
+resized.  Slots may not overlap in guest physical address space.
+
+Memory for the region is taken starting at the address denoted by the
+field userspace_addr, which must point at user addressable memory for
+the entire memory slot size.  Any object may back this memory, including
+anonymous memory, ordinary files, and hugetlbfs.
+
+It is recommended that the lower 21 bits of guest_phys_addr and userspace_addr
+be identical.  This allows large pages in the guest to be backed by large
+pages in the host.
+
+The flags field supports just one flag, KVM_MEM_LOG_DIRTY_PAGES, which
+instructs kvm to keep track of writes to memory within the slot.  See
+the KVM_GET_DIRTY_LOG ioctl.
+
+When the KVM_CAP_SYNC_MMU capability, changes in the backing of the memory
+region are automatically reflected into the guest.  For example, an mmap()
+that affects the region will be made visible immediately.  Another example
+is madvise(MADV_DROP).
+
+It is recommended to use this API instead of the KVM_SET_MEMORY_REGION ioctl.
+The KVM_SET_MEMORY_REGION does not allow fine grained control over memory
+allocation and is deprecated.
+
+4.36 KVM_SET_TSS_ADDR
+
+Capability: KVM_CAP_SET_TSS_ADDR
+Architectures: x86
+Type: vm ioctl
+Parameters: unsigned long tss_address (in)
+Returns: 0 on success, -1 on error
+
+This ioctl defines the physical address of a three-page region in the guest
+physical address space.  The region must be within the first 4GB of the
+guest physical address space and must not conflict with any memory slot
+or any mmio address.  The guest may malfunction if it accesses this memory
+region.
+
+This ioctl is required on Intel-based hosts.  This is needed on Intel hardware
+because of a quirk in the virtualization implementation (see the internals
+documentation when it pops into existence).
+
+4.37 KVM_ENABLE_CAP
+
+Capability: KVM_CAP_ENABLE_CAP
+Architectures: ppc
+Type: vcpu ioctl
+Parameters: struct kvm_enable_cap (in)
+Returns: 0 on success; -1 on error
+
++Not all extensions are enabled by default. Using this ioctl the application
+can enable an extension, making it available to the guest.
+
+On systems that do not support this ioctl, it always fails. On systems that
+do support it, it only works for extensions that are supported for enablement.
+
+To check if a capability can be enabled, the KVM_CHECK_EXTENSION ioctl should
+be used.
+
+struct kvm_enable_cap {
+       /* in */
+       __u32 cap;
+
+The capability that is supposed to get enabled.
+
+       __u32 flags;
+
+A bitfield indicating future enhancements. Has to be 0 for now.
+
+       __u64 args[4];
+
+Arguments for enabling a feature. If a feature needs initial values to
+function properly, this is the place to put them.
+
+       __u8  pad[64];
+};
+
+4.38 KVM_GET_MP_STATE
+
+Capability: KVM_CAP_MP_STATE
+Architectures: x86, ia64
+Type: vcpu ioctl
+Parameters: struct kvm_mp_state (out)
+Returns: 0 on success; -1 on error
+
+struct kvm_mp_state {
+        __u32 mp_state;
+};
+
+Returns the vcpu's current "multiprocessing state" (though also valid on
+uniprocessor guests).
+
+Possible values are:
+
+ - KVM_MP_STATE_RUNNABLE:        the vcpu is currently running
+ - KVM_MP_STATE_UNINITIALIZED:   the vcpu is an application processor (AP)
+                                 which has not yet received an INIT signal
+ - KVM_MP_STATE_INIT_RECEIVED:   the vcpu has received an INIT signal, and is
+                                 now ready for a SIPI
+ - KVM_MP_STATE_HALTED:          the vcpu has executed a HLT instruction and
+                                 is waiting for an interrupt
+ - KVM_MP_STATE_SIPI_RECEIVED:   the vcpu has just received a SIPI (vector
+                                 accessible via KVM_GET_VCPU_EVENTS)
+
+This ioctl is only useful after KVM_CREATE_IRQCHIP.  Without an in-kernel
+irqchip, the multiprocessing state must be maintained by userspace.
+
+4.39 KVM_SET_MP_STATE
+
+Capability: KVM_CAP_MP_STATE
+Architectures: x86, ia64
+Type: vcpu ioctl
+Parameters: struct kvm_mp_state (in)
+Returns: 0 on success; -1 on error
+
+Sets the vcpu's current "multiprocessing state"; see KVM_GET_MP_STATE for
+arguments.
+
+This ioctl is only useful after KVM_CREATE_IRQCHIP.  Without an in-kernel
+irqchip, the multiprocessing state must be maintained by userspace.
+
+4.40 KVM_SET_IDENTITY_MAP_ADDR
+
+Capability: KVM_CAP_SET_IDENTITY_MAP_ADDR
+Architectures: x86
+Type: vm ioctl
+Parameters: unsigned long identity (in)
+Returns: 0 on success, -1 on error
+
+This ioctl defines the physical address of a one-page region in the guest
+physical address space.  The region must be within the first 4GB of the
+guest physical address space and must not conflict with any memory slot
+or any mmio address.  The guest may malfunction if it accesses this memory
+region.
+
+This ioctl is required on Intel-based hosts.  This is needed on Intel hardware
+because of a quirk in the virtualization implementation (see the internals
+documentation when it pops into existence).
+
+4.41 KVM_SET_BOOT_CPU_ID
+
+Capability: KVM_CAP_SET_BOOT_CPU_ID
+Architectures: x86, ia64
+Type: vm ioctl
+Parameters: unsigned long vcpu_id
+Returns: 0 on success, -1 on error
+
+Define which vcpu is the Bootstrap Processor (BSP).  Values are the same
+as the vcpu id in KVM_CREATE_VCPU.  If this ioctl is not called, the default
+is vcpu 0.
+
+4.42 KVM_GET_XSAVE
+
+Capability: KVM_CAP_XSAVE
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_xsave (out)
+Returns: 0 on success, -1 on error
+
+struct kvm_xsave {
+        __u32 region[1024];
+};
+
+This ioctl would copy current vcpu's xsave struct to the userspace.
+
+4.43 KVM_SET_XSAVE
+
+Capability: KVM_CAP_XSAVE
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_xsave (in)
+Returns: 0 on success, -1 on error
+
+struct kvm_xsave {
+        __u32 region[1024];
+};
+
+This ioctl would copy userspace's xsave struct to the kernel.
+
+4.44 KVM_GET_XCRS
+
+Capability: KVM_CAP_XCRS
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_xcrs (out)
+Returns: 0 on success, -1 on error
+
+struct kvm_xcr {
+        __u32 xcr;
+        __u32 reserved;
+        __u64 value;
+};
+
+struct kvm_xcrs {
+        __u32 nr_xcrs;
+        __u32 flags;
+        struct kvm_xcr xcrs[KVM_MAX_XCRS];
+        __u64 padding[16];
+};
+
+This ioctl would copy current vcpu's xcrs to the userspace.
+
+4.45 KVM_SET_XCRS
+
+Capability: KVM_CAP_XCRS
+Architectures: x86
+Type: vcpu ioctl
+Parameters: struct kvm_xcrs (in)
+Returns: 0 on success, -1 on error
+
+struct kvm_xcr {
+        __u32 xcr;
+        __u32 reserved;
+        __u64 value;
+};
+
+struct kvm_xcrs {
+        __u32 nr_xcrs;
+        __u32 flags;
+        struct kvm_xcr xcrs[KVM_MAX_XCRS];
+        __u64 padding[16];
+};
+
+This ioctl would set vcpu's xcr to the value userspace specified.
+
+4.46 KVM_GET_SUPPORTED_CPUID
+
+Capability: KVM_CAP_EXT_CPUID
+Architectures: x86
+Type: system ioctl
+Parameters: struct kvm_cpuid2 (in/out)
+Returns: 0 on success, -1 on error
+
+struct kvm_cpuid2 {
+        __u32 nent;
+        __u32 padding;
+        struct kvm_cpuid_entry2 entries[0];
+};
+
+#define KVM_CPUID_FLAG_SIGNIFCANT_INDEX 1
+#define KVM_CPUID_FLAG_STATEFUL_FUNC    2
+#define KVM_CPUID_FLAG_STATE_READ_NEXT  4
+
+struct kvm_cpuid_entry2 {
+        __u32 function;
+        __u32 index;
+        __u32 flags;
+        __u32 eax;
+        __u32 ebx;
+        __u32 ecx;
+        __u32 edx;
+        __u32 padding[3];
+};
+
+This ioctl returns x86 cpuid features which are supported by both the hardware
+and kvm.  Userspace can use the information returned by this ioctl to
+construct cpuid information (for KVM_SET_CPUID2) that is consistent with
+hardware, kernel, and userspace capabilities, and with user requirements (for
+example, the user may wish to constrain cpuid to emulate older hardware,
+or for feature consistency across a cluster).
+
+Userspace invokes KVM_GET_SUPPORTED_CPUID by passing a kvm_cpuid2 structure
+with the 'nent' field indicating the number of entries in the variable-size
+array 'entries'.  If the number of entries is too low to describe the cpu
+capabilities, an error (E2BIG) is returned.  If the number is too high,
+the 'nent' field is adjusted and an error (ENOMEM) is returned.  If the
+number is just right, the 'nent' field is adjusted to the number of valid
+entries in the 'entries' array, which is then filled.
+
+The entries returned are the host cpuid as returned by the cpuid instruction,
+with unknown or unsupported features masked out.  Some features (for example,
+x2apic), may not be present in the host cpu, but are exposed by kvm if it can
+emulate them efficiently. The fields in each entry are defined as follows:
+
+  function: the eax value used to obtain the entry
+  index: the ecx value used to obtain the entry (for entries that are
+         affected by ecx)
+  flags: an OR of zero or more of the following:
+        KVM_CPUID_FLAG_SIGNIFCANT_INDEX:
+           if the index field is valid
+        KVM_CPUID_FLAG_STATEFUL_FUNC:
+           if cpuid for this function returns different values for successive
+           invocations; there will be several entries with the same function,
+           all with this flag set
+        KVM_CPUID_FLAG_STATE_READ_NEXT:
+           for KVM_CPUID_FLAG_STATEFUL_FUNC entries, set if this entry is
+           the first entry to be read by a cpu
+   eax, ebx, ecx, edx: the values returned by the cpuid instruction for
+         this function/index combination
+
+4.47 KVM_PPC_GET_PVINFO
+
+Capability: KVM_CAP_PPC_GET_PVINFO
+Architectures: ppc
+Type: vm ioctl
+Parameters: struct kvm_ppc_pvinfo (out)
+Returns: 0 on success, !0 on error
+
+struct kvm_ppc_pvinfo {
+        __u32 flags;
+        __u32 hcall[4];
+        __u8  pad[108];
+};
+
+This ioctl fetches PV specific information that need to be passed to the guest
+using the device tree or other means from vm context.
+
+For now the only implemented piece of information distributed here is an array
+of 4 instructions that make up a hypercall.
+
+If any additional field gets added to this structure later on, a bit for that
+additional piece of information will be set in the flags bitmap.
+
+4.48 KVM_ASSIGN_PCI_DEVICE
+
+Capability: KVM_CAP_DEVICE_ASSIGNMENT
+Architectures: x86 ia64
+Type: vm ioctl
+Parameters: struct kvm_assigned_pci_dev (in)
+Returns: 0 on success, -1 on error
+
+Assigns a host PCI device to the VM.
+
+struct kvm_assigned_pci_dev {
+        __u32 assigned_dev_id;
+        __u32 busnr;
+        __u32 devfn;
+        __u32 flags;
+        __u32 segnr;
+        union {
+                __u32 reserved[11];
+        };
+};
+
+The PCI device is specified by the triple segnr, busnr, and devfn.
+Identification in succeeding service requests is done via assigned_dev_id. The
+following flags are specified:
+
+/* Depends on KVM_CAP_IOMMU */
+#define KVM_DEV_ASSIGN_ENABLE_IOMMU     (1 << 0)
+
+4.49 KVM_DEASSIGN_PCI_DEVICE
+
+Capability: KVM_CAP_DEVICE_DEASSIGNMENT
+Architectures: x86 ia64
+Type: vm ioctl
+Parameters: struct kvm_assigned_pci_dev (in)
+Returns: 0 on success, -1 on error
+
+Ends PCI device assignment, releasing all associated resources.
+
+See KVM_CAP_DEVICE_ASSIGNMENT for the data structure. Only assigned_dev_id is
+used in kvm_assigned_pci_dev to identify the device.
+
+4.50 KVM_ASSIGN_DEV_IRQ
+
+Capability: KVM_CAP_ASSIGN_DEV_IRQ
+Architectures: x86 ia64
+Type: vm ioctl
+Parameters: struct kvm_assigned_irq (in)
+Returns: 0 on success, -1 on error
+
+Assigns an IRQ to a passed-through device.
+
+struct kvm_assigned_irq {
+        __u32 assigned_dev_id;
+        __u32 host_irq;
+        __u32 guest_irq;
+        __u32 flags;
+        union {
+                struct {
+                        __u32 addr_lo;
+                        __u32 addr_hi;
+                        __u32 data;
+                } guest_msi;
+                __u32 reserved[12];
+        };
+};
+
+The following flags are defined:
+
+#define KVM_DEV_IRQ_HOST_INTX    (1 << 0)
+#define KVM_DEV_IRQ_HOST_MSI     (1 << 1)
+#define KVM_DEV_IRQ_HOST_MSIX    (1 << 2)
+
+#define KVM_DEV_IRQ_GUEST_INTX   (1 << 8)
+#define KVM_DEV_IRQ_GUEST_MSI    (1 << 9)
+#define KVM_DEV_IRQ_GUEST_MSIX   (1 << 10)
+
+It is not valid to specify multiple types per host or guest IRQ. However, the
+IRQ type of host and guest can differ or can even be null.
+
+4.51 KVM_DEASSIGN_DEV_IRQ
+
+Capability: KVM_CAP_ASSIGN_DEV_IRQ
+Architectures: x86 ia64
+Type: vm ioctl
+Parameters: struct kvm_assigned_irq (in)
+Returns: 0 on success, -1 on error
+
+Ends an IRQ assignment to a passed-through device.
+
+See KVM_ASSIGN_DEV_IRQ for the data structure. The target device is specified
+by assigned_dev_id, flags must correspond to the IRQ type specified on
+KVM_ASSIGN_DEV_IRQ. Partial deassignment of host or guest IRQ is allowed.
+
+4.52 KVM_SET_GSI_ROUTING
+
+Capability: KVM_CAP_IRQ_ROUTING
+Architectures: x86 ia64
+Type: vm ioctl
+Parameters: struct kvm_irq_routing (in)
+Returns: 0 on success, -1 on error
+
+Sets the GSI routing table entries, overwriting any previously set entries.
+
+struct kvm_irq_routing {
+        __u32 nr;
+        __u32 flags;
+        struct kvm_irq_routing_entry entries[0];
+};
+
+No flags are specified so far, the corresponding field must be set to zero.
+
+struct kvm_irq_routing_entry {
+        __u32 gsi;
+        __u32 type;
+        __u32 flags;
+        __u32 pad;
+        union {
+                struct kvm_irq_routing_irqchip irqchip;
+                struct kvm_irq_routing_msi msi;
+                __u32 pad[8];
+        } u;
+};
+
+/* gsi routing entry types */
+#define KVM_IRQ_ROUTING_IRQCHIP 1
+#define KVM_IRQ_ROUTING_MSI 2
+
+No flags are specified so far, the corresponding field must be set to zero.
+
+struct kvm_irq_routing_irqchip {
+        __u32 irqchip;
+        __u32 pin;
+};
+
+struct kvm_irq_routing_msi {
+        __u32 address_lo;
+        __u32 address_hi;
+        __u32 data;
+        __u32 pad;
+};
+
+4.53 KVM_ASSIGN_SET_MSIX_NR
+
+Capability: KVM_CAP_DEVICE_MSIX
+Architectures: x86 ia64
+Type: vm ioctl
+Parameters: struct kvm_assigned_msix_nr (in)
+Returns: 0 on success, -1 on error
+
+Set the number of MSI-X interrupts for an assigned device. This service can
+only be called once in the lifetime of an assigned device.
+
+struct kvm_assigned_msix_nr {
+        __u32 assigned_dev_id;
+        __u16 entry_nr;
+        __u16 padding;
+};
+
+#define KVM_MAX_MSIX_PER_DEV            256
+
+4.54 KVM_ASSIGN_SET_MSIX_ENTRY
+
+Capability: KVM_CAP_DEVICE_MSIX
+Architectures: x86 ia64
+Type: vm ioctl
+Parameters: struct kvm_assigned_msix_entry (in)
+Returns: 0 on success, -1 on error
+
+Specifies the routing of an MSI-X assigned device interrupt to a GSI. Setting
+the GSI vector to zero means disabling the interrupt.
+
+struct kvm_assigned_msix_entry {
+        __u32 assigned_dev_id;
+        __u32 gsi;
+        __u16 entry; /* The index of entry in the MSI-X table */
+        __u16 padding[3];
+};
+
+5. The kvm_run structure
+
+Application code obtains a pointer to the kvm_run structure by
+mmap()ing a vcpu fd.  From that point, application code can control
+execution by changing fields in kvm_run prior to calling the KVM_RUN
+ioctl, and obtain information about the reason KVM_RUN returned by
+looking up structure members.
+
+struct kvm_run {
+        /* in */
+        __u8 request_interrupt_window;
+
+Request that KVM_RUN return when it becomes possible to inject external
+interrupts into the guest.  Useful in conjunction with KVM_INTERRUPT.
+
+        __u8 padding1[7];
+
+        /* out */
+        __u32 exit_reason;
+
+When KVM_RUN has returned successfully (return value 0), this informs
+application code why KVM_RUN has returned.  Allowable values for this
+field are detailed below.
+
+        __u8 ready_for_interrupt_injection;
+
+If request_interrupt_window has been specified, this field indicates
+an interrupt can be injected now with KVM_INTERRUPT.
+
+        __u8 if_flag;
+
+The value of the current interrupt flag.  Only valid if in-kernel
+local APIC is not used.
+
+        __u8 padding2[2];
+
+        /* in (pre_kvm_run), out (post_kvm_run) */
+        __u64 cr8;
+
+The value of the cr8 register.  Only valid if in-kernel local APIC is
+not used.  Both input and output.
+
+        __u64 apic_base;
+
+The value of the APIC BASE msr.  Only valid if in-kernel local
+APIC is not used.  Both input and output.
+
+        union {
+                /* KVM_EXIT_UNKNOWN */
+                struct {
+                        __u64 hardware_exit_reason;
+                } hw;
+
+If exit_reason is KVM_EXIT_UNKNOWN, the vcpu has exited due to unknown
+reasons.  Further architecture-specific information is available in
+hardware_exit_reason.
+
+                /* KVM_EXIT_FAIL_ENTRY */
+                struct {
+                        __u64 hardware_entry_failure_reason;
+                } fail_entry;
+
+If exit_reason is KVM_EXIT_FAIL_ENTRY, the vcpu could not be run due
+to unknown reasons.  Further architecture-specific information is
+available in hardware_entry_failure_reason.
+
+                /* KVM_EXIT_EXCEPTION */
+                struct {
+                        __u32 exception;
+                        __u32 error_code;
+                } ex;
+
+Unused.
+
+                /* KVM_EXIT_IO */
+                struct {
+#define KVM_EXIT_IO_IN  0
+#define KVM_EXIT_IO_OUT 1
+                        __u8 direction;
+                        __u8 size; /* bytes */
+                        __u16 port;
+                        __u32 count;
+                        __u64 data_offset; /* relative to kvm_run start */
+                } io;
+
+If exit_reason is KVM_EXIT_IO, then the vcpu has
+executed a port I/O instruction which could not be satisfied by kvm.
+data_offset describes where the data is located (KVM_EXIT_IO_OUT) or
+where kvm expects application code to place the data for the next
+KVM_RUN invocation (KVM_EXIT_IO_IN).  Data format is a packed array.
+
+                struct {
+                        struct kvm_debug_exit_arch arch;
+                } debug;
+
+Unused.
+
+                /* KVM_EXIT_MMIO */
+                struct {
+                        __u64 phys_addr;
+                        __u8  data[8];
+                        __u32 len;
+                        __u8  is_write;
+                } mmio;
+
+If exit_reason is KVM_EXIT_MMIO, then the vcpu has
+executed a memory-mapped I/O instruction which could not be satisfied
+by kvm.  The 'data' member contains the written data if 'is_write' is
+true, and should be filled by application code otherwise.
+
+NOTE: For KVM_EXIT_IO, KVM_EXIT_MMIO and KVM_EXIT_OSI, the corresponding
+operations are complete (and guest state is consistent) only after userspace
+has re-entered the kernel with KVM_RUN.  The kernel side will first finish
+incomplete operations and then check for pending signals.  Userspace
+can re-enter the guest with an unmasked signal pending to complete
+pending operations.
+
+                /* KVM_EXIT_HYPERCALL */
+                struct {
+                        __u64 nr;
+                        __u64 args[6];
+                        __u64 ret;
+                        __u32 longmode;
+                        __u32 pad;
+                } hypercall;
+
+Unused.  This was once used for 'hypercall to userspace'.  To implement
+such functionality, use KVM_EXIT_IO (x86) or KVM_EXIT_MMIO (all except s390).
+Note KVM_EXIT_IO is significantly faster than KVM_EXIT_MMIO.
+
+                /* KVM_EXIT_TPR_ACCESS */
+                struct {
+                        __u64 rip;
+                        __u32 is_write;
+                        __u32 pad;
+                } tpr_access;
+
+To be documented (KVM_TPR_ACCESS_REPORTING).
+
+                /* KVM_EXIT_S390_SIEIC */
+                struct {
+                        __u8 icptcode;
+                        __u64 mask; /* psw upper half */
+                        __u64 addr; /* psw lower half */
+                        __u16 ipa;
+                        __u32 ipb;
+                } s390_sieic;
+
+s390 specific.
+
+                /* KVM_EXIT_S390_RESET */
+#define KVM_S390_RESET_POR       1
+#define KVM_S390_RESET_CLEAR     2
+#define KVM_S390_RESET_SUBSYSTEM 4
+#define KVM_S390_RESET_CPU_INIT  8
+#define KVM_S390_RESET_IPL       16
+                __u64 s390_reset_flags;
+
+s390 specific.
+
+                /* KVM_EXIT_DCR */
+                struct {
+                        __u32 dcrn;
+                        __u32 data;
+                        __u8  is_write;
+                } dcr;
+
+powerpc specific.
+
+                /* KVM_EXIT_OSI */
+                struct {
+                        __u64 gprs[32];
+                } osi;
+
+MOL uses a special hypercall interface it calls 'OSI'. To enable it, we catch
+hypercalls and exit with this exit struct that contains all the guest gprs.
+
+If exit_reason is KVM_EXIT_OSI, then the vcpu has triggered such a hypercall.
+Userspace can now handle the hypercall and when it's done modify the gprs as
+necessary. Upon guest entry all guest GPRs will then be replaced by the values
+in this struct.
+
+                /* Fix the size of the union. */
+                char padding[256];
+        };
+};
diff --git a/Documentation/virtual/kvm/cpuid.txt b/Documentation/virtual/kvm/cpuid.txt
new file mode 100644
index 000000000000..882068538c9c
--- /dev/null
+++ b/Documentation/virtual/kvm/cpuid.txt
@@ -0,0 +1,45 @@
+KVM CPUID bits
+Glauber Costa <glommer@redhat.com>, Red Hat Inc, 2010
+=====================================================
+
+A guest running on a kvm host, can check some of its features using
+cpuid. This is not always guaranteed to work, since userspace can
+mask-out some, or even all KVM-related cpuid features before launching
+a guest.
+
+KVM cpuid functions are:
+
+function: KVM_CPUID_SIGNATURE (0x40000000)
+returns : eax = 0,
+          ebx = 0x4b4d564b,
+          ecx = 0x564b4d56,
+          edx = 0x4d.
+Note that this value in ebx, ecx and edx corresponds to the string "KVMKVMKVM".
+This function queries the presence of KVM cpuid leafs.
+
+
+function: define KVM_CPUID_FEATURES (0x40000001)
+returns : ebx, ecx, edx = 0
+          eax = and OR'ed group of (1 << flag), where each flags is:
+
+
+flag                               || value || meaning
+=============================================================================
+KVM_FEATURE_CLOCKSOURCE            ||     0 || kvmclock available at msrs
+                                   ||       || 0x11 and 0x12.
+------------------------------------------------------------------------------
+KVM_FEATURE_NOP_IO_DELAY           ||     1 || not necessary to perform delays
+                                   ||       || on PIO operations.
+------------------------------------------------------------------------------
+KVM_FEATURE_MMU_OP                 ||     2 || deprecated.
+------------------------------------------------------------------------------
+KVM_FEATURE_CLOCKSOURCE2           ||     3 || kvmclock available at msrs
+                                   ||       || 0x4b564d00 and 0x4b564d01
+------------------------------------------------------------------------------
+KVM_FEATURE_ASYNC_PF               ||     4 || async pf can be enabled by
+                                   ||       || writing to msr 0x4b564d02
+------------------------------------------------------------------------------
+KVM_FEATURE_CLOCKSOURCE_STABLE_BIT ||    24 || host will warn if no guest-side
+                                   ||       || per-cpu warps are expected in
+                                   ||       || kvmclock.
+------------------------------------------------------------------------------
diff --git a/Documentation/virtual/kvm/locking.txt b/Documentation/virtual/kvm/locking.txt
new file mode 100644
index 000000000000..3b4cd3bf5631
--- /dev/null
+++ b/Documentation/virtual/kvm/locking.txt
@@ -0,0 +1,25 @@
+KVM Lock Overview
+=================
+
+1. Acquisition Orders
+---------------------
+
+(to be written)
+
+2. Reference
+------------
+
+Name:           kvm_lock
+Type:           raw_spinlock
+Arch:           any
+Protects:       - vm_list
+                - hardware virtualization enable/disable
+Comment:        'raw' because hardware enabling/disabling must be atomic /wrt
+                migration.
+
+Name:           kvm_arch::tsc_write_lock
+Type:           raw_spinlock
+Arch:           x86
+Protects:       - kvm_arch::{last_tsc_write,last_tsc_nsec,last_tsc_offset}
+                - tsc offset in vmcb
+Comment:        'raw' because updating the tsc offsets must not be preempted.
diff --git a/Documentation/virtual/kvm/mmu.txt b/Documentation/virtual/kvm/mmu.txt
new file mode 100644
index 000000000000..f46aa58389ca
--- /dev/null
+++ b/Documentation/virtual/kvm/mmu.txt
@@ -0,0 +1,348 @@
+The x86 kvm shadow mmu
+======================
+
+The mmu (in arch/x86/kvm, files mmu.[ch] and paging_tmpl.h) is responsible
+for presenting a standard x86 mmu to the guest, while translating guest
+physical addresses to host physical addresses.
+
+The mmu code attempts to satisfy the following requirements:
+
+- correctness: the guest should not be able to determine that it is running
+               on an emulated mmu except for timing (we attempt to comply
+               with the specification, not emulate the characteristics of
+               a particular implementation such as tlb size)
+- security:    the guest must not be able to touch host memory not assigned
+               to it
+- performance: minimize the performance penalty imposed by the mmu
+- scaling:     need to scale to large memory and large vcpu guests
+- hardware:    support the full range of x86 virtualization hardware
+- integration: Linux memory management code must be in control of guest memory
+               so that swapping, page migration, page merging, transparent
+               hugepages, and similar features work without change
+- dirty tracking: report writes to guest memory to enable live migration
+               and framebuffer-based displays
+- footprint:   keep the amount of pinned kernel memory low (most memory
+               should be shrinkable)
+- reliability:  avoid multipage or GFP_ATOMIC allocations
+
+Acronyms
+========
+
+pfn   host page frame number
+hpa   host physical address
+hva   host virtual address
+gfn   guest frame number
+gpa   guest physical address
+gva   guest virtual address
+ngpa  nested guest physical address
+ngva  nested guest virtual address
+pte   page table entry (used also to refer generically to paging structure
+      entries)
+gpte  guest pte (referring to gfns)
+spte  shadow pte (referring to pfns)
+tdp   two dimensional paging (vendor neutral term for NPT and EPT)
+
+Virtual and real hardware supported
+===================================
+
+The mmu supports first-generation mmu hardware, which allows an atomic switch
+of the current paging mode and cr3 during guest entry, as well as
+two-dimensional paging (AMD's NPT and Intel's EPT).  The emulated hardware
+it exposes is the traditional 2/3/4 level x86 mmu, with support for global
+pages, pae, pse, pse36, cr0.wp, and 1GB pages.  Work is in progress to support
+exposing NPT capable hardware on NPT capable hosts.
+
+Translation
+===========
+
+The primary job of the mmu is to program the processor's mmu to translate
+addresses for the guest.  Different translations are required at different
+times:
+
+- when guest paging is disabled, we translate guest physical addresses to
+  host physical addresses (gpa->hpa)
+- when guest paging is enabled, we translate guest virtual addresses, to
+  guest physical addresses, to host physical addresses (gva->gpa->hpa)
+- when the guest launches a guest of its own, we translate nested guest
+  virtual addresses, to nested guest physical addresses, to guest physical
+  addresses, to host physical addresses (ngva->ngpa->gpa->hpa)
+
+The primary challenge is to encode between 1 and 3 translations into hardware
+that support only 1 (traditional) and 2 (tdp) translations.  When the
+number of required translations matches the hardware, the mmu operates in
+direct mode; otherwise it operates in shadow mode (see below).
+
+Memory
+======
+
+Guest memory (gpa) is part of the user address space of the process that is
+using kvm.  Userspace defines the translation between guest addresses and user
+addresses (gpa->hva); note that two gpas may alias to the same hva, but not
+vice versa.
+
+These hvas may be backed using any method available to the host: anonymous
+memory, file backed memory, and device memory.  Memory might be paged by the
+host at any time.
+
+Events
+======
+
+The mmu is driven by events, some from the guest, some from the host.
+
+Guest generated events:
+- writes to control registers (especially cr3)
+- invlpg/invlpga instruction execution
+- access to missing or protected translations
+
+Host generated events:
+- changes in the gpa->hpa translation (either through gpa->hva changes or
+  through hva->hpa changes)
+- memory pressure (the shrinker)
+
+Shadow pages
+============
+
+The principal data structure is the shadow page, 'struct kvm_mmu_page'.  A
+shadow page contains 512 sptes, which can be either leaf or nonleaf sptes.  A
+shadow page may contain a mix of leaf and nonleaf sptes.
+
+A nonleaf spte allows the hardware mmu to reach the leaf pages and
+is not related to a translation directly.  It points to other shadow pages.
+
+A leaf spte corresponds to either one or two translations encoded into
+one paging structure entry.  These are always the lowest level of the
+translation stack, with optional higher level translations left to NPT/EPT.
+Leaf ptes point at guest pages.
+
+The following table shows translations encoded by leaf ptes, with higher-level
+translations in parentheses:
+
+ Non-nested guests:
+  nonpaging:     gpa->hpa
+  paging:        gva->gpa->hpa
+  paging, tdp:   (gva->)gpa->hpa
+ Nested guests:
+  non-tdp:       ngva->gpa->hpa  (*)
+  tdp:           (ngva->)ngpa->gpa->hpa
+
+(*) the guest hypervisor will encode the ngva->gpa translation into its page
+    tables if npt is not present
+
+Shadow pages contain the following information:
+  role.level:
+    The level in the shadow paging hierarchy that this shadow page belongs to.
+    1=4k sptes, 2=2M sptes, 3=1G sptes, etc.
+  role.direct:
+    If set, leaf sptes reachable from this page are for a linear range.
+    Examples include real mode translation, large guest pages backed by small
+    host pages, and gpa->hpa translations when NPT or EPT is active.
+    The linear range starts at (gfn << PAGE_SHIFT) and its size is determined
+    by role.level (2MB for first level, 1GB for second level, 0.5TB for third
+    level, 256TB for fourth level)
+    If clear, this page corresponds to a guest page table denoted by the gfn
+    field.
+  role.quadrant:
+    When role.cr4_pae=0, the guest uses 32-bit gptes while the host uses 64-bit
+    sptes.  That means a guest page table contains more ptes than the host,
+    so multiple shadow pages are needed to shadow one guest page.
+    For first-level shadow pages, role.quadrant can be 0 or 1 and denotes the
+    first or second 512-gpte block in the guest page table.  For second-level
+    page tables, each 32-bit gpte is converted to two 64-bit sptes
+    (since each first-level guest page is shadowed by two first-level
+    shadow pages) so role.quadrant takes values in the range 0..3.  Each
+    quadrant maps 1GB virtual address space.
+  role.access:
+    Inherited guest access permissions in the form uwx.  Note execute
+    permission is positive, not negative.
+  role.invalid:
+    The page is invalid and should not be used.  It is a root page that is
+    currently pinned (by a cpu hardware register pointing to it); once it is
+    unpinned it will be destroyed.
+  role.cr4_pae:
+    Contains the value of cr4.pae for which the page is valid (e.g. whether
+    32-bit or 64-bit gptes are in use).
+  role.nxe:
+    Contains the value of efer.nxe for which the page is valid.
+  role.cr0_wp:
+    Contains the value of cr0.wp for which the page is valid.
+  gfn:
+    Either the guest page table containing the translations shadowed by this
+    page, or the base page frame for linear translations.  See role.direct.
+  spt:
+    A pageful of 64-bit sptes containing the translations for this page.
+    Accessed by both kvm and hardware.
+    The page pointed to by spt will have its page->private pointing back
+    at the shadow page structure.
+    sptes in spt point either at guest pages, or at lower-level shadow pages.
+    Specifically, if sp1 and sp2 are shadow pages, then sp1->spt[n] may point
+    at __pa(sp2->spt).  sp2 will point back at sp1 through parent_pte.
+    The spt array forms a DAG structure with the shadow page as a node, and
+    guest pages as leaves.
+  gfns:
+    An array of 512 guest frame numbers, one for each present pte.  Used to
+    perform a reverse map from a pte to a gfn. When role.direct is set, any
+    element of this array can be calculated from the gfn field when used, in
+    this case, the array of gfns is not allocated. See role.direct and gfn.
+  slot_bitmap:
+    A bitmap containing one bit per memory slot.  If the page contains a pte
+    mapping a page from memory slot n, then bit n of slot_bitmap will be set
+    (if a page is aliased among several slots, then it is not guaranteed that
+    all slots will be marked).
+    Used during dirty logging to avoid scanning a shadow page if none if its
+    pages need tracking.
+  root_count:
+    A counter keeping track of how many hardware registers (guest cr3 or
+    pdptrs) are now pointing at the page.  While this counter is nonzero, the
+    page cannot be destroyed.  See role.invalid.
+  multimapped:
+    Whether there exist multiple sptes pointing at this page.
+  parent_pte/parent_ptes:
+    If multimapped is zero, parent_pte points at the single spte that points at
+    this page's spt.  Otherwise, parent_ptes points at a data structure
+    with a list of parent_ptes.
+  unsync:
+    If true, then the translations in this page may not match the guest's
+    translation.  This is equivalent to the state of the tlb when a pte is
+    changed but before the tlb entry is flushed.  Accordingly, unsync ptes
+    are synchronized when the guest executes invlpg or flushes its tlb by
+    other means.  Valid for leaf pages.
+  unsync_children:
+    How many sptes in the page point at pages that are unsync (or have
+    unsynchronized children).
+  unsync_child_bitmap:
+    A bitmap indicating which sptes in spt point (directly or indirectly) at
+    pages that may be unsynchronized.  Used to quickly locate all unsychronized
+    pages reachable from a given page.
+
+Reverse map
+===========
+
+The mmu maintains a reverse mapping whereby all ptes mapping a page can be
+reached given its gfn.  This is used, for example, when swapping out a page.
+
+Synchronized and unsynchronized pages
+=====================================
+
+The guest uses two events to synchronize its tlb and page tables: tlb flushes
+and page invalidations (invlpg).
+
+A tlb flush means that we need to synchronize all sptes reachable from the
+guest's cr3.  This is expensive, so we keep all guest page tables write
+protected, and synchronize sptes to gptes when a gpte is written.
+
+A special case is when a guest page table is reachable from the current
+guest cr3.  In this case, the guest is obliged to issue an invlpg instruction
+before using the translation.  We take advantage of that by removing write
+protection from the guest page, and allowing the guest to modify it freely.
+We synchronize modified gptes when the guest invokes invlpg.  This reduces
+the amount of emulation we have to do when the guest modifies multiple gptes,
+or when the a guest page is no longer used as a page table and is used for
+random guest data.
+
+As a side effect we have to resynchronize all reachable unsynchronized shadow
+pages on a tlb flush.
+
+
+Reaction to events
+==================
+
+- guest page fault (or npt page fault, or ept violation)
+
+This is the most complicated event.  The cause of a page fault can be:
+
+  - a true guest fault (the guest translation won't allow the access) (*)
+  - access to a missing translation
+  - access to a protected translation
+    - when logging dirty pages, memory is write protected
+    - synchronized shadow pages are write protected (*)
+  - access to untranslatable memory (mmio)
+
+  (*) not applicable in direct mode
+
+Handling a page fault is performed as follows:
+
+ - if needed, walk the guest page tables to determine the guest translation
+   (gva->gpa or ngpa->gpa)
+   - if permissions are insufficient, reflect the fault back to the guest
+ - determine the host page
+   - if this is an mmio request, there is no host page; call the emulator
+     to emulate the instruction instead
+ - walk the shadow page table to find the spte for the translation,
+   instantiating missing intermediate page tables as necessary
+ - try to unsynchronize the page
+   - if successful, we can let the guest continue and modify the gpte
+ - emulate the instruction
+   - if failed, unshadow the page and let the guest continue
+ - update any translations that were modified by the instruction
+
+invlpg handling:
+
+  - walk the shadow page hierarchy and drop affected translations
+  - try to reinstantiate the indicated translation in the hope that the
+    guest will use it in the near future
+
+Guest control register updates:
+
+- mov to cr3
+  - look up new shadow roots
+  - synchronize newly reachable shadow pages
+
+- mov to cr0/cr4/efer
+  - set up mmu context for new paging mode
+  - look up new shadow roots
+  - synchronize newly reachable shadow pages
+
+Host translation updates:
+
+  - mmu notifier called with updated hva
+  - look up affected sptes through reverse map
+  - drop (or update) translations
+
+Emulating cr0.wp
+================
+
+If tdp is not enabled, the host must keep cr0.wp=1 so page write protection
+works for the guest kernel, not guest guest userspace.  When the guest
+cr0.wp=1, this does not present a problem.  However when the guest cr0.wp=0,
+we cannot map the permissions for gpte.u=1, gpte.w=0 to any spte (the
+semantics require allowing any guest kernel access plus user read access).
+
+We handle this by mapping the permissions to two possible sptes, depending
+on fault type:
+
+- kernel write fault: spte.u=0, spte.w=1 (allows full kernel access,
+  disallows user access)
+- read fault: spte.u=1, spte.w=0 (allows full read access, disallows kernel
+  write access)
+
+(user write faults generate a #PF)
+
+Large pages
+===========
+
+The mmu supports all combinations of large and small guest and host pages.
+Supported page sizes include 4k, 2M, 4M, and 1G.  4M pages are treated as
+two separate 2M pages, on both guest and host, since the mmu always uses PAE
+paging.
+
+To instantiate a large spte, four constraints must be satisfied:
+
+- the spte must point to a large host page
+- the guest pte must be a large pte of at least equivalent size (if tdp is
+  enabled, there is no guest pte and this condition is satisified)
+- if the spte will be writeable, the large page frame may not overlap any
+  write-protected pages
+- the guest page must be wholly contained by a single memory slot
+
+To check the last two conditions, the mmu maintains a ->write_count set of
+arrays for each memory slot and large page size.  Every write protected page
+causes its write_count to be incremented, thus preventing instantiation of
+a large spte.  The frames at the end of an unaligned memory slot have
+artificically inflated ->write_counts so they can never be instantiated.
+
+Further reading
+===============
+
+- NPT presentation from KVM Forum 2008
+  http://www.linux-kvm.org/wiki/images/c/c8/KvmForum2008%24kdf2008_21.pdf
+
diff --git a/Documentation/virtual/kvm/msr.txt b/Documentation/virtual/kvm/msr.txt
new file mode 100644
index 000000000000..d079aed27e03
--- /dev/null
+++ b/Documentation/virtual/kvm/msr.txt
@@ -0,0 +1,187 @@
+KVM-specific MSRs.
+Glauber Costa <glommer@redhat.com>, Red Hat Inc, 2010
+=====================================================
+
+KVM makes use of some custom MSRs to service some requests.
+
+Custom MSRs have a range reserved for them, that goes from
+0x4b564d00 to 0x4b564dff. There are MSRs outside this area,
+but they are deprecated and their use is discouraged.
+
+Custom MSR list
+--------
+
+The current supported Custom MSR list is:
+
+MSR_KVM_WALL_CLOCK_NEW:   0x4b564d00
+
+        data: 4-byte alignment physical address of a memory area which must be
+        in guest RAM. This memory is expected to hold a copy of the following
+        structure:
+
+        struct pvclock_wall_clock {
+                u32   version;
+                u32   sec;
+                u32   nsec;
+        } __attribute__((__packed__));
+
+        whose data will be filled in by the hypervisor. The hypervisor is only
+        guaranteed to update this data at the moment of MSR write.
+        Users that want to reliably query this information more than once have
+        to write more than once to this MSR. Fields have the following meanings:
+
+                version: guest has to check version before and after grabbing
+                time information and check that they are both equal and even.
+                An odd version indicates an in-progress update.
+
+                sec: number of seconds for wallclock.
+
+                nsec: number of nanoseconds for wallclock.
+
+        Note that although MSRs are per-CPU entities, the effect of this
+        particular MSR is global.
+
+        Availability of this MSR must be checked via bit 3 in 0x4000001 cpuid
+        leaf prior to usage.
+
+MSR_KVM_SYSTEM_TIME_NEW:  0x4b564d01
+
+        data: 4-byte aligned physical address of a memory area which must be in
+        guest RAM, plus an enable bit in bit 0. This memory is expected to hold
+        a copy of the following structure:
+
+        struct pvclock_vcpu_time_info {
+                u32   version;
+                u32   pad0;
+                u64   tsc_timestamp;
+                u64   system_time;
+                u32   tsc_to_system_mul;
+                s8    tsc_shift;
+                u8    flags;
+                u8    pad[2];
+        } __attribute__((__packed__)); /* 32 bytes */
+
+        whose data will be filled in by the hypervisor periodically. Only one
+        write, or registration, is needed for each VCPU. The interval between
+        updates of this structure is arbitrary and implementation-dependent.
+        The hypervisor may update this structure at any time it sees fit until
+        anything with bit0 == 0 is written to it.
+
+        Fields have the following meanings:
+
+                version: guest has to check version before and after grabbing
+                time information and check that they are both equal and even.
+                An odd version indicates an in-progress update.
+
+                tsc_timestamp: the tsc value at the current VCPU at the time
+                of the update of this structure. Guests can subtract this value
+                from current tsc to derive a notion of elapsed time since the
+                structure update.
+
+                system_time: a host notion of monotonic time, including sleep
+                time at the time this structure was last updated. Unit is
+                nanoseconds.
+
+                tsc_to_system_mul: a function of the tsc frequency. One has
+                to multiply any tsc-related quantity by this value to get
+                a value in nanoseconds, besides dividing by 2^tsc_shift
+
+                tsc_shift: cycle to nanosecond divider, as a power of two, to
+                allow for shift rights. One has to shift right any tsc-related
+                quantity by this value to get a value in nanoseconds, besides
+                multiplying by tsc_to_system_mul.
+
+                With this information, guests can derive per-CPU time by
+                doing:
+
+                        time = (current_tsc - tsc_timestamp)
+                        time = (time * tsc_to_system_mul) >> tsc_shift
+                        time = time + system_time
+
+                flags: bits in this field indicate extended capabilities
+                coordinated between the guest and the hypervisor. Availability
+                of specific flags has to be checked in 0x40000001 cpuid leaf.
+                Current flags are:
+
+                 flag bit   | cpuid bit    | meaning
+                -------------------------------------------------------------
+                            |              | time measures taken across
+                     0      |      24      | multiple cpus are guaranteed to
+                            |              | be monotonic
+                -------------------------------------------------------------
+
+        Availability of this MSR must be checked via bit 3 in 0x4000001 cpuid
+        leaf prior to usage.
+
+
+MSR_KVM_WALL_CLOCK:  0x11
+
+        data and functioning: same as MSR_KVM_WALL_CLOCK_NEW. Use that instead.
+
+        This MSR falls outside the reserved KVM range and may be removed in the
+        future. Its usage is deprecated.
+
+        Availability of this MSR must be checked via bit 0 in 0x4000001 cpuid
+        leaf prior to usage.
+
+MSR_KVM_SYSTEM_TIME: 0x12
+
+        data and functioning: same as MSR_KVM_SYSTEM_TIME_NEW. Use that instead.
+
+        This MSR falls outside the reserved KVM range and may be removed in the
+        future. Its usage is deprecated.
+
+        Availability of this MSR must be checked via bit 0 in 0x4000001 cpuid
+        leaf prior to usage.
+
+        The suggested algorithm for detecting kvmclock presence is then:
+
+                if (!kvm_para_available())    /* refer to cpuid.txt */
+                        return NON_PRESENT;
+
+                flags = cpuid_eax(0x40000001);
+                if (flags & 3) {
+                        msr_kvm_system_time = MSR_KVM_SYSTEM_TIME_NEW;
+                        msr_kvm_wall_clock = MSR_KVM_WALL_CLOCK_NEW;
+                        return PRESENT;
+                } else if (flags & 0) {
+                        msr_kvm_system_time = MSR_KVM_SYSTEM_TIME;
+                        msr_kvm_wall_clock = MSR_KVM_WALL_CLOCK;
+                        return PRESENT;
+                } else
+                        return NON_PRESENT;
+
+MSR_KVM_ASYNC_PF_EN: 0x4b564d02
+        data: Bits 63-6 hold 64-byte aligned physical address of a
+        64 byte memory area which must be in guest RAM and must be
+        zeroed. Bits 5-2 are reserved and should be zero. Bit 0 is 1
+        when asynchronous page faults are enabled on the vcpu 0 when
+        disabled. Bit 2 is 1 if asynchronous page faults can be injected
+        when vcpu is in cpl == 0.
+
+        First 4 byte of 64 byte memory location will be written to by
+        the hypervisor at the time of asynchronous page fault (APF)
+        injection to indicate type of asynchronous page fault. Value
+        of 1 means that the page referred to by the page fault is not
+        present. Value 2 means that the page is now available. Disabling
+        interrupt inhibits APFs. Guest must not enable interrupt
+        before the reason is read, or it may be overwritten by another
+        APF. Since APF uses the same exception vector as regular page
+        fault guest must reset the reason to 0 before it does
+        something that can generate normal page fault.  If during page
+        fault APF reason is 0 it means that this is regular page
+        fault.
+
+        During delivery of type 1 APF cr2 contains a token that will
+        be used to notify a guest when missing page becomes
+        available. When page becomes available type 2 APF is sent with
+        cr2 set to the token associated with the page. There is special
+        kind of token 0xffffffff which tells vcpu that it should wake
+        up all processes waiting for APFs and no individual type 2 APFs
+        will be sent.
+
+        If APF is disabled while there are outstanding APFs, they will
+        not be delivered.
+
+        Currently type 2 APF will be always delivered on the same vcpu as
+        type 1 was, but guest should not rely on that.
diff --git a/Documentation/virtual/kvm/ppc-pv.txt b/Documentation/virtual/kvm/ppc-pv.txt
new file mode 100644
index 000000000000..3ab969c59046
--- /dev/null
+++ b/Documentation/virtual/kvm/ppc-pv.txt
@@ -0,0 +1,196 @@
+The PPC KVM paravirtual interface
+=================================
+
+The basic execution principle by which KVM on PowerPC works is to run all kernel
+space code in PR=1 which is user space. This way we trap all privileged
+instructions and can emulate them accordingly.
+
+Unfortunately that is also the downfall. There are quite some privileged
+instructions that needlessly return us to the hypervisor even though they
+could be handled differently.
+
+This is what the PPC PV interface helps with. It takes privileged instructions
+and transforms them into unprivileged ones with some help from the hypervisor.
+This cuts down virtualization costs by about 50% on some of my benchmarks.
+
+The code for that interface can be found in arch/powerpc/kernel/kvm*
+
+Querying for existence
+======================
+
+To find out if we're running on KVM or not, we leverage the device tree. When
+Linux is running on KVM, a node /hypervisor exists. That node contains a
+compatible property with the value "linux,kvm".
+
+Once you determined you're running under a PV capable KVM, you can now use
+hypercalls as described below.
+
+KVM hypercalls
+==============
+
+Inside the device tree's /hypervisor node there's a property called
+'hypercall-instructions'. This property contains at most 4 opcodes that make
+up the hypercall. To call a hypercall, just call these instructions.
+
+The parameters are as follows:
+
+        Register        IN                      OUT
+
+        r0              -                       volatile
+        r3              1st parameter           Return code
+        r4              2nd parameter           1st output value
+        r5              3rd parameter           2nd output value
+        r6              4th parameter           3rd output value
+        r7              5th parameter           4th output value
+        r8              6th parameter           5th output value
+        r9              7th parameter           6th output value
+        r10             8th parameter           7th output value
+        r11             hypercall number        8th output value
+        r12             -                       volatile
+
+Hypercall definitions are shared in generic code, so the same hypercall numbers
+apply for x86 and powerpc alike with the exception that each KVM hypercall
+also needs to be ORed with the KVM vendor code which is (42 << 16).
+
+Return codes can be as follows:
+
+        Code            Meaning
+
+        0               Success
+        12              Hypercall not implemented
+        <0              Error
+
+The magic page
+==============
+
+To enable communication between the hypervisor and guest there is a new shared
+page that contains parts of supervisor visible register state. The guest can
+map this shared page using the KVM hypercall KVM_HC_PPC_MAP_MAGIC_PAGE.
+
+With this hypercall issued the guest always gets the magic page mapped at the
+desired location in effective and physical address space. For now, we always
+map the page to -4096. This way we can access it using absolute load and store
+functions. The following instruction reads the first field of the magic page:
+
+        ld      rX, -4096(0)
+
+The interface is designed to be extensible should there be need later to add
+additional registers to the magic page. If you add fields to the magic page,
+also define a new hypercall feature to indicate that the host can give you more
+registers. Only if the host supports the additional features, make use of them.
+
+The magic page has the following layout as described in
+arch/powerpc/include/asm/kvm_para.h:
+
+struct kvm_vcpu_arch_shared {
+        __u64 scratch1;
+        __u64 scratch2;
+        __u64 scratch3;
+        __u64 critical;         /* Guest may not get interrupts if == r1 */
+        __u64 sprg0;
+        __u64 sprg1;
+        __u64 sprg2;
+        __u64 sprg3;
+        __u64 srr0;
+        __u64 srr1;
+        __u64 dar;
+        __u64 msr;
+        __u32 dsisr;
+        __u32 int_pending;      /* Tells the guest if we have an interrupt */
+};
+
+Additions to the page must only occur at the end. Struct fields are always 32
+or 64 bit aligned, depending on them being 32 or 64 bit wide respectively.
+
+Magic page features
+===================
+
+When mapping the magic page using the KVM hypercall KVM_HC_PPC_MAP_MAGIC_PAGE,
+a second return value is passed to the guest. This second return value contains
+a bitmap of available features inside the magic page.
+
+The following enhancements to the magic page are currently available:
+
+  KVM_MAGIC_FEAT_SR             Maps SR registers r/w in the magic page
+
+For enhanced features in the magic page, please check for the existence of the
+feature before using them!
+
+MSR bits
+========
+
+The MSR contains bits that require hypervisor intervention and bits that do
+not require direct hypervisor intervention because they only get interpreted
+when entering the guest or don't have any impact on the hypervisor's behavior.
+
+The following bits are safe to be set inside the guest:
+
+  MSR_EE
+  MSR_RI
+  MSR_CR
+  MSR_ME
+
+If any other bit changes in the MSR, please still use mtmsr(d).
+
+Patched instructions
+====================
+
+The "ld" and "std" instructions are transormed to "lwz" and "stw" instructions
+respectively on 32 bit systems with an added offset of 4 to accommodate for big
+endianness.
+
+The following is a list of mapping the Linux kernel performs when running as
+guest. Implementing any of those mappings is optional, as the instruction traps
+also act on the shared page. So calling privileged instructions still works as
+before.
+
+From                    To
+====                    ==
+
+mfmsr   rX              ld      rX, magic_page->msr
+mfsprg  rX, 0           ld      rX, magic_page->sprg0
+mfsprg  rX, 1           ld      rX, magic_page->sprg1
+mfsprg  rX, 2           ld      rX, magic_page->sprg2
+mfsprg  rX, 3           ld      rX, magic_page->sprg3
+mfsrr0  rX              ld      rX, magic_page->srr0
+mfsrr1  rX              ld      rX, magic_page->srr1
+mfdar   rX              ld      rX, magic_page->dar
+mfdsisr rX              lwz     rX, magic_page->dsisr
+
+mtmsr   rX              std     rX, magic_page->msr
+mtsprg  0, rX           std     rX, magic_page->sprg0
+mtsprg  1, rX           std     rX, magic_page->sprg1
+mtsprg  2, rX           std     rX, magic_page->sprg2
+mtsprg  3, rX           std     rX, magic_page->sprg3
+mtsrr0  rX              std     rX, magic_page->srr0
+mtsrr1  rX              std     rX, magic_page->srr1
+mtdar   rX              std     rX, magic_page->dar
+mtdsisr rX              stw     rX, magic_page->dsisr
+
+tlbsync                 nop
+
+mtmsrd  rX, 0           b       <special mtmsr section>
+mtmsr   rX              b       <special mtmsr section>
+
+mtmsrd  rX, 1           b       <special mtmsrd section>
+
+[Book3S only]
+mtsrin  rX, rY          b       <special mtsrin section>
+
+[BookE only]
+wrteei  [0|1]           b       <special wrteei section>
+
+
+Some instructions require more logic to determine what's going on than a load
+or store instruction can deliver. To enable patching of those, we keep some
+RAM around where we can live translate instructions to. What happens is the
+following:
+
+        1) copy emulation code to memory
+        2) patch that code to fit the emulated instruction
+        3) patch that code to return to the original pc + 4
+        4) patch the original instruction to branch to the new code
+
+That way we can inject an arbitrary amount of code as replacement for a single
+instruction. This allows us to check for pending interrupts when setting EE=1
+for example.
diff --git a/Documentation/virtual/kvm/review-checklist.txt b/Documentation/virtual/kvm/review-checklist.txt
new file mode 100644
index 000000000000..a850986ed684
--- /dev/null
+++ b/Documentation/virtual/kvm/review-checklist.txt
@@ -0,0 +1,38 @@
+Review checklist for kvm patches
+================================
+
+1.  The patch must follow Documentation/CodingStyle and
+    Documentation/SubmittingPatches.
+
+2.  Patches should be against kvm.git master branch.
+
+3.  If the patch introduces or modifies a new userspace API:
+    - the API must be documented in Documentation/virtual/kvm/api.txt
+    - the API must be discoverable using KVM_CHECK_EXTENSION
+
+4.  New state must include support for save/restore.
+
+5.  New features must default to off (userspace should explicitly request them).
+    Performance improvements can and should default to on.
+
+6.  New cpu features should be exposed via KVM_GET_SUPPORTED_CPUID2
+
+7.  Emulator changes should be accompanied by unit tests for qemu-kvm.git
+    kvm/test directory.
+
+8.  Changes should be vendor neutral when possible.  Changes to common code
+    are better than duplicating changes to vendor code.
+
+9.  Similarly, prefer changes to arch independent code than to arch dependent
+    code.
+
+10. User/kernel interfaces and guest/host interfaces must be 64-bit clean
+    (all variables and sizes naturally aligned on 64-bit; use specific types
+    only - u64 rather than ulong).
+
+11. New guest visible features must either be documented in a hardware manual
+    or be accompanied by documentation.
+
+12. Features must be robust against reset and kexec - for example, shared
+    host/guest memory must be unshared to prevent the host from writing to
+    guest memory that the guest has not reserved for this purpose.
diff --git a/Documentation/virtual/kvm/timekeeping.txt b/Documentation/virtual/kvm/timekeeping.txt
new file mode 100644
index 000000000000..df8946377cb6
--- /dev/null
+++ b/Documentation/virtual/kvm/timekeeping.txt
@@ -0,0 +1,612 @@
+
+        Timekeeping Virtualization for X86-Based Architectures
+
+        Zachary Amsden <zamsden@redhat.com>
+        Copyright (c) 2010, Red Hat.  All rights reserved.
+
+1) Overview
+2) Timing Devices
+3) TSC Hardware
+4) Virtualization Problems
+
+=========================================================================
+
+1) Overview
+
+One of the most complicated parts of the X86 platform, and specifically,
+the virtualization of this platform is the plethora of timing devices available
+and the complexity of emulating those devices.  In addition, virtualization of
+time introduces a new set of challenges because it introduces a multiplexed
+division of time beyond the control of the guest CPU.
+
+First, we will describe the various timekeeping hardware available, then
+present some of the problems which arise and solutions available, giving
+specific recommendations for certain classes of KVM guests.
+
+The purpose of this document is to collect data and information relevant to
+timekeeping which may be difficult to find elsewhere, specifically,
+information relevant to KVM and hardware-based virtualization.
+
+=========================================================================
+
+2) Timing Devices
+
+First we discuss the basic hardware devices available.  TSC and the related
+KVM clock are special enough to warrant a full exposition and are described in
+the following section.
+
+2.1) i8254 - PIT
+
+One of the first timer devices available is the programmable interrupt timer,
+or PIT.  The PIT has a fixed frequency 1.193182 MHz base clock and three
+channels which can be programmed to deliver periodic or one-shot interrupts.
+These three channels can be configured in different modes and have individual
+counters.  Channel 1 and 2 were not available for general use in the original
+IBM PC, and historically were connected to control RAM refresh and the PC
+speaker.  Now the PIT is typically integrated as part of an emulated chipset
+and a separate physical PIT is not used.
+
+The PIT uses I/O ports 0x40 - 0x43.  Access to the 16-bit counters is done
+using single or multiple byte access to the I/O ports.  There are 6 modes
+available, but not all modes are available to all timers, as only timer 2
+has a connected gate input, required for modes 1 and 5.  The gate line is
+controlled by port 61h, bit 0, as illustrated in the following diagram.
+
+ --------------             ----------------
+|              |           |                |
+|  1.1932 MHz  |---------->| CLOCK      OUT | ---------> IRQ 0
+|    Clock     |   |       |                |
+ --------------    |    +->| GATE  TIMER 0  |
+                   |        ----------------
+                   |
+                   |        ----------------
+                   |       |                |
+                   |------>| CLOCK      OUT | ---------> 66.3 KHZ DRAM
+                   |       |                |            (aka /dev/null)
+                   |    +->| GATE  TIMER 1  |
+                   |        ----------------
+                   |
+                   |        ----------------
+                   |       |                |
+                   |------>| CLOCK      OUT | ---------> Port 61h, bit 5
+                           |                |      |
+Port 61h, bit 0 ---------->| GATE  TIMER 2  |       \_.----   ____
+                            ----------------         _|    )--|LPF|---Speaker
+                                                    / *----   \___/
+Port 61h, bit 1 -----------------------------------/
+
+The timer modes are now described.
+
+Mode 0: Single Timeout.   This is a one-shot software timeout that counts down
+ when the gate is high (always true for timers 0 and 1).  When the count
+ reaches zero, the output goes high.
+
+Mode 1: Triggered One-shot.  The output is initially set high.  When the gate
+ line is set high, a countdown is initiated (which does not stop if the gate is
+ lowered), during which the output is set low.  When the count reaches zero,
+ the output goes high.
+
+Mode 2: Rate Generator.  The output is initially set high.  When the countdown
+ reaches 1, the output goes low for one count and then returns high.  The value
+ is reloaded and the countdown automatically resumes.  If the gate line goes
+ low, the count is halted.  If the output is low when the gate is lowered, the
+ output automatically goes high (this only affects timer 2).
+
+Mode 3: Square Wave.   This generates a high / low square wave.  The count
+ determines the length of the pulse, which alternates between high and low
+ when zero is reached.  The count only proceeds when gate is high and is
+ automatically reloaded on reaching zero.  The count is decremented twice at
+ each clock to generate a full high / low cycle at the full periodic rate.
+ If the count is even, the clock remains high for N/2 counts and low for N/2
+ counts; if the clock is odd, the clock is high for (N+1)/2 counts and low
+ for (N-1)/2 counts.  Only even values are latched by the counter, so odd
+ values are not observed when reading.  This is the intended mode for timer 2,
+ which generates sine-like tones by low-pass filtering the square wave output.
+
+Mode 4: Software Strobe.  After programming this mode and loading the counter,
+ the output remains high until the counter reaches zero.  Then the output
+ goes low for 1 clock cycle and returns high.  The counter is not reloaded.
+ Counting only occurs when gate is high.
+
+Mode 5: Hardware Strobe.  After programming and loading the counter, the
+ output remains high.  When the gate is raised, a countdown is initiated
+ (which does not stop if the gate is lowered).  When the counter reaches zero,
+ the output goes low for 1 clock cycle and then returns high.  The counter is
+ not reloaded.
+
+In addition to normal binary counting, the PIT supports BCD counting.  The
+command port, 0x43 is used to set the counter and mode for each of the three
+timers.
+
+PIT commands, issued to port 0x43, using the following bit encoding:
+
+Bit 7-4: Command (See table below)
+Bit 3-1: Mode (000 = Mode 0, 101 = Mode 5, 11X = undefined)
+Bit 0  : Binary (0) / BCD (1)
+
+Command table:
+
+0000 - Latch Timer 0 count for port 0x40
+        sample and hold the count to be read in port 0x40;
+        additional commands ignored until counter is read;
+        mode bits ignored.
+
+0001 - Set Timer 0 LSB mode for port 0x40
+        set timer to read LSB only and force MSB to zero;
+        mode bits set timer mode
+
+0010 - Set Timer 0 MSB mode for port 0x40
+        set timer to read MSB only and force LSB to zero;
+        mode bits set timer mode
+
+0011 - Set Timer 0 16-bit mode for port 0x40
+        set timer to read / write LSB first, then MSB;
+        mode bits set timer mode
+
+0100 - Latch Timer 1 count for port 0x41 - as described above
+0101 - Set Timer 1 LSB mode for port 0x41 - as described above
+0110 - Set Timer 1 MSB mode for port 0x41 - as described above
+0111 - Set Timer 1 16-bit mode for port 0x41 - as described above
+
+1000 - Latch Timer 2 count for port 0x42 - as described above
+1001 - Set Timer 2 LSB mode for port 0x42 - as described above
+1010 - Set Timer 2 MSB mode for port 0x42 - as described above
+1011 - Set Timer 2 16-bit mode for port 0x42 as described above
+
+1101 - General counter latch
+        Latch combination of counters into corresponding ports
+        Bit 3 = Counter 2
+        Bit 2 = Counter 1
+        Bit 1 = Counter 0
+        Bit 0 = Unused
+
+1110 - Latch timer status
+        Latch combination of counter mode into corresponding ports
+        Bit 3 = Counter 2
+        Bit 2 = Counter 1
+        Bit 1 = Counter 0
+
+        The output of ports 0x40-0x42 following this command will be:
+
+        Bit 7 = Output pin
+        Bit 6 = Count loaded (0 if timer has expired)
+        Bit 5-4 = Read / Write mode
+            01 = MSB only
+            10 = LSB only
+            11 = LSB / MSB (16-bit)
+        Bit 3-1 = Mode
+        Bit 0 = Binary (0) / BCD mode (1)
+
+2.2) RTC
+
+The second device which was available in the original PC was the MC146818 real
+time clock.  The original device is now obsolete, and usually emulated by the
+system chipset, sometimes by an HPET and some frankenstein IRQ routing.
+
+The RTC is accessed through CMOS variables, which uses an index register to
+control which bytes are read.  Since there is only one index register, read
+of the CMOS and read of the RTC require lock protection (in addition, it is
+dangerous to allow userspace utilities such as hwclock to have direct RTC
+access, as they could corrupt kernel reads and writes of CMOS memory).
+
+The RTC generates an interrupt which is usually routed to IRQ 8.  The interrupt
+can function as a periodic timer, an additional once a day alarm, and can issue
+interrupts after an update of the CMOS registers by the MC146818 is complete.
+The type of interrupt is signalled in the RTC status registers.
+
+The RTC will update the current time fields by battery power even while the
+system is off.  The current time fields should not be read while an update is
+in progress, as indicated in the status register.
+
+The clock uses a 32.768kHz crystal, so bits 6-4 of register A should be
+programmed to a 32kHz divider if the RTC is to count seconds.
+
+This is the RAM map originally used for the RTC/CMOS:
+
+Location    Size    Description
+------------------------------------------
+00h         byte    Current second (BCD)
+01h         byte    Seconds alarm (BCD)
+02h         byte    Current minute (BCD)
+03h         byte    Minutes alarm (BCD)
+04h         byte    Current hour (BCD)
+05h         byte    Hours alarm (BCD)
+06h         byte    Current day of week (BCD)
+07h         byte    Current day of month (BCD)
+08h         byte    Current month (BCD)
+09h         byte    Current year (BCD)
+0Ah         byte    Register A
+                       bit 7   = Update in progress
+                       bit 6-4 = Divider for clock
+                                  000 = 4.194 MHz
+                                  001 = 1.049 MHz
+                                  010 = 32 kHz
+                                  10X = test modes
+                                  110 = reset / disable
+                                  111 = reset / disable
+                       bit 3-0 = Rate selection for periodic interrupt
+                                  000 = periodic timer disabled
+                                  001 = 3.90625 uS
+                                  010 = 7.8125 uS
+                                  011 = .122070 mS
+                                  100 = .244141 mS
+                                     ...
+                                 1101 = 125 mS
+                                 1110 = 250 mS
+                                 1111 = 500 mS
+0Bh         byte    Register B
+                       bit 7   = Run (0) / Halt (1)
+                       bit 6   = Periodic interrupt enable
+                       bit 5   = Alarm interrupt enable
+                       bit 4   = Update-ended interrupt enable
+                       bit 3   = Square wave interrupt enable
+                       bit 2   = BCD calendar (0) / Binary (1)
+                       bit 1   = 12-hour mode (0) / 24-hour mode (1)
+                       bit 0   = 0 (DST off) / 1 (DST enabled)
+OCh         byte    Register C (read only)
+                       bit 7   = interrupt request flag (IRQF)
+                       bit 6   = periodic interrupt flag (PF)
+                       bit 5   = alarm interrupt flag (AF)
+                       bit 4   = update interrupt flag (UF)
+                       bit 3-0 = reserved
+ODh         byte    Register D (read only)
+                       bit 7   = RTC has power
+                       bit 6-0 = reserved
+32h         byte    Current century BCD (*)
+  (*) location vendor specific and now determined from ACPI global tables
+
+2.3) APIC
+
+On Pentium and later processors, an on-board timer is available to each CPU
+as part of the Advanced Programmable Interrupt Controller.  The APIC is
+accessed through memory-mapped registers and provides interrupt service to each
+CPU, used for IPIs and local timer interrupts.
+
+Although in theory the APIC is a safe and stable source for local interrupts,
+in practice, many bugs and glitches have occurred due to the special nature of
+the APIC CPU-local memory-mapped hardware.  Beware that CPU errata may affect
+the use of the APIC and that workarounds may be required.  In addition, some of
+these workarounds pose unique constraints for virtualization - requiring either
+extra overhead incurred from extra reads of memory-mapped I/O or additional
+functionality that may be more computationally expensive to implement.
+
+Since the APIC is documented quite well in the Intel and AMD manuals, we will
+avoid repetition of the detail here.  It should be pointed out that the APIC
+timer is programmed through the LVT (local vector timer) register, is capable
+of one-shot or periodic operation, and is based on the bus clock divided down
+by the programmable divider register.
+
+2.4) HPET
+
+HPET is quite complex, and was originally intended to replace the PIT / RTC
+support of the X86 PC.  It remains to be seen whether that will be the case, as
+the de facto standard of PC hardware is to emulate these older devices.  Some
+systems designated as legacy free may support only the HPET as a hardware timer
+device.
+
+The HPET spec is rather loose and vague, requiring at least 3 hardware timers,
+but allowing implementation freedom to support many more.  It also imposes no
+fixed rate on the timer frequency, but does impose some extremal values on
+frequency, error and slew.
+
+In general, the HPET is recommended as a high precision (compared to PIT /RTC)
+time source which is independent of local variation (as there is only one HPET
+in any given system).  The HPET is also memory-mapped, and its presence is
+indicated through ACPI tables by the BIOS.
+
+Detailed specification of the HPET is beyond the current scope of this
+document, as it is also very well documented elsewhere.
+
+2.5) Offboard Timers
+
+Several cards, both proprietary (watchdog boards) and commonplace (e1000) have
+timing chips built into the cards which may have registers which are accessible
+to kernel or user drivers.  To the author's knowledge, using these to generate
+a clocksource for a Linux or other kernel has not yet been attempted and is in
+general frowned upon as not playing by the agreed rules of the game.  Such a
+timer device would require additional support to be virtualized properly and is
+not considered important at this time as no known operating system does this.
+
+=========================================================================
+
+3) TSC Hardware
+
+The TSC or time stamp counter is relatively simple in theory; it counts
+instruction cycles issued by the processor, which can be used as a measure of
+time.  In practice, due to a number of problems, it is the most complicated
+timekeeping device to use.
+
+The TSC is represented internally as a 64-bit MSR which can be read with the
+RDMSR, RDTSC, or RDTSCP (when available) instructions.  In the past, hardware
+limitations made it possible to write the TSC, but generally on old hardware it
+was only possible to write the low 32-bits of the 64-bit counter, and the upper
+32-bits of the counter were cleared.  Now, however, on Intel processors family
+0Fh, for models 3, 4 and 6, and family 06h, models e and f, this restriction
+has been lifted and all 64-bits are writable.  On AMD systems, the ability to
+write the TSC MSR is not an architectural guarantee.
+
+The TSC is accessible from CPL-0 and conditionally, for CPL > 0 software by
+means of the CR4.TSD bit, which when enabled, disables CPL > 0 TSC access.
+
+Some vendors have implemented an additional instruction, RDTSCP, which returns
+atomically not just the TSC, but an indicator which corresponds to the
+processor number.  This can be used to index into an array of TSC variables to
+determine offset information in SMP systems where TSCs are not synchronized.
+The presence of this instruction must be determined by consulting CPUID feature
+bits.
+
+Both VMX and SVM provide extension fields in the virtualization hardware which
+allows the guest visible TSC to be offset by a constant.  Newer implementations
+promise to allow the TSC to additionally be scaled, but this hardware is not
+yet widely available.
+
+3.1) TSC synchronization
+
+The TSC is a CPU-local clock in most implementations.  This means, on SMP
+platforms, the TSCs of different CPUs may start at different times depending
+on when the CPUs are powered on.  Generally, CPUs on the same die will share
+the same clock, however, this is not always the case.
+
+The BIOS may attempt to resynchronize the TSCs during the poweron process and
+the operating system or other system software may attempt to do this as well.
+Several hardware limitations make the problem worse - if it is not possible to
+write the full 64-bits of the TSC, it may be impossible to match the TSC in
+newly arriving CPUs to that of the rest of the system, resulting in
+unsynchronized TSCs.  This may be done by BIOS or system software, but in
+practice, getting a perfectly synchronized TSC will not be possible unless all
+values are read from the same clock, which generally only is possible on single
+socket systems or those with special hardware support.
+
+3.2) TSC and CPU hotplug
+
+As touched on already, CPUs which arrive later than the boot time of the system
+may not have a TSC value that is synchronized with the rest of the system.
+Either system software, BIOS, or SMM code may actually try to establish the TSC
+to a value matching the rest of the system, but a perfect match is usually not
+a guarantee.  This can have the effect of bringing a system from a state where
+TSC is synchronized back to a state where TSC synchronization flaws, however
+small, may be exposed to the OS and any virtualization environment.
+
+3.3) TSC and multi-socket / NUMA
+
+Multi-socket systems, especially large multi-socket systems are likely to have
+individual clocksources rather than a single, universally distributed clock.
+Since these clocks are driven by different crystals, they will not have
+perfectly matched frequency, and temperature and electrical variations will
+cause the CPU clocks, and thus the TSCs to drift over time.  Depending on the
+exact clock and bus design, the drift may or may not be fixed in absolute
+error, and may accumulate over time.
+
+In addition, very large systems may deliberately slew the clocks of individual
+cores.  This technique, known as spread-spectrum clocking, reduces EMI at the
+clock frequency and harmonics of it, which may be required to pass FCC
+standards for telecommunications and computer equipment.
+
+It is recommended not to trust the TSCs to remain synchronized on NUMA or
+multiple socket systems for these reasons.
+
+3.4) TSC and C-states
+
+C-states, or idling states of the processor, especially C1E and deeper sleep
+states may be problematic for TSC as well.  The TSC may stop advancing in such
+a state, resulting in a TSC which is behind that of other CPUs when execution
+is resumed.  Such CPUs must be detected and flagged by the operating system
+based on CPU and chipset identifications.
+
+The TSC in such a case may be corrected by catching it up to a known external
+clocksource.
+
+3.5) TSC frequency change / P-states
+
+To make things slightly more interesting, some CPUs may change frequency.  They
+may or may not run the TSC at the same rate, and because the frequency change
+may be staggered or slewed, at some points in time, the TSC rate may not be
+known other than falling within a range of values.  In this case, the TSC will
+not be a stable time source, and must be calibrated against a known, stable,
+external clock to be a usable source of time.
+
+Whether the TSC runs at a constant rate or scales with the P-state is model
+dependent and must be determined by inspecting CPUID, chipset or vendor
+specific MSR fields.
+
+In addition, some vendors have known bugs where the P-state is actually
+compensated for properly during normal operation, but when the processor is
+inactive, the P-state may be raised temporarily to service cache misses from
+other processors.  In such cases, the TSC on halted CPUs could advance faster
+than that of non-halted processors.  AMD Turion processors are known to have
+this problem.
+
+3.6) TSC and STPCLK / T-states
+
+External signals given to the processor may also have the effect of stopping
+the TSC.  This is typically done for thermal emergency power control to prevent
+an overheating condition, and typically, there is no way to detect that this
+condition has happened.
+
+3.7) TSC virtualization - VMX
+
+VMX provides conditional trapping of RDTSC, RDMSR, WRMSR and RDTSCP
+instructions, which is enough for full virtualization of TSC in any manner.  In
+addition, VMX allows passing through the host TSC plus an additional TSC_OFFSET
+field specified in the VMCS.  Special instructions must be used to read and
+write the VMCS field.
+
+3.8) TSC virtualization - SVM
+
+SVM provides conditional trapping of RDTSC, RDMSR, WRMSR and RDTSCP
+instructions, which is enough for full virtualization of TSC in any manner.  In
+addition, SVM allows passing through the host TSC plus an additional offset
+field specified in the SVM control block.
+
+3.9) TSC feature bits in Linux
+
+In summary, there is no way to guarantee the TSC remains in perfect
+synchronization unless it is explicitly guaranteed by the architecture.  Even
+if so, the TSCs in multi-sockets or NUMA systems may still run independently
+despite being locally consistent.
+
+The following feature bits are used by Linux to signal various TSC attributes,
+but they can only be taken to be meaningful for UP or single node systems.
+
+X86_FEATURE_TSC                 : The TSC is available in hardware
+X86_FEATURE_RDTSCP              : The RDTSCP instruction is available
+X86_FEATURE_CONSTANT_TSC        : The TSC rate is unchanged with P-states
+X86_FEATURE_NONSTOP_TSC         : The TSC does not stop in C-states
+X86_FEATURE_TSC_RELIABLE        : TSC sync checks are skipped (VMware)
+
+4) Virtualization Problems
+
+Timekeeping is especially problematic for virtualization because a number of
+challenges arise.  The most obvious problem is that time is now shared between
+the host and, potentially, a number of virtual machines.  Thus the virtual
+operating system does not run with 100% usage of the CPU, despite the fact that
+it may very well make that assumption.  It may expect it to remain true to very
+exacting bounds when interrupt sources are disabled, but in reality only its
+virtual interrupt sources are disabled, and the machine may still be preempted
+at any time.  This causes problems as the passage of real time, the injection
+of machine interrupts and the associated clock sources are no longer completely
+synchronized with real time.
+
+This same problem can occur on native harware to a degree, as SMM mode may
+steal cycles from the naturally on X86 systems when SMM mode is used by the
+BIOS, but not in such an extreme fashion.  However, the fact that SMM mode may
+cause similar problems to virtualization makes it a good justification for
+solving many of these problems on bare metal.
+
+4.1) Interrupt clocking
+
+One of the most immediate problems that occurs with legacy operating systems
+is that the system timekeeping routines are often designed to keep track of
+time by counting periodic interrupts.  These interrupts may come from the PIT
+or the RTC, but the problem is the same: the host virtualization engine may not
+be able to deliver the proper number of interrupts per second, and so guest
+time may fall behind.  This is especially problematic if a high interrupt rate
+is selected, such as 1000 HZ, which is unfortunately the default for many Linux
+guests.
+
+There are three approaches to solving this problem; first, it may be possible
+to simply ignore it.  Guests which have a separate time source for tracking
+'wall clock' or 'real time' may not need any adjustment of their interrupts to
+maintain proper time.  If this is not sufficient, it may be necessary to inject
+additional interrupts into the guest in order to increase the effective
+interrupt rate.  This approach leads to complications in extreme conditions,
+where host load or guest lag is too much to compensate for, and thus another
+solution to the problem has risen: the guest may need to become aware of lost
+ticks and compensate for them internally.  Although promising in theory, the
+implementation of this policy in Linux has been extremely error prone, and a
+number of buggy variants of lost tick compensation are distributed across
+commonly used Linux systems.
+
+Windows uses periodic RTC clocking as a means of keeping time internally, and
+thus requires interrupt slewing to keep proper time.  It does use a low enough
+rate (ed: is it 18.2 Hz?) however that it has not yet been a problem in
+practice.
+
+4.2) TSC sampling and serialization
+
+As the highest precision time source available, the cycle counter of the CPU
+has aroused much interest from developers.  As explained above, this timer has
+many problems unique to its nature as a local, potentially unstable and
+potentially unsynchronized source.  One issue which is not unique to the TSC,
+but is highlighted because of its very precise nature is sampling delay.  By
+definition, the counter, once read is already old.  However, it is also
+possible for the counter to be read ahead of the actual use of the result.
+This is a consequence of the superscalar execution of the instruction stream,
+which may execute instructions out of order.  Such execution is called
+non-serialized.  Forcing serialized execution is necessary for precise
+measurement with the TSC, and requires a serializing instruction, such as CPUID
+or an MSR read.
+
+Since CPUID may actually be virtualized by a trap and emulate mechanism, this
+serialization can pose a performance issue for hardware virtualization.  An
+accurate time stamp counter reading may therefore not always be available, and
+it may be necessary for an implementation to guard against "backwards" reads of
+the TSC as seen from other CPUs, even in an otherwise perfectly synchronized
+system.
+
+4.3) Timespec aliasing
+
+Additionally, this lack of serialization from the TSC poses another challenge
+when using results of the TSC when measured against another time source.  As
+the TSC is much higher precision, many possible values of the TSC may be read
+while another clock is still expressing the same value.
+
+That is, you may read (T,T+10) while external clock C maintains the same value.
+Due to non-serialized reads, you may actually end up with a range which
+fluctuates - from (T-1.. T+10).  Thus, any time calculated from a TSC, but
+calibrated against an external value may have a range of valid values.
+Re-calibrating this computation may actually cause time, as computed after the
+calibration, to go backwards, compared with time computed before the
+calibration.
+
+This problem is particularly pronounced with an internal time source in Linux,
+the kernel time, which is expressed in the theoretically high resolution
+timespec - but which advances in much larger granularity intervals, sometimes
+at the rate of jiffies, and possibly in catchup modes, at a much larger step.
+
+This aliasing requires care in the computation and recalibration of kvmclock
+and any other values derived from TSC computation (such as TSC virtualization
+itself).
+
+4.4) Migration
+
+Migration of a virtual machine raises problems for timekeeping in two ways.
+First, the migration itself may take time, during which interrupts cannot be
+delivered, and after which, the guest time may need to be caught up.  NTP may
+be able to help to some degree here, as the clock correction required is
+typically small enough to fall in the NTP-correctable window.
+
+An additional concern is that timers based off the TSC (or HPET, if the raw bus
+clock is exposed) may now be running at different rates, requiring compensation
+in some way in the hypervisor by virtualizing these timers.  In addition,
+migrating to a faster machine may preclude the use of a passthrough TSC, as a
+faster clock cannot be made visible to a guest without the potential of time
+advancing faster than usual.  A slower clock is less of a problem, as it can
+always be caught up to the original rate.  KVM clock avoids these problems by
+simply storing multipliers and offsets against the TSC for the guest to convert
+back into nanosecond resolution values.
+
+4.5) Scheduling
+
+Since scheduling may be based on precise timing and firing of interrupts, the
+scheduling algorithms of an operating system may be adversely affected by
+virtualization.  In theory, the effect is random and should be universally
+distributed, but in contrived as well as real scenarios (guest device access,
+causes of virtualization exits, possible context switch), this may not always
+be the case.  The effect of this has not been well studied.
+
+In an attempt to work around this, several implementations have provided a
+paravirtualized scheduler clock, which reveals the true amount of CPU time for
+which a virtual machine has been running.
+
+4.6) Watchdogs
+
+Watchdog timers, such as the lock detector in Linux may fire accidentally when
+running under hardware virtualization due to timer interrupts being delayed or
+misinterpretation of the passage of real time.  Usually, these warnings are
+spurious and can be ignored, but in some circumstances it may be necessary to
+disable such detection.
+
+4.7) Delays and precision timing
+
+Precise timing and delays may not be possible in a virtualized system.  This
+can happen if the system is controlling physical hardware, or issues delays to
+compensate for slower I/O to and from devices.  The first issue is not solvable
+in general for a virtualized system; hardware control software can't be
+adequately virtualized without a full real-time operating system, which would
+require an RT aware virtualization platform.
+
+The second issue may cause performance problems, but this is unlikely to be a
+significant issue.  In many cases these delays may be eliminated through
+configuration or paravirtualization.
+
+4.8) Covert channels and leaks
+
+In addition to the above problems, time information will inevitably leak to the
+guest about the host in anything but a perfect implementation of virtualized
+time.  This may allow the guest to infer the presence of a hypervisor (as in a
+red-pill type detection), and it may allow information to leak between guests
+by using CPU utilization itself as a signalling channel.  Preventing such
+problems would require completely isolated virtual time which may not track
+real time any longer.  This may be useful in certain security or QA contexts,
+but in general isn't recommended for real-world deployment scenarios.
diff --git a/Documentation/virtual/lguest/.gitignore b/Documentation/virtual/lguest/.gitignore
new file mode 100644
index 000000000000..115587fd5f65
--- /dev/null
+++ b/Documentation/virtual/lguest/.gitignore
@@ -0,0 +1 @@
+lguest
diff --git a/Documentation/virtual/lguest/Makefile b/Documentation/virtual/lguest/Makefile
new file mode 100644
index 000000000000..bebac6b4f332
--- /dev/null
+++ b/Documentation/virtual/lguest/Makefile
@@ -0,0 +1,8 @@
+# This creates the demonstration utility "lguest" which runs a Linux guest.
+# Missing headers?  Add "-I../../include -I../../arch/x86/include"
+CFLAGS:=-m32 -Wall -Wmissing-declarations -Wmissing-prototypes -O3 -U_FORTIFY_SOURCE
+
+all: lguest
+
+clean:
+        rm -f lguest
diff --git a/Documentation/virtual/lguest/extract b/Documentation/virtual/lguest/extract
new file mode 100644
index 000000000000..7730bb6e4b94
--- /dev/null
+++ b/Documentation/virtual/lguest/extract
@@ -0,0 +1,58 @@
+#! /bin/sh
+
+set -e
+
+PREFIX=$1
+shift
+
+trap 'rm -r $TMPDIR' 0
+TMPDIR=`mktemp -d`
+
+exec 3>/dev/null
+for f; do
+    while IFS="
+" read -r LINE; do
+        case "$LINE" in
+            *$PREFIX:[0-9]*:\**)
+                NUM=`echo "$LINE" | sed "s/.*$PREFIX:\([0-9]*\).*/\1/"`
+                if [ -f $TMPDIR/$NUM ]; then
+                    echo "$TMPDIR/$NUM already exits prior to $f"
+                    exit 1
+                fi
+                exec 3>>$TMPDIR/$NUM
+                echo $f | sed 's,\.\./,,g' > $TMPDIR/.$NUM
+                /bin/echo "$LINE" | sed -e "s/$PREFIX:[0-9]*//" -e "s/:\*/*/" >&3
+                ;;
+            *$PREFIX:[0-9]*)
+                NUM=`echo "$LINE" | sed "s/.*$PREFIX:\([0-9]*\).*/\1/"`
+                if [ -f $TMPDIR/$NUM ]; then
+                    echo "$TMPDIR/$NUM already exits prior to $f"
+                    exit 1
+                fi
+                exec 3>>$TMPDIR/$NUM
+                echo $f | sed 's,\.\./,,g' > $TMPDIR/.$NUM
+                /bin/echo "$LINE" | sed "s/$PREFIX:[0-9]*//" >&3
+                ;;
+            *:\**)
+                /bin/echo "$LINE" | sed -e "s/:\*/*/" -e "s,/\*\*/,," >&3
+                echo >&3
+                exec 3>/dev/null
+                ;;
+            *)
+                /bin/echo "$LINE" >&3
+                ;;
+        esac
+    done < $f
+    echo >&3
+    exec 3>/dev/null
+done
+
+LASTFILE=""
+for f in $TMPDIR/*; do
+    if [ "$LASTFILE" != $(cat $TMPDIR/.$(basename $f) ) ]; then
+        LASTFILE=$(cat $TMPDIR/.$(basename $f) )
+        echo "[ $LASTFILE ]"
+    fi
+    cat $f
+done
+
diff --git a/Documentation/virtual/lguest/lguest.c b/Documentation/virtual/lguest/lguest.c
new file mode 100644
index 000000000000..d9da7e148538
--- /dev/null
+++ b/Documentation/virtual/lguest/lguest.c
@@ -0,0 +1,2095 @@
+/*P:100
+ * This is the Launcher code, a simple program which lays out the "physical"
+ * memory for the new Guest by mapping the kernel image and the virtual
+ * devices, then opens /dev/lguest to tell the kernel about the Guest and
+ * control it.
+:*/
+#define _LARGEFILE64_SOURCE
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+#include <err.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <elf.h>
+#include <sys/mman.h>
+#include <sys/param.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+#include <sys/eventfd.h>
+#include <fcntl.h>
+#include <stdbool.h>
+#include <errno.h>
+#include <ctype.h>
+#include <sys/socket.h>
+#include <sys/ioctl.h>
+#include <sys/time.h>
+#include <time.h>
+#include <netinet/in.h>
+#include <net/if.h>
+#include <linux/sockios.h>
+#include <linux/if_tun.h>
+#include <sys/uio.h>
+#include <termios.h>
+#include <getopt.h>
+#include <assert.h>
+#include <sched.h>
+#include <limits.h>
+#include <stddef.h>
+#include <signal.h>
+#include <pwd.h>
+#include <grp.h>
+
+#include <linux/virtio_config.h>
+#include <linux/virtio_net.h>
+#include <linux/virtio_blk.h>
+#include <linux/virtio_console.h>
+#include <linux/virtio_rng.h>
+#include <linux/virtio_ring.h>
+#include <asm/bootparam.h>
+#include "../../include/linux/lguest_launcher.h"
+/*L:110
+ * We can ignore the 42 include files we need for this program, but I do want
+ * to draw attention to the use of kernel-style types.
+ *
+ * As Linus said, "C is a Spartan language, and so should your naming be."  I
+ * like these abbreviations, so we define them here.  Note that u64 is always
+ * unsigned long long, which works on all Linux systems: this means that we can
+ * use %llu in printf for any u64.
+ */
+typedef unsigned long long u64;
+typedef uint32_t u32;
+typedef uint16_t u16;
+typedef uint8_t u8;
+/*:*/
+
+#define PAGE_PRESENT 0x7        /* Present, RW, Execute */
+#define BRIDGE_PFX "bridge:"
+#ifndef SIOCBRADDIF
+#define SIOCBRADDIF     0x89a2          /* add interface to bridge      */
+#endif
+/* We can have up to 256 pages for devices. */
+#define DEVICE_PAGES 256
+/* This will occupy 3 pages: it must be a power of 2. */
+#define VIRTQUEUE_NUM 256
+
+/*L:120
+ * verbose is both a global flag and a macro.  The C preprocessor allows
+ * this, and although I wouldn't recommend it, it works quite nicely here.
+ */
+static bool verbose;
+#define verbose(args...) \
+        do { if (verbose) printf(args); } while(0)
+/*:*/
+
+/* The pointer to the start of guest memory. */
+static void *guest_base;
+/* The maximum guest physical address allowed, and maximum possible. */
+static unsigned long guest_limit, guest_max;
+/* The /dev/lguest file descriptor. */
+static int lguest_fd;
+
+/* a per-cpu variable indicating whose vcpu is currently running */
+static unsigned int __thread cpu_id;
+
+/* This is our list of devices. */
+struct device_list {
+        /* Counter to assign interrupt numbers. */
+        unsigned int next_irq;
+
+        /* Counter to print out convenient device numbers. */
+        unsigned int device_num;
+
+        /* The descriptor page for the devices. */
+        u8 *descpage;
+
+        /* A single linked list of devices. */
+        struct device *dev;
+        /* And a pointer to the last device for easy append. */
+        struct device *lastdev;
+};
+
+/* The list of Guest devices, based on command line arguments. */
+static struct device_list devices;
+
+/* The device structure describes a single device. */
+struct device {
+        /* The linked-list pointer. */
+        struct device *next;
+
+        /* The device's descriptor, as mapped into the Guest. */
+        struct lguest_device_desc *desc;
+
+        /* We can't trust desc values once Guest has booted: we use these. */
+        unsigned int feature_len;
+        unsigned int num_vq;
+
+        /* The name of this device, for --verbose. */
+        const char *name;
+
+        /* Any queues attached to this device */
+        struct virtqueue *vq;
+
+        /* Is it operational */
+        bool running;
+
+        /* Does Guest want an intrrupt on empty? */
+        bool irq_on_empty;
+
+        /* Device-specific data. */
+        void *priv;
+};
+
+/* The virtqueue structure describes a queue attached to a device. */
+struct virtqueue {
+        struct virtqueue *next;
+
+        /* Which device owns me. */
+        struct device *dev;
+
+        /* The configuration for this queue. */
+        struct lguest_vqconfig config;
+
+        /* The actual ring of buffers. */
+        struct vring vring;
+
+        /* Last available index we saw. */
+        u16 last_avail_idx;
+
+        /* How many are used since we sent last irq? */
+        unsigned int pending_used;
+
+        /* Eventfd where Guest notifications arrive. */
+        int eventfd;
+
+        /* Function for the thread which is servicing this virtqueue. */
+        void (*service)(struct virtqueue *vq);
+        pid_t thread;
+};
+
+/* Remember the arguments to the program so we can "reboot" */
+static char **main_args;
+
+/* The original tty settings to restore on exit. */
+static struct termios orig_term;
+
+/*
+ * We have to be careful with barriers: our devices are all run in separate
+ * threads and so we need to make sure that changes visible to the Guest happen
+ * in precise order.
+ */
+#define wmb() __asm__ __volatile__("" : : : "memory")
+#define mb() __asm__ __volatile__("" : : : "memory")
+
+/*
+ * Convert an iovec element to the given type.
+ *
+ * This is a fairly ugly trick: we need to know the size of the type and
+ * alignment requirement to check the pointer is kosher.  It's also nice to
+ * have the name of the type in case we report failure.
+ *
+ * Typing those three things all the time is cumbersome and error prone, so we
+ * have a macro which sets them all up and passes to the real function.
+ */
+#define convert(iov, type) \
+        ((type *)_convert((iov), sizeof(type), __alignof__(type), #type))
+
+static void *_convert(struct iovec *iov, size_t size, size_t align,
+                      const char *name)
+{
+        if (iov->iov_len != size)
+                errx(1, "Bad iovec size %zu for %s", iov->iov_len, name);
+        if ((unsigned long)iov->iov_base % align != 0)
+                errx(1, "Bad alignment %p for %s", iov->iov_base, name);
+        return iov->iov_base;
+}
+
+/* Wrapper for the last available index.  Makes it easier to change. */
+#define lg_last_avail(vq)       ((vq)->last_avail_idx)
+
+/*
+ * The virtio configuration space is defined to be little-endian.  x86 is
+ * little-endian too, but it's nice to be explicit so we have these helpers.
+ */
+#define cpu_to_le16(v16) (v16)
+#define cpu_to_le32(v32) (v32)
+#define cpu_to_le64(v64) (v64)
+#define le16_to_cpu(v16) (v16)
+#define le32_to_cpu(v32) (v32)
+#define le64_to_cpu(v64) (v64)
+
+/* Is this iovec empty? */
+static bool iov_empty(const struct iovec iov[], unsigned int num_iov)
+{
+        unsigned int i;
+
+        for (i = 0; i < num_iov; i++)
+                if (iov[i].iov_len)
+                        return false;
+        return true;
+}
+
+/* Take len bytes from the front of this iovec. */
+static void iov_consume(struct iovec iov[], unsigned num_iov, unsigned len)
+{
+        unsigned int i;
+
+        for (i = 0; i < num_iov; i++) {
+                unsigned int used;
+
+                used = iov[i].iov_len < len ? iov[i].iov_len : len;
+                iov[i].iov_base += used;
+                iov[i].iov_len -= used;
+                len -= used;
+        }
+        assert(len == 0);
+}
+
+/* The device virtqueue descriptors are followed by feature bitmasks. */
+static u8 *get_feature_bits(struct device *dev)
+{
+        return (u8 *)(dev->desc + 1)
+                + dev->num_vq * sizeof(struct lguest_vqconfig);
+}
+
+/*L:100
+ * The Launcher code itself takes us out into userspace, that scary place where
+ * pointers run wild and free!  Unfortunately, like most userspace programs,
+ * it's quite boring (which is why everyone likes to hack on the kernel!).
+ * Perhaps if you make up an Lguest Drinking Game at this point, it will get
+ * you through this section.  Or, maybe not.
+ *
+ * The Launcher sets up a big chunk of memory to be the Guest's "physical"
+ * memory and stores it in "guest_base".  In other words, Guest physical ==
+ * Launcher virtual with an offset.
+ *
+ * This can be tough to get your head around, but usually it just means that we
+ * use these trivial conversion functions when the Guest gives us its
+ * "physical" addresses:
+ */
+static void *from_guest_phys(unsigned long addr)
+{
+        return guest_base + addr;
+}
+
+static unsigned long to_guest_phys(const void *addr)
+{
+        return (addr - guest_base);
+}
+
+/*L:130
+ * Loading the Kernel.
+ *
+ * We start with couple of simple helper routines.  open_or_die() avoids
+ * error-checking code cluttering the callers:
+ */
+static int open_or_die(const char *name, int flags)
+{
+        int fd = open(name, flags);
+        if (fd < 0)
+                err(1, "Failed to open %s", name);
+        return fd;
+}
+
+/* map_zeroed_pages() takes a number of pages. */
+static void *map_zeroed_pages(unsigned int num)
+{
+        int fd = open_or_die("/dev/zero", O_RDONLY);
+        void *addr;
+
+        /*
+         * We use a private mapping (ie. if we write to the page, it will be
+         * copied). We allocate an extra two pages PROT_NONE to act as guard
+         * pages against read/write attempts that exceed allocated space.
+         */
+        addr = mmap(NULL, getpagesize() * (num+2),
+                    PROT_NONE, MAP_PRIVATE, fd, 0);
+
+        if (addr == MAP_FAILED)
+                err(1, "Mmapping %u pages of /dev/zero", num);
+
+        if (mprotect(addr + getpagesize(), getpagesize() * num,
+                     PROT_READ|PROT_WRITE) == -1)
+                err(1, "mprotect rw %u pages failed", num);
+
+        /*
+         * One neat mmap feature is that you can close the fd, and it
+         * stays mapped.
+         */
+        close(fd);
+
+        /* Return address after PROT_NONE page */
+        return addr + getpagesize();
+}
+
+/* Get some more pages for a device. */
+static void *get_pages(unsigned int num)
+{
+        void *addr = from_guest_phys(guest_limit);
+
+        guest_limit += num * getpagesize();
+        if (guest_limit > guest_max)
+                errx(1, "Not enough memory for devices");
+        return addr;
+}
+
+/*
+ * This routine is used to load the kernel or initrd.  It tries mmap, but if
+ * that fails (Plan 9's kernel file isn't nicely aligned on page boundaries),
+ * it falls back to reading the memory in.
+ */
+static void map_at(int fd, void *addr, unsigned long offset, unsigned long len)
+{
+        ssize_t r;
+
+        /*
+         * We map writable even though for some segments are marked read-only.
+         * The kernel really wants to be writable: it patches its own
+         * instructions.
+         *
+         * MAP_PRIVATE means that the page won't be copied until a write is
+         * done to it.  This allows us to share untouched memory between
+         * Guests.
+         */
+        if (mmap(addr, len, PROT_READ|PROT_WRITE,
+                 MAP_FIXED|MAP_PRIVATE, fd, offset) != MAP_FAILED)
+                return;
+
+        /* pread does a seek and a read in one shot: saves a few lines. */
+        r = pread(fd, addr, len, offset);
+        if (r != len)
+                err(1, "Reading offset %lu len %lu gave %zi", offset, len, r);
+}
+
+/*
+ * This routine takes an open vmlinux image, which is in ELF, and maps it into
+ * the Guest memory.  ELF = Embedded Linking Format, which is the format used
+ * by all modern binaries on Linux including the kernel.
+ *
+ * The ELF headers give *two* addresses: a physical address, and a virtual
+ * address.  We use the physical address; the Guest will map itself to the
+ * virtual address.
+ *
+ * We return the starting address.
+ */
+static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr)
+{
+        Elf32_Phdr phdr[ehdr->e_phnum];
+        unsigned int i;
+
+        /*
+         * Sanity checks on the main ELF header: an x86 executable with a
+         * reasonable number of correctly-sized program headers.
+         */
+        if (ehdr->e_type != ET_EXEC
+            || ehdr->e_machine != EM_386
+            || ehdr->e_phentsize != sizeof(Elf32_Phdr)
+            || ehdr->e_phnum < 1 || ehdr->e_phnum > 65536U/sizeof(Elf32_Phdr))
+                errx(1, "Malformed elf header");
+
+        /*
+         * An ELF executable contains an ELF header and a number of "program"
+         * headers which indicate which parts ("segments") of the program to
+         * load where.
+         */
+
+        /* We read in all the program headers at once: */
+        if (lseek(elf_fd, ehdr->e_phoff, SEEK_SET) < 0)
+                err(1, "Seeking to program headers");
+        if (read(elf_fd, phdr, sizeof(phdr)) != sizeof(phdr))
+                err(1, "Reading program headers");
+
+        /*
+         * Try all the headers: there are usually only three.  A read-only one,
+         * a read-write one, and a "note" section which we don't load.
+         */
+        for (i = 0; i < ehdr->e_phnum; i++) {
+                /* If this isn't a loadable segment, we ignore it */
+                if (phdr[i].p_type != PT_LOAD)
+                        continue;
+
+                verbose("Section %i: size %i addr %p\n",
+                        i, phdr[i].p_memsz, (void *)phdr[i].p_paddr);
+
+                /* We map this section of the file at its physical address. */
+                map_at(elf_fd, from_guest_phys(phdr[i].p_paddr),
+                       phdr[i].p_offset, phdr[i].p_filesz);
+        }
+
+        /* The entry point is given in the ELF header. */
+        return ehdr->e_entry;
+}
+
+/*L:150
+ * A bzImage, unlike an ELF file, is not meant to be loaded.  You're supposed
+ * to jump into it and it will unpack itself.  We used to have to perform some
+ * hairy magic because the unpacking code scared me.
+ *
+ * Fortunately, Jeremy Fitzhardinge convinced me it wasn't that hard and wrote
+ * a small patch to jump over the tricky bits in the Guest, so now we just read
+ * the funky header so we know where in the file to load, and away we go!
+ */
+static unsigned long load_bzimage(int fd)
+{
+        struct boot_params boot;
+        int r;
+        /* Modern bzImages get loaded at 1M. */
+        void *p = from_guest_phys(0x100000);
+
+        /*
+         * Go back to the start of the file and read the header.  It should be
+         * a Linux boot header (see Documentation/x86/i386/boot.txt)
+         */
+        lseek(fd, 0, SEEK_SET);
+        read(fd, &boot, sizeof(boot));
+
+        /* Inside the setup_hdr, we expect the magic "HdrS" */
+        if (memcmp(&boot.hdr.header, "HdrS", 4) != 0)
+                errx(1, "This doesn't look like a bzImage to me");
+
+        /* Skip over the extra sectors of the header. */
+        lseek(fd, (boot.hdr.setup_sects+1) * 512, SEEK_SET);
+
+        /* Now read everything into memory. in nice big chunks. */
+        while ((r = read(fd, p, 65536)) > 0)
+                p += r;
+
+        /* Finally, code32_start tells us where to enter the kernel. */
+        return boot.hdr.code32_start;
+}
+
+/*L:140
+ * Loading the kernel is easy when it's a "vmlinux", but most kernels
+ * come wrapped up in the self-decompressing "bzImage" format.  With a little
+ * work, we can load those, too.
+ */
+static unsigned long load_kernel(int fd)
+{
+        Elf32_Ehdr hdr;
+
+        /* Read in the first few bytes. */
+        if (read(fd, &hdr, sizeof(hdr)) != sizeof(hdr))
+                err(1, "Reading kernel");
+
+        /* If it's an ELF file, it starts with "\177ELF" */
+        if (memcmp(hdr.e_ident, ELFMAG, SELFMAG) == 0)
+                return map_elf(fd, &hdr);
+
+        /* Otherwise we assume it's a bzImage, and try to load it. */
+        return load_bzimage(fd);
+}
+
+/*
+ * This is a trivial little helper to align pages.  Andi Kleen hated it because
+ * it calls getpagesize() twice: "it's dumb code."
+ *
+ * Kernel guys get really het up about optimization, even when it's not
+ * necessary.  I leave this code as a reaction against that.
+ */
+static inline unsigned long page_align(unsigned long addr)
+{
+        /* Add upwards and truncate downwards. */
+        return ((addr + getpagesize()-1) & ~(getpagesize()-1));
+}
+
+/*L:180
+ * An "initial ram disk" is a disk image loaded into memory along with the
+ * kernel which the kernel can use to boot from without needing any drivers.
+ * Most distributions now use this as standard: the initrd contains the code to
+ * load the appropriate driver modules for the current machine.
+ *
+ * Importantly, James Morris works for RedHat, and Fedora uses initrds for its
+ * kernels.  He sent me this (and tells me when I break it).
+ */
+static unsigned long load_initrd(const char *name, unsigned long mem)
+{
+        int ifd;
+        struct stat st;
+        unsigned long len;
+
+        ifd = open_or_die(name, O_RDONLY);
+        /* fstat() is needed to get the file size. */
+        if (fstat(ifd, &st) < 0)
+                err(1, "fstat() on initrd '%s'", name);
+
+        /*
+         * We map the initrd at the top of memory, but mmap wants it to be
+         * page-aligned, so we round the size up for that.
+         */
+        len = page_align(st.st_size);
+        map_at(ifd, from_guest_phys(mem - len), 0, st.st_size);
+        /*
+         * Once a file is mapped, you can close the file descriptor.  It's a
+         * little odd, but quite useful.
+         */
+        close(ifd);
+        verbose("mapped initrd %s size=%lu @ %p\n", name, len, (void*)mem-len);
+
+        /* We return the initrd size. */
+        return len;
+}
+/*:*/
+
+/*
+ * Simple routine to roll all the commandline arguments together with spaces
+ * between them.
+ */
+static void concat(char *dst, char *args[])
+{
+        unsigned int i, len = 0;
+
+        for (i = 0; args[i]; i++) {
+                if (i) {
+                        strcat(dst+len, " ");
+                        len++;
+                }
+                strcpy(dst+len, args[i]);
+                len += strlen(args[i]);
+        }
+        /* In case it's empty. */
+        dst[len] = '\0';
+}
+
+/*L:185
+ * This is where we actually tell the kernel to initialize the Guest.  We
+ * saw the arguments it expects when we looked at initialize() in lguest_user.c:
+ * the base of Guest "physical" memory, the top physical page to allow and the
+ * entry point for the Guest.
+ */
+static void tell_kernel(unsigned long start)
+{
+        unsigned long args[] = { LHREQ_INITIALIZE,
+                                 (unsigned long)guest_base,
+                                 guest_limit / getpagesize(), start };
+        verbose("Guest: %p - %p (%#lx)\n",
+                guest_base, guest_base + guest_limit, guest_limit);
+        lguest_fd = open_or_die("/dev/lguest", O_RDWR);
+        if (write(lguest_fd, args, sizeof(args)) < 0)
+                err(1, "Writing to /dev/lguest");
+}
+/*:*/
+
+/*L:200
+ * Device Handling.
+ *
+ * When the Guest gives us a buffer, it sends an array of addresses and sizes.
+ * We need to make sure it's not trying to reach into the Launcher itself, so
+ * we have a convenient routine which checks it and exits with an error message
+ * if something funny is going on:
+ */
+static void *_check_pointer(unsigned long addr, unsigned int size,
+                            unsigned int line)
+{
+        /*
+         * Check if the requested address and size exceeds the allocated memory,
+         * or addr + size wraps around.
+         */
+        if ((addr + size) > guest_limit || (addr + size) < addr)
+                errx(1, "%s:%i: Invalid address %#lx", __FILE__, line, addr);
+        /*
+         * We return a pointer for the caller's convenience, now we know it's
+         * safe to use.
+         */
+        return from_guest_phys(addr);
+}
+/* A macro which transparently hands the line number to the real function. */
+#define check_pointer(addr,size) _check_pointer(addr, size, __LINE__)
+
+/*
+ * Each buffer in the virtqueues is actually a chain of descriptors.  This
+ * function returns the next descriptor in the chain, or vq->vring.num if we're
+ * at the end.
+ */
+static unsigned next_desc(struct vring_desc *desc,
+                          unsigned int i, unsigned int max)
+{
+        unsigned int next;
+
+        /* If this descriptor says it doesn't chain, we're done. */
+        if (!(desc[i].flags & VRING_DESC_F_NEXT))
+                return max;
+
+        /* Check they're not leading us off end of descriptors. */
+        next = desc[i].next;
+        /* Make sure compiler knows to grab that: we don't want it changing! */
+        wmb();
+
+        if (next >= max)
+                errx(1, "Desc next is %u", next);
+
+        return next;
+}
+
+/*
+ * This actually sends the interrupt for this virtqueue, if we've used a
+ * buffer.
+ */
+static void trigger_irq(struct virtqueue *vq)
+{
+        unsigned long buf[] = { LHREQ_IRQ, vq->config.irq };
+
+        /* Don't inform them if nothing used. */
+        if (!vq->pending_used)
+                return;
+        vq->pending_used = 0;
+
+        /* If they don't want an interrupt, don't send one... */
+        if (vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT) {
+                /* ... unless they've asked us to force one on empty. */
+                if (!vq->dev->irq_on_empty
+                    || lg_last_avail(vq) != vq->vring.avail->idx)
+                        return;
+        }
+
+        /* Send the Guest an interrupt tell them we used something up. */
+        if (write(lguest_fd, buf, sizeof(buf)) != 0)
+                err(1, "Triggering irq %i", vq->config.irq);
+}
+
+/*
+ * This looks in the virtqueue for the first available buffer, and converts
+ * it to an iovec for convenient access.  Since descriptors consist of some
+ * number of output then some number of input descriptors, it's actually two
+ * iovecs, but we pack them into one and note how many of each there were.
+ *
+ * This function waits if necessary, and returns the descriptor number found.
+ */
+static unsigned wait_for_vq_desc(struct virtqueue *vq,
+                                 struct iovec iov[],
+                                 unsigned int *out_num, unsigned int *in_num)
+{
+        unsigned int i, head, max;
+        struct vring_desc *desc;
+        u16 last_avail = lg_last_avail(vq);
+
+        /* There's nothing available? */
+        while (last_avail == vq->vring.avail->idx) {
+                u64 event;
+
+                /*
+                 * Since we're about to sleep, now is a good time to tell the
+                 * Guest about what we've used up to now.
+                 */
+                trigger_irq(vq);
+
+                /* OK, now we need to know about added descriptors. */
+                vq->vring.used->flags &= ~VRING_USED_F_NO_NOTIFY;
+
+                /*
+                 * They could have slipped one in as we were doing that: make
+                 * sure it's written, then check again.
+                 */
+                mb();
+                if (last_avail != vq->vring.avail->idx) {
+                        vq->vring.used->flags |= VRING_USED_F_NO_NOTIFY;
+                        break;
+                }
+
+                /* Nothing new?  Wait for eventfd to tell us they refilled. */
+                if (read(vq->eventfd, &event, sizeof(event)) != sizeof(event))
+                        errx(1, "Event read failed?");
+
+                /* We don't need to be notified again. */
+                vq->vring.used->flags |= VRING_USED_F_NO_NOTIFY;
+        }
+
+        /* Check it isn't doing very strange things with descriptor numbers. */
+        if ((u16)(vq->vring.avail->idx - last_avail) > vq->vring.num)
+                errx(1, "Guest moved used index from %u to %u",
+                     last_avail, vq->vring.avail->idx);
+
+        /*
+         * Grab the next descriptor number they're advertising, and increment
+         * the index we've seen.
+         */
+        head = vq->vring.avail->ring[last_avail % vq->vring.num];
+        lg_last_avail(vq)++;
+
+        /* If their number is silly, that's a fatal mistake. */
+        if (head >= vq->vring.num)
+                errx(1, "Guest says index %u is available", head);
+
+        /* When we start there are none of either input nor output. */
+        *out_num = *in_num = 0;
+
+        max = vq->vring.num;
+        desc = vq->vring.desc;
+        i = head;
+
+        /*
+         * If this is an indirect entry, then this buffer contains a descriptor
+         * table which we handle as if it's any normal descriptor chain.
+         */
+        if (desc[i].flags & VRING_DESC_F_INDIRECT) {
+                if (desc[i].len % sizeof(struct vring_desc))
+                        errx(1, "Invalid size for indirect buffer table");
+
+                max = desc[i].len / sizeof(struct vring_desc);
+                desc = check_pointer(desc[i].addr, desc[i].len);
+                i = 0;
+        }
+
+        do {
+                /* Grab the first descriptor, and check it's OK. */
+                iov[*out_num + *in_num].iov_len = desc[i].len;
+                iov[*out_num + *in_num].iov_base
+                        = check_pointer(desc[i].addr, desc[i].len);
+                /* If this is an input descriptor, increment that count. */
+                if (desc[i].flags & VRING_DESC_F_WRITE)
+                        (*in_num)++;
+                else {
+                        /*
+                         * If it's an output descriptor, they're all supposed
+                         * to come before any input descriptors.
+                         */
+                        if (*in_num)
+                                errx(1, "Descriptor has out after in");
+                        (*out_num)++;
+                }
+
+                /* If we've got too many, that implies a descriptor loop. */
+                if (*out_num + *in_num > max)
+                        errx(1, "Looped descriptor");
+        } while ((i = next_desc(desc, i, max)) != max);
+
+        return head;
+}
+
+/*
+ * After we've used one of their buffers, we tell the Guest about it.  Sometime
+ * later we'll want to send them an interrupt using trigger_irq(); note that
+ * wait_for_vq_desc() does that for us if it has to wait.
+ */
+static void add_used(struct virtqueue *vq, unsigned int head, int len)
+{
+        struct vring_used_elem *used;
+
+        /*
+         * The virtqueue contains a ring of used buffers.  Get a pointer to the
+         * next entry in that used ring.
+         */
+        used = &vq->vring.used->ring[vq->vring.used->idx % vq->vring.num];
+        used->id = head;
+        used->len = len;
+        /* Make sure buffer is written before we update index. */
+        wmb();
+        vq->vring.used->idx++;
+        vq->pending_used++;
+}
+
+/* And here's the combo meal deal.  Supersize me! */
+static void add_used_and_trigger(struct virtqueue *vq, unsigned head, int len)
+{
+        add_used(vq, head, len);
+        trigger_irq(vq);
+}
+
+/*
+ * The Console
+ *
+ * We associate some data with the console for our exit hack.
+ */
+struct console_abort {
+        /* How many times have they hit ^C? */
+        int count;
+        /* When did they start? */
+        struct timeval start;
+};
+
+/* This is the routine which handles console input (ie. stdin). */
+static void console_input(struct virtqueue *vq)
+{
+        int len;
+        unsigned int head, in_num, out_num;
+        struct console_abort *abort = vq->dev->priv;
+        struct iovec iov[vq->vring.num];
+
+        /* Make sure there's a descriptor available. */
+        head = wait_for_vq_desc(vq, iov, &out_num, &in_num);
+        if (out_num)
+                errx(1, "Output buffers in console in queue?");
+
+        /* Read into it.  This is where we usually wait. */
+        len = readv(STDIN_FILENO, iov, in_num);
+        if (len <= 0) {
+                /* Ran out of input? */
+                warnx("Failed to get console input, ignoring console.");
+                /*
+                 * For simplicity, dying threads kill the whole Launcher.  So
+                 * just nap here.
+                 */
+                for (;;)
+                        pause();
+        }
+
+        /* Tell the Guest we used a buffer. */
+        add_used_and_trigger(vq, head, len);
+
+        /*
+         * Three ^C within one second?  Exit.
+         *
+         * This is such a hack, but works surprisingly well.  Each ^C has to
+         * be in a buffer by itself, so they can't be too fast.  But we check
+         * that we get three within about a second, so they can't be too
+         * slow.
+         */
+        if (len != 1 || ((char *)iov[0].iov_base)[0] != 3) {
+                abort->count = 0;
+                return;
+        }
+
+        abort->count++;
+        if (abort->count == 1)
+                gettimeofday(&abort->start, NULL);
+        else if (abort->count == 3) {
+                struct timeval now;
+                gettimeofday(&now, NULL);
+                /* Kill all Launcher processes with SIGINT, like normal ^C */
+                if (now.tv_sec <= abort->start.tv_sec+1)
+                        kill(0, SIGINT);
+                abort->count = 0;
+        }
+}
+
+/* This is the routine which handles console output (ie. stdout). */
+static void console_output(struct virtqueue *vq)
+{
+        unsigned int head, out, in;
+        struct iovec iov[vq->vring.num];
+
+        /* We usually wait in here, for the Guest to give us something. */
+        head = wait_for_vq_desc(vq, iov, &out, &in);
+        if (in)
+                errx(1, "Input buffers in console output queue?");
+
+        /* writev can return a partial write, so we loop here. */
+        while (!iov_empty(iov, out)) {
+                int len = writev(STDOUT_FILENO, iov, out);
+                if (len <= 0)
+                        err(1, "Write to stdout gave %i", len);
+                iov_consume(iov, out, len);
+        }
+
+        /*
+         * We're finished with that buffer: if we're going to sleep,
+         * wait_for_vq_desc() will prod the Guest with an interrupt.
+         */
+        add_used(vq, head, 0);
+}
+
+/*
+ * The Network
+ *
+ * Handling output for network is also simple: we get all the output buffers
+ * and write them to /dev/net/tun.
+ */
+struct net_info {
+        int tunfd;
+};
+
+static void net_output(struct virtqueue *vq)
+{
+        struct net_info *net_info = vq->dev->priv;
+        unsigned int head, out, in;
+        struct iovec iov[vq->vring.num];
+
+        /* We usually wait in here for the Guest to give us a packet. */
+        head = wait_for_vq_desc(vq, iov, &out, &in);
+        if (in)
+                errx(1, "Input buffers in net output queue?");
+        /*
+         * Send the whole thing through to /dev/net/tun.  It expects the exact
+         * same format: what a coincidence!
+         */
+        if (writev(net_info->tunfd, iov, out) < 0)
+                errx(1, "Write to tun failed?");
+
+        /*
+         * Done with that one; wait_for_vq_desc() will send the interrupt if
+         * all packets are processed.
+         */
+        add_used(vq, head, 0);
+}
+
+/*
+ * Handling network input is a bit trickier, because I've tried to optimize it.
+ *
+ * First we have a helper routine which tells is if from this file descriptor
+ * (ie. the /dev/net/tun device) will block:
+ */
+static bool will_block(int fd)
+{
+        fd_set fdset;
+        struct timeval zero = { 0, 0 };
+        FD_ZERO(&fdset);
+        FD_SET(fd, &fdset);
+        return select(fd+1, &fdset, NULL, NULL, &zero) != 1;
+}
+
+/*
+ * This handles packets coming in from the tun device to our Guest.  Like all
+ * service routines, it gets called again as soon as it returns, so you don't
+ * see a while(1) loop here.
+ */
+static void net_input(struct virtqueue *vq)
+{
+        int len;
+        unsigned int head, out, in;
+        struct iovec iov[vq->vring.num];
+        struct net_info *net_info = vq->dev->priv;
+
+        /*
+         * Get a descriptor to write an incoming packet into.  This will also
+         * send an interrupt if they're out of descriptors.
+         */
+        head = wait_for_vq_desc(vq, iov, &out, &in);
+        if (out)
+                errx(1, "Output buffers in net input queue?");
+
+        /*
+         * If it looks like we'll block reading from the tun device, send them
+         * an interrupt.
+         */
+        if (vq->pending_used && will_block(net_info->tunfd))
+                trigger_irq(vq);
+
+        /*
+         * Read in the packet.  This is where we normally wait (when there's no
+         * incoming network traffic).
+         */
+        len = readv(net_info->tunfd, iov, in);
+        if (len <= 0)
+                err(1, "Failed to read from tun.");
+
+        /*
+         * Mark that packet buffer as used, but don't interrupt here.  We want
+         * to wait until we've done as much work as we can.
+         */
+        add_used(vq, head, len);
+}
+/*:*/
+
+/* This is the helper to create threads: run the service routine in a loop. */
+static int do_thread(void *_vq)
+{
+        struct virtqueue *vq = _vq;
+
+        for (;;)
+                vq->service(vq);
+        return 0;
+}
+
+/*
+ * When a child dies, we kill our entire process group with SIGTERM.  This
+ * also has the side effect that the shell restores the console for us!
+ */
+static void kill_launcher(int signal)
+{
+        kill(0, SIGTERM);
+}
+
+static void reset_device(struct device *dev)
+{
+        struct virtqueue *vq;
+
+        verbose("Resetting device %s\n", dev->name);
+
+        /* Clear any features they've acked. */
+        memset(get_feature_bits(dev) + dev->feature_len, 0, dev->feature_len);
+
+        /* We're going to be explicitly killing threads, so ignore them. */
+        signal(SIGCHLD, SIG_IGN);
+
+        /* Zero out the virtqueues, get rid of their threads */
+        for (vq = dev->vq; vq; vq = vq->next) {
+                if (vq->thread != (pid_t)-1) {
+                        kill(vq->thread, SIGTERM);
+                        waitpid(vq->thread, NULL, 0);
+                        vq->thread = (pid_t)-1;
+                }
+                memset(vq->vring.desc, 0,
+                       vring_size(vq->config.num, LGUEST_VRING_ALIGN));
+                lg_last_avail(vq) = 0;
+        }
+        dev->running = false;
+
+        /* Now we care if threads die. */
+        signal(SIGCHLD, (void *)kill_launcher);
+}
+
+/*L:216
+ * This actually creates the thread which services the virtqueue for a device.
+ */
+static void create_thread(struct virtqueue *vq)
+{
+        /*
+         * Create stack for thread.  Since the stack grows upwards, we point
+         * the stack pointer to the end of this region.
+         */
+        char *stack = malloc(32768);
+        unsigned long args[] = { LHREQ_EVENTFD,
+                                 vq->config.pfn*getpagesize(), 0 };
+
+        /* Create a zero-initialized eventfd. */
+        vq->eventfd = eventfd(0, 0);
+        if (vq->eventfd < 0)
+                err(1, "Creating eventfd");
+        args[2] = vq->eventfd;
+
+        /*
+         * Attach an eventfd to this virtqueue: it will go off when the Guest
+         * does an LHCALL_NOTIFY for this vq.
+         */
+        if (write(lguest_fd, &args, sizeof(args)) != 0)
+                err(1, "Attaching eventfd");
+
+        /*
+         * CLONE_VM: because it has to access the Guest memory, and SIGCHLD so
+         * we get a signal if it dies.
+         */
+        vq->thread = clone(do_thread, stack + 32768, CLONE_VM | SIGCHLD, vq);
+        if (vq->thread == (pid_t)-1)
+                err(1, "Creating clone");
+
+        /* We close our local copy now the child has it. */
+        close(vq->eventfd);
+}
+
+static bool accepted_feature(struct device *dev, unsigned int bit)
+{
+        const u8 *features = get_feature_bits(dev) + dev->feature_len;
+
+        if (dev->feature_len < bit / CHAR_BIT)
+                return false;
+        return features[bit / CHAR_BIT] & (1 << (bit % CHAR_BIT));
+}
+
+static void start_device(struct device *dev)
+{
+        unsigned int i;
+        struct virtqueue *vq;
+
+        verbose("Device %s OK: offered", dev->name);
+        for (i = 0; i < dev->feature_len; i++)
+                verbose(" %02x", get_feature_bits(dev)[i]);
+        verbose(", accepted");
+        for (i = 0; i < dev->feature_len; i++)
+                verbose(" %02x", get_feature_bits(dev)
+                        [dev->feature_len+i]);
+
+        dev->irq_on_empty = accepted_feature(dev, VIRTIO_F_NOTIFY_ON_EMPTY);
+
+        for (vq = dev->vq; vq; vq = vq->next) {
+                if (vq->service)
+                        create_thread(vq);
+        }
+        dev->running = true;
+}
+
+static void cleanup_devices(void)
+{
+        struct device *dev;
+
+        for (dev = devices.dev; dev; dev = dev->next)
+                reset_device(dev);
+
+        /* If we saved off the original terminal settings, restore them now. */
+        if (orig_term.c_lflag & (ISIG|ICANON|ECHO))
+                tcsetattr(STDIN_FILENO, TCSANOW, &orig_term);
+}
+
+/* When the Guest tells us they updated the status field, we handle it. */
+static void update_device_status(struct device *dev)
+{
+        /* A zero status is a reset, otherwise it's a set of flags. */
+        if (dev->desc->status == 0)
+                reset_device(dev);
+        else if (dev->desc->status & VIRTIO_CONFIG_S_FAILED) {
+                warnx("Device %s configuration FAILED", dev->name);
+                if (dev->running)
+                        reset_device(dev);
+        } else if (dev->desc->status & VIRTIO_CONFIG_S_DRIVER_OK) {
+                if (!dev->running)
+                        start_device(dev);
+        }
+}
+
+/*L:215
+ * This is the generic routine we call when the Guest uses LHCALL_NOTIFY.  In
+ * particular, it's used to notify us of device status changes during boot.
+ */
+static void handle_output(unsigned long addr)
+{
+        struct device *i;
+
+        /* Check each device. */
+        for (i = devices.dev; i; i = i->next) {
+                struct virtqueue *vq;
+
+                /*
+                 * Notifications to device descriptors mean they updated the
+                 * device status.
+                 */
+                if (from_guest_phys(addr) == i->desc) {
+                        update_device_status(i);
+                        return;
+                }
+
+                /*
+                 * Devices *can* be used before status is set to DRIVER_OK.
+                 * The original plan was that they would never do this: they
+                 * would always finish setting up their status bits before
+                 * actually touching the virtqueues.  In practice, we allowed
+                 * them to, and they do (eg. the disk probes for partition
+                 * tables as part of initialization).
+                 *
+                 * If we see this, we start the device: once it's running, we
+                 * expect the device to catch all the notifications.
+                 */
+                for (vq = i->vq; vq; vq = vq->next) {
+                        if (addr != vq->config.pfn*getpagesize())
+                                continue;
+                        if (i->running)
+                                errx(1, "Notification on running %s", i->name);
+                        /* This just calls create_thread() for each virtqueue */
+                        start_device(i);
+                        return;
+                }
+        }
+
+        /*
+         * Early console write is done using notify on a nul-terminated string
+         * in Guest memory.  It's also great for hacking debugging messages
+         * into a Guest.
+         */
+        if (addr >= guest_limit)
+                errx(1, "Bad NOTIFY %#lx", addr);
+
+        write(STDOUT_FILENO, from_guest_phys(addr),
+              strnlen(from_guest_phys(addr), guest_limit - addr));
+}
+
+/*L:190
+ * Device Setup
+ *
+ * All devices need a descriptor so the Guest knows it exists, and a "struct
+ * device" so the Launcher can keep track of it.  We have common helper
+ * routines to allocate and manage them.
+ */
+
+/*
+ * The layout of the device page is a "struct lguest_device_desc" followed by a
+ * number of virtqueue descriptors, then two sets of feature bits, then an
+ * array of configuration bytes.  This routine returns the configuration
+ * pointer.
+ */
+static u8 *device_config(const struct device *dev)
+{
+        return (void *)(dev->desc + 1)
+                + dev->num_vq * sizeof(struct lguest_vqconfig)
+                + dev->feature_len * 2;
+}
+
+/*
+ * This routine allocates a new "struct lguest_device_desc" from descriptor
+ * table page just above the Guest's normal memory.  It returns a pointer to
+ * that descriptor.
+ */
+static struct lguest_device_desc *new_dev_desc(u16 type)
+{
+        struct lguest_device_desc d = { .type = type };
+        void *p;
+
+        /* Figure out where the next device config is, based on the last one. */
+        if (devices.lastdev)
+                p = device_config(devices.lastdev)
+                        + devices.lastdev->desc->config_len;
+        else
+                p = devices.descpage;
+
+        /* We only have one page for all the descriptors. */
+        if (p + sizeof(d) > (void *)devices.descpage + getpagesize())
+                errx(1, "Too many devices");
+
+        /* p might not be aligned, so we memcpy in. */
+        return memcpy(p, &d, sizeof(d));
+}
+
+/*
+ * Each device descriptor is followed by the description of its virtqueues.  We
+ * specify how many descriptors the virtqueue is to have.
+ */
+static void add_virtqueue(struct device *dev, unsigned int num_descs,
+                          void (*service)(struct virtqueue *))
+{
+        unsigned int pages;
+        struct virtqueue **i, *vq = malloc(sizeof(*vq));
+        void *p;
+
+        /* First we need some memory for this virtqueue. */
+        pages = (vring_size(num_descs, LGUEST_VRING_ALIGN) + getpagesize() - 1)
+                / getpagesize();
+        p = get_pages(pages);
+
+        /* Initialize the virtqueue */
+        vq->next = NULL;
+        vq->last_avail_idx = 0;
+        vq->dev = dev;
+
+        /*
+         * This is the routine the service thread will run, and its Process ID
+         * once it's running.
+         */
+        vq->service = service;
+        vq->thread = (pid_t)-1;
+
+        /* Initialize the configuration. */
+        vq->config.num = num_descs;
+        vq->config.irq = devices.next_irq++;
+        vq->config.pfn = to_guest_phys(p) / getpagesize();
+
+        /* Initialize the vring. */
+        vring_init(&vq->vring, num_descs, p, LGUEST_VRING_ALIGN);
+
+        /*
+         * Append virtqueue to this device's descriptor.  We use
+         * device_config() to get the end of the device's current virtqueues;
+         * we check that we haven't added any config or feature information
+         * yet, otherwise we'd be overwriting them.
+         */
+        assert(dev->desc->config_len == 0 && dev->desc->feature_len == 0);
+        memcpy(device_config(dev), &vq->config, sizeof(vq->config));
+        dev->num_vq++;
+        dev->desc->num_vq++;
+
+        verbose("Virtqueue page %#lx\n", to_guest_phys(p));
+
+        /*
+         * Add to tail of list, so dev->vq is first vq, dev->vq->next is
+         * second.
+         */
+        for (i = &dev->vq; *i; i = &(*i)->next);
+        *i = vq;
+}
+
+/*
+ * The first half of the feature bitmask is for us to advertise features.  The
+ * second half is for the Guest to accept features.
+ */
+static void add_feature(struct device *dev, unsigned bit)
+{
+        u8 *features = get_feature_bits(dev);
+
+        /* We can't extend the feature bits once we've added config bytes */
+        if (dev->desc->feature_len <= bit / CHAR_BIT) {
+                assert(dev->desc->config_len == 0);
+                dev->feature_len = dev->desc->feature_len = (bit/CHAR_BIT) + 1;
+        }
+
+        features[bit / CHAR_BIT] |= (1 << (bit % CHAR_BIT));
+}
+
+/*
+ * This routine sets the configuration fields for an existing device's
+ * descriptor.  It only works for the last device, but that's OK because that's
+ * how we use it.
+ */
+static void set_config(struct device *dev, unsigned len, const void *conf)
+{
+        /* Check we haven't overflowed our single page. */
+        if (device_config(dev) + len > devices.descpage + getpagesize())
+                errx(1, "Too many devices");
+
+        /* Copy in the config information, and store the length. */
+        memcpy(device_config(dev), conf, len);
+        dev->desc->config_len = len;
+
+        /* Size must fit in config_len field (8 bits)! */
+        assert(dev->desc->config_len == len);
+}
+
+/*
+ * This routine does all the creation and setup of a new device, including
+ * calling new_dev_desc() to allocate the descriptor and device memory.  We
+ * don't actually start the service threads until later.
+ *
+ * See what I mean about userspace being boring?
+ */
+static struct device *new_device(const char *name, u16 type)
+{
+        struct device *dev = malloc(sizeof(*dev));
+
+        /* Now we populate the fields one at a time. */
+        dev->desc = new_dev_desc(type);
+        dev->name = name;
+        dev->vq = NULL;
+        dev->feature_len = 0;
+        dev->num_vq = 0;
+        dev->running = false;
+
+        /*
+         * Append to device list.  Prepending to a single-linked list is
+         * easier, but the user expects the devices to be arranged on the bus
+         * in command-line order.  The first network device on the command line
+         * is eth0, the first block device /dev/vda, etc.
+         */
+        if (devices.lastdev)
+                devices.lastdev->next = dev;
+        else
+                devices.dev = dev;
+        devices.lastdev = dev;
+
+        return dev;
+}
+
+/*
+ * Our first setup routine is the console.  It's a fairly simple device, but
+ * UNIX tty handling makes it uglier than it could be.
+ */
+static void setup_console(void)
+{
+        struct device *dev;
+
+        /* If we can save the initial standard input settings... */
+        if (tcgetattr(STDIN_FILENO, &orig_term) == 0) {
+                struct termios term = orig_term;
+                /*
+                 * Then we turn off echo, line buffering and ^C etc: We want a
+                 * raw input stream to the Guest.
+                 */
+                term.c_lflag &= ~(ISIG|ICANON|ECHO);
+                tcsetattr(STDIN_FILENO, TCSANOW, &term);
+        }
+
+        dev = new_device("console", VIRTIO_ID_CONSOLE);
+
+        /* We store the console state in dev->priv, and initialize it. */
+        dev->priv = malloc(sizeof(struct console_abort));
+        ((struct console_abort *)dev->priv)->count = 0;
+
+        /*
+         * The console needs two virtqueues: the input then the output.  When
+         * they put something the input queue, we make sure we're listening to
+         * stdin.  When they put something in the output queue, we write it to
+         * stdout.
+         */
+        add_virtqueue(dev, VIRTQUEUE_NUM, console_input);
+        add_virtqueue(dev, VIRTQUEUE_NUM, console_output);
+
+        verbose("device %u: console\n", ++devices.device_num);
+}
+/*:*/
+
+/*M:010
+ * Inter-guest networking is an interesting area.  Simplest is to have a
+ * --sharenet=<name> option which opens or creates a named pipe.  This can be
+ * used to send packets to another guest in a 1:1 manner.
+ *
+ * More sopisticated is to use one of the tools developed for project like UML
+ * to do networking.
+ *
+ * Faster is to do virtio bonding in kernel.  Doing this 1:1 would be
+ * completely generic ("here's my vring, attach to your vring") and would work
+ * for any traffic.  Of course, namespace and permissions issues need to be
+ * dealt with.  A more sophisticated "multi-channel" virtio_net.c could hide
+ * multiple inter-guest channels behind one interface, although it would
+ * require some manner of hotplugging new virtio channels.
+ *
+ * Finally, we could implement a virtio network switch in the kernel.
+:*/
+
+static u32 str2ip(const char *ipaddr)
+{
+        unsigned int b[4];
+
+        if (sscanf(ipaddr, "%u.%u.%u.%u", &b[0], &b[1], &b[2], &b[3]) != 4)
+                errx(1, "Failed to parse IP address '%s'", ipaddr);
+        return (b[0] << 24) | (b[1] << 16) | (b[2] << 8) | b[3];
+}
+
+static void str2mac(const char *macaddr, unsigned char mac[6])
+{
+        unsigned int m[6];
+        if (sscanf(macaddr, "%02x:%02x:%02x:%02x:%02x:%02x",
+                   &m[0], &m[1], &m[2], &m[3], &m[4], &m[5]) != 6)
+                errx(1, "Failed to parse mac address '%s'", macaddr);
+        mac[0] = m[0];
+        mac[1] = m[1];
+        mac[2] = m[2];
+        mac[3] = m[3];
+        mac[4] = m[4];
+        mac[5] = m[5];
+}
+
+/*
+ * This code is "adapted" from libbridge: it attaches the Host end of the
+ * network device to the bridge device specified by the command line.
+ *
+ * This is yet another James Morris contribution (I'm an IP-level guy, so I
+ * dislike bridging), and I just try not to break it.
+ */
+static void add_to_bridge(int fd, const char *if_name, const char *br_name)
+{
+        int ifidx;
+        struct ifreq ifr;
+
+        if (!*br_name)
+                errx(1, "must specify bridge name");
+
+        ifidx = if_nametoindex(if_name);
+        if (!ifidx)
+                errx(1, "interface %s does not exist!", if_name);
+
+        strncpy(ifr.ifr_name, br_name, IFNAMSIZ);
+        ifr.ifr_name[IFNAMSIZ-1] = '\0';
+        ifr.ifr_ifindex = ifidx;
+        if (ioctl(fd, SIOCBRADDIF, &ifr) < 0)
+                err(1, "can't add %s to bridge %s", if_name, br_name);
+}
+
+/*
+ * This sets up the Host end of the network device with an IP address, brings
+ * it up so packets will flow, the copies the MAC address into the hwaddr
+ * pointer.
+ */
+static void configure_device(int fd, const char *tapif, u32 ipaddr)
+{
+        struct ifreq ifr;
+        struct sockaddr_in sin;
+
+        memset(&ifr, 0, sizeof(ifr));
+        strcpy(ifr.ifr_name, tapif);
+
+        /* Don't read these incantations.  Just cut & paste them like I did! */
+        sin.sin_family = AF_INET;
+        sin.sin_addr.s_addr = htonl(ipaddr);
+        memcpy(&ifr.ifr_addr, &sin, sizeof(sin));
+        if (ioctl(fd, SIOCSIFADDR, &ifr) != 0)
+                err(1, "Setting %s interface address", tapif);
+        ifr.ifr_flags = IFF_UP;
+        if (ioctl(fd, SIOCSIFFLAGS, &ifr) != 0)
+                err(1, "Bringing interface %s up", tapif);
+}
+
+static int get_tun_device(char tapif[IFNAMSIZ])
+{
+        struct ifreq ifr;
+        int netfd;
+
+        /* Start with this zeroed.  Messy but sure. */
+        memset(&ifr, 0, sizeof(ifr));
+
+        /*
+         * We open the /dev/net/tun device and tell it we want a tap device.  A
+         * tap device is like a tun device, only somehow different.  To tell
+         * the truth, I completely blundered my way through this code, but it
+         * works now!
+         */
+        netfd = open_or_die("/dev/net/tun", O_RDWR);
+        ifr.ifr_flags = IFF_TAP | IFF_NO_PI | IFF_VNET_HDR;
+        strcpy(ifr.ifr_name, "tap%d");
+        if (ioctl(netfd, TUNSETIFF, &ifr) != 0)
+                err(1, "configuring /dev/net/tun");
+
+        if (ioctl(netfd, TUNSETOFFLOAD,
+                  TUN_F_CSUM|TUN_F_TSO4|TUN_F_TSO6|TUN_F_TSO_ECN) != 0)
+                err(1, "Could not set features for tun device");
+
+        /*
+         * We don't need checksums calculated for packets coming in this
+         * device: trust us!
+         */
+        ioctl(netfd, TUNSETNOCSUM, 1);
+
+        memcpy(tapif, ifr.ifr_name, IFNAMSIZ);
+        return netfd;
+}
+
+/*L:195
+ * Our network is a Host<->Guest network.  This can either use bridging or
+ * routing, but the principle is the same: it uses the "tun" device to inject
+ * packets into the Host as if they came in from a normal network card.  We
+ * just shunt packets between the Guest and the tun device.
+ */
+static void setup_tun_net(char *arg)
+{
+        struct device *dev;
+        struct net_info *net_info = malloc(sizeof(*net_info));
+        int ipfd;
+        u32 ip = INADDR_ANY;
+        bool bridging = false;
+        char tapif[IFNAMSIZ], *p;
+        struct virtio_net_config conf;
+
+        net_info->tunfd = get_tun_device(tapif);
+
+        /* First we create a new network device. */
+        dev = new_device("net", VIRTIO_ID_NET);
+        dev->priv = net_info;
+
+        /* Network devices need a recv and a send queue, just like console. */
+        add_virtqueue(dev, VIRTQUEUE_NUM, net_input);
+        add_virtqueue(dev, VIRTQUEUE_NUM, net_output);
+
+        /*
+         * We need a socket to perform the magic network ioctls to bring up the
+         * tap interface, connect to the bridge etc.  Any socket will do!
+         */
+        ipfd = socket(PF_INET, SOCK_DGRAM, IPPROTO_IP);
+        if (ipfd < 0)
+                err(1, "opening IP socket");
+
+        /* If the command line was --tunnet=bridge:<name> do bridging. */
+        if (!strncmp(BRIDGE_PFX, arg, strlen(BRIDGE_PFX))) {
+                arg += strlen(BRIDGE_PFX);
+                bridging = true;
+        }
+
+        /* A mac address may follow the bridge name or IP address */
+        p = strchr(arg, ':');
+        if (p) {
+                str2mac(p+1, conf.mac);
+                add_feature(dev, VIRTIO_NET_F_MAC);
+                *p = '\0';
+        }
+
+        /* arg is now either an IP address or a bridge name */
+        if (bridging)
+                add_to_bridge(ipfd, tapif, arg);
+        else
+                ip = str2ip(arg);
+
+        /* Set up the tun device. */
+        configure_device(ipfd, tapif, ip);
+
+        add_feature(dev, VIRTIO_F_NOTIFY_ON_EMPTY);
+        /* Expect Guest to handle everything except UFO */
+        add_feature(dev, VIRTIO_NET_F_CSUM);
+        add_feature(dev, VIRTIO_NET_F_GUEST_CSUM);
+        add_feature(dev, VIRTIO_NET_F_GUEST_TSO4);
+        add_feature(dev, VIRTIO_NET_F_GUEST_TSO6);
+        add_feature(dev, VIRTIO_NET_F_GUEST_ECN);
+        add_feature(dev, VIRTIO_NET_F_HOST_TSO4);
+        add_feature(dev, VIRTIO_NET_F_HOST_TSO6);
+        add_feature(dev, VIRTIO_NET_F_HOST_ECN);
+        /* We handle indirect ring entries */
+        add_feature(dev, VIRTIO_RING_F_INDIRECT_DESC);
+        set_config(dev, sizeof(conf), &conf);
+
+        /* We don't need the socket any more; setup is done. */
+        close(ipfd);
+
+        devices.device_num++;
+
+        if (bridging)
+                verbose("device %u: tun %s attached to bridge: %s\n",
+                        devices.device_num, tapif, arg);
+        else
+                verbose("device %u: tun %s: %s\n",
+                        devices.device_num, tapif, arg);
+}
+/*:*/
+
+/* This hangs off device->priv. */
+struct vblk_info {
+        /* The size of the file. */
+        off64_t len;
+
+        /* The file descriptor for the file. */
+        int fd;
+
+};
+
+/*L:210
+ * The Disk
+ *
+ * The disk only has one virtqueue, so it only has one thread.  It is really
+ * simple: the Guest asks for a block number and we read or write that position
+ * in the file.
+ *
+ * Before we serviced each virtqueue in a separate thread, that was unacceptably
+ * slow: the Guest waits until the read is finished before running anything
+ * else, even if it could have been doing useful work.
+ *
+ * We could have used async I/O, except it's reputed to suck so hard that
+ * characters actually go missing from your code when you try to use it.
+ */
+static void blk_request(struct virtqueue *vq)
+{
+        struct vblk_info *vblk = vq->dev->priv;
+        unsigned int head, out_num, in_num, wlen;
+        int ret;
+        u8 *in;
+        struct virtio_blk_outhdr *out;
+        struct iovec iov[vq->vring.num];
+        off64_t off;
+
+        /*
+         * Get the next request, where we normally wait.  It triggers the
+         * interrupt to acknowledge previously serviced requests (if any).
+         */
+        head = wait_for_vq_desc(vq, iov, &out_num, &in_num);
+
+        /*
+         * Every block request should contain at least one output buffer
+         * (detailing the location on disk and the type of request) and one
+         * input buffer (to hold the result).
+         */
+        if (out_num == 0 || in_num == 0)
+                errx(1, "Bad virtblk cmd %u out=%u in=%u",
+                     head, out_num, in_num);
+
+        out = convert(&iov[0], struct virtio_blk_outhdr);
+        in = convert(&iov[out_num+in_num-1], u8);
+        /*
+         * For historical reasons, block operations are expressed in 512 byte
+         * "sectors".
+         */
+        off = out->sector * 512;
+
+        /*
+         * In general the virtio block driver is allowed to try SCSI commands.
+         * It'd be nice if we supported eject, for example, but we don't.
+         */
+        if (out->type & VIRTIO_BLK_T_SCSI_CMD) {
+                fprintf(stderr, "Scsi commands unsupported\n");
+                *in = VIRTIO_BLK_S_UNSUPP;
+                wlen = sizeof(*in);
+        } else if (out->type & VIRTIO_BLK_T_OUT) {
+                /*
+                 * Write
+                 *
+                 * Move to the right location in the block file.  This can fail
+                 * if they try to write past end.
+                 */
+                if (lseek64(vblk->fd, off, SEEK_SET) != off)
+                        err(1, "Bad seek to sector %llu", out->sector);
+
+                ret = writev(vblk->fd, iov+1, out_num-1);
+                verbose("WRITE to sector %llu: %i\n", out->sector, ret);
+
+                /*
+                 * Grr... Now we know how long the descriptor they sent was, we
+                 * make sure they didn't try to write over the end of the block
+                 * file (possibly extending it).
+                 */
+                if (ret > 0 && off + ret > vblk->len) {
+                        /* Trim it back to the correct length */
+                        ftruncate64(vblk->fd, vblk->len);
+                        /* Die, bad Guest, die. */
+                        errx(1, "Write past end %llu+%u", off, ret);
+                }
+
+                wlen = sizeof(*in);
+                *in = (ret >= 0 ? VIRTIO_BLK_S_OK : VIRTIO_BLK_S_IOERR);
+        } else if (out->type & VIRTIO_BLK_T_FLUSH) {
+                /* Flush */
+                ret = fdatasync(vblk->fd);
+                verbose("FLUSH fdatasync: %i\n", ret);
+                wlen = sizeof(*in);
+                *in = (ret >= 0 ? VIRTIO_BLK_S_OK : VIRTIO_BLK_S_IOERR);
+        } else {
+                /*
+                 * Read
+                 *
+                 * Move to the right location in the block file.  This can fail
+                 * if they try to read past end.
+                 */
+                if (lseek64(vblk->fd, off, SEEK_SET) != off)
+                        err(1, "Bad seek to sector %llu", out->sector);
+
+                ret = readv(vblk->fd, iov+1, in_num-1);
+                verbose("READ from sector %llu: %i\n", out->sector, ret);
+                if (ret >= 0) {
+                        wlen = sizeof(*in) + ret;
+                        *in = VIRTIO_BLK_S_OK;
+                } else {
+                        wlen = sizeof(*in);
+                        *in = VIRTIO_BLK_S_IOERR;
+                }
+        }
+
+        /* Finished that request. */
+        add_used(vq, head, wlen);
+}
+
+/*L:198 This actually sets up a virtual block device. */
+static void setup_block_file(const char *filename)
+{
+        struct device *dev;
+        struct vblk_info *vblk;
+        struct virtio_blk_config conf;
+
+        /* Creat the device. */
+        dev = new_device("block", VIRTIO_ID_BLOCK);
+
+        /* The device has one virtqueue, where the Guest places requests. */
+        add_virtqueue(dev, VIRTQUEUE_NUM, blk_request);
+
+        /* Allocate the room for our own bookkeeping */
+        vblk = dev->priv = malloc(sizeof(*vblk));
+
+        /* First we open the file and store the length. */
+        vblk->fd = open_or_die(filename, O_RDWR|O_LARGEFILE);
+        vblk->len = lseek64(vblk->fd, 0, SEEK_END);
+
+        /* We support FLUSH. */
+        add_feature(dev, VIRTIO_BLK_F_FLUSH);
+
+        /* Tell Guest how many sectors this device has. */
+        conf.capacity = cpu_to_le64(vblk->len / 512);
+
+        /*
+         * Tell Guest not to put in too many descriptors at once: two are used
+         * for the in and out elements.
+         */
+        add_feature(dev, VIRTIO_BLK_F_SEG_MAX);
+        conf.seg_max = cpu_to_le32(VIRTQUEUE_NUM - 2);
+
+        /* Don't try to put whole struct: we have 8 bit limit. */
+        set_config(dev, offsetof(struct virtio_blk_config, geometry), &conf);
+
+        verbose("device %u: virtblock %llu sectors\n",
+                ++devices.device_num, le64_to_cpu(conf.capacity));
+}
+
+/*L:211
+ * Our random number generator device reads from /dev/random into the Guest's
+ * input buffers.  The usual case is that the Guest doesn't want random numbers
+ * and so has no buffers although /dev/random is still readable, whereas
+ * console is the reverse.
+ *
+ * The same logic applies, however.
+ */
+struct rng_info {
+        int rfd;
+};
+
+static void rng_input(struct virtqueue *vq)
+{
+        int len;
+        unsigned int head, in_num, out_num, totlen = 0;
+        struct rng_info *rng_info = vq->dev->priv;
+        struct iovec iov[vq->vring.num];
+
+        /* First we need a buffer from the Guests's virtqueue. */
+        head = wait_for_vq_desc(vq, iov, &out_num, &in_num);
+        if (out_num)
+                errx(1, "Output buffers in rng?");
+
+        /*
+         * Just like the console write, we loop to cover the whole iovec.
+         * In this case, short reads actually happen quite a bit.
+         */
+        while (!iov_empty(iov, in_num)) {
+                len = readv(rng_info->rfd, iov, in_num);
+                if (len <= 0)
+                        err(1, "Read from /dev/random gave %i", len);
+                iov_consume(iov, in_num, len);
+                totlen += len;
+        }
+
+        /* Tell the Guest about the new input. */
+        add_used(vq, head, totlen);
+}
+
+/*L:199
+ * This creates a "hardware" random number device for the Guest.
+ */
+static void setup_rng(void)
+{
+        struct device *dev;
+        struct rng_info *rng_info = malloc(sizeof(*rng_info));
+
+        /* Our device's privat info simply contains the /dev/random fd. */
+        rng_info->rfd = open_or_die("/dev/random", O_RDONLY);
+
+        /* Create the new device. */
+        dev = new_device("rng", VIRTIO_ID_RNG);
+        dev->priv = rng_info;
+
+        /* The device has one virtqueue, where the Guest places inbufs. */
+        add_virtqueue(dev, VIRTQUEUE_NUM, rng_input);
+
+        verbose("device %u: rng\n", devices.device_num++);
+}
+/* That's the end of device setup. */
+
+/*L:230 Reboot is pretty easy: clean up and exec() the Launcher afresh. */
+static void __attribute__((noreturn)) restart_guest(void)
+{
+        unsigned int i;
+
+        /*
+         * Since we don't track all open fds, we simply close everything beyond
+         * stderr.
+         */
+        for (i = 3; i < FD_SETSIZE; i++)
+                close(i);
+
+        /* Reset all the devices (kills all threads). */
+        cleanup_devices();
+
+        execv(main_args[0], main_args);
+        err(1, "Could not exec %s", main_args[0]);
+}
+
+/*L:220
+ * Finally we reach the core of the Launcher which runs the Guest, serves
+ * its input and output, and finally, lays it to rest.
+ */
+static void __attribute__((noreturn)) run_guest(void)
+{
+        for (;;) {
+                unsigned long notify_addr;
+                int readval;
+
+                /* We read from the /dev/lguest device to run the Guest. */
+                readval = pread(lguest_fd, &notify_addr,
+                                sizeof(notify_addr), cpu_id);
+
+                /* One unsigned long means the Guest did HCALL_NOTIFY */
+                if (readval == sizeof(notify_addr)) {
+                        verbose("Notify on address %#lx\n", notify_addr);
+                        handle_output(notify_addr);
+                /* ENOENT means the Guest died.  Reading tells us why. */
+                } else if (errno == ENOENT) {
+                        char reason[1024] = { 0 };
+                        pread(lguest_fd, reason, sizeof(reason)-1, cpu_id);
+                        errx(1, "%s", reason);
+                /* ERESTART means that we need to reboot the guest */
+                } else if (errno == ERESTART) {
+                        restart_guest();
+                /* Anything else means a bug or incompatible change. */
+                } else
+                        err(1, "Running guest failed");
+        }
+}
+/*L:240
+ * This is the end of the Launcher.  The good news: we are over halfway
+ * through!  The bad news: the most fiendish part of the code still lies ahead
+ * of us.
+ *
+ * Are you ready?  Take a deep breath and join me in the core of the Host, in
+ * "make Host".
+:*/
+
+static struct option opts[] = {
+        { "verbose", 0, NULL, 'v' },
+        { "tunnet", 1, NULL, 't' },
+        { "block", 1, NULL, 'b' },
+        { "rng", 0, NULL, 'r' },
+        { "initrd", 1, NULL, 'i' },
+        { "username", 1, NULL, 'u' },
+        { "chroot", 1, NULL, 'c' },
+        { NULL },
+};
+static void usage(void)
+{
+        errx(1, "Usage: lguest [--verbose] "
+             "[--tunnet=(<ipaddr>:<macaddr>|bridge:<bridgename>:<macaddr>)\n"
+             "|--block=<filename>|--initrd=<filename>]...\n"
+             "<mem-in-mb> vmlinux [args...]");
+}
+
+/*L:105 The main routine is where the real work begins: */
+int main(int argc, char *argv[])
+{
+        /* Memory, code startpoint and size of the (optional) initrd. */
+        unsigned long mem = 0, start, initrd_size = 0;
+        /* Two temporaries. */
+        int i, c;
+        /* The boot information for the Guest. */
+        struct boot_params *boot;
+        /* If they specify an initrd file to load. */
+        const char *initrd_name = NULL;
+
+        /* Password structure for initgroups/setres[gu]id */
+        struct passwd *user_details = NULL;
+
+        /* Directory to chroot to */
+        char *chroot_path = NULL;
+
+        /* Save the args: we "reboot" by execing ourselves again. */
+        main_args = argv;
+
+        /*
+         * First we initialize the device list.  We keep a pointer to the last
+         * device, and the next interrupt number to use for devices (1:
+         * remember that 0 is used by the timer).
+         */
+        devices.lastdev = NULL;
+        devices.next_irq = 1;
+
+        /* We're CPU 0.  In fact, that's the only CPU possible right now. */
+        cpu_id = 0;
+
+        /*
+         * We need to know how much memory so we can set up the device
+         * descriptor and memory pages for the devices as we parse the command
+         * line.  So we quickly look through the arguments to find the amount
+         * of memory now.
+         */
+        for (i = 1; i < argc; i++) {
+                if (argv[i][0] != '-') {
+                        mem = atoi(argv[i]) * 1024 * 1024;
+                        /*
+                         * We start by mapping anonymous pages over all of
+                         * guest-physical memory range.  This fills it with 0,
+                         * and ensures that the Guest won't be killed when it
+                         * tries to access it.
+                         */
+                        guest_base = map_zeroed_pages(mem / getpagesize()
+                                                      + DEVICE_PAGES);
+                        guest_limit = mem;
+                        guest_max = mem + DEVICE_PAGES*getpagesize();
+                        devices.descpage = get_pages(1);
+                        break;
+                }
+        }
+
+        /* The options are fairly straight-forward */
+        while ((c = getopt_long(argc, argv, "v", opts, NULL)) != EOF) {
+                switch (c) {
+                case 'v':
+                        verbose = true;
+                        break;
+                case 't':
+                        setup_tun_net(optarg);
+                        break;
+                case 'b':
+                        setup_block_file(optarg);
+                        break;
+                case 'r':
+                        setup_rng();
+                        break;
+                case 'i':
+                        initrd_name = optarg;
+                        break;
+                case 'u':
+                        user_details = getpwnam(optarg);
+                        if (!user_details)
+                                err(1, "getpwnam failed, incorrect username?");
+                        break;
+                case 'c':
+                        chroot_path = optarg;
+                        break;
+                default:
+                        warnx("Unknown argument %s", argv[optind]);
+                        usage();
+                }
+        }
+        /*
+         * After the other arguments we expect memory and kernel image name,
+         * followed by command line arguments for the kernel.
+         */
+        if (optind + 2 > argc)
+                usage();
+
+        verbose("Guest base is at %p\n", guest_base);
+
+        /* We always have a console device */
+        setup_console();
+
+        /* Now we load the kernel */
+        start = load_kernel(open_or_die(argv[optind+1], O_RDONLY));
+
+        /* Boot information is stashed at physical address 0 */
+        boot = from_guest_phys(0);
+
+        /* Map the initrd image if requested (at top of physical memory) */
+        if (initrd_name) {
+                initrd_size = load_initrd(initrd_name, mem);
+                /*
+                 * These are the location in the Linux boot header where the
+                 * start and size of the initrd are expected to be found.
+                 */
+                boot->hdr.ramdisk_image = mem - initrd_size;
+                boot->hdr.ramdisk_size = initrd_size;
+                /* The bootloader type 0xFF means "unknown"; that's OK. */
+                boot->hdr.type_of_loader = 0xFF;
+        }
+
+        /*
+         * The Linux boot header contains an "E820" memory map: ours is a
+         * simple, single region.
+         */
+        boot->e820_entries = 1;
+        boot->e820_map[0] = ((struct e820entry) { 0, mem, E820_RAM });
+        /*
+         * The boot header contains a command line pointer: we put the command
+         * line after the boot header.
+         */
+        boot->hdr.cmd_line_ptr = to_guest_phys(boot + 1);
+        /* We use a simple helper to copy the arguments separated by spaces. */
+        concat((char *)(boot + 1), argv+optind+2);
+
+        /* Boot protocol version: 2.07 supports the fields for lguest. */
+        boot->hdr.version = 0x207;
+
+        /* The hardware_subarch value of "1" tells the Guest it's an lguest. */
+        boot->hdr.hardware_subarch = 1;
+
+        /* Tell the entry path not to try to reload segment registers. */
+        boot->hdr.loadflags |= KEEP_SEGMENTS;
+
+        /*
+         * We tell the kernel to initialize the Guest: this returns the open
+         * /dev/lguest file descriptor.
+         */
+        tell_kernel(start);
+
+        /* Ensure that we terminate if a device-servicing child dies. */
+        signal(SIGCHLD, kill_launcher);
+
+        /* If we exit via err(), this kills all the threads, restores tty. */
+        atexit(cleanup_devices);
+
+        /* If requested, chroot to a directory */
+        if (chroot_path) {
+                if (chroot(chroot_path) != 0)
+                        err(1, "chroot(\"%s\") failed", chroot_path);
+
+                if (chdir("/") != 0)
+                        err(1, "chdir(\"/\") failed");
+
+                verbose("chroot done\n");
+        }
+
+        /* If requested, drop privileges */
+        if (user_details) {
+                uid_t u;
+                gid_t g;
+
+                u = user_details->pw_uid;
+                g = user_details->pw_gid;
+
+                if (initgroups(user_details->pw_name, g) != 0)
+                        err(1, "initgroups failed");
+
+                if (setresgid(g, g, g) != 0)
+                        err(1, "setresgid failed");
+
+                if (setresuid(u, u, u) != 0)
+                        err(1, "setresuid failed");
+
+                verbose("Dropping privileges completed\n");
+        }
+
+        /* Finally, run the Guest.  This doesn't return. */
+        run_guest();
+}
+/*:*/
+
+/*M:999
+ * Mastery is done: you now know everything I do.
+ *
+ * But surely you have seen code, features and bugs in your wanderings which
+ * you now yearn to attack?  That is the real game, and I look forward to you
+ * patching and forking lguest into the Your-Name-Here-visor.
+ *
+ * Farewell, and good coding!
+ * Rusty Russell.
+ */
diff --git a/Documentation/virtual/lguest/lguest.txt b/Documentation/virtual/lguest/lguest.txt
new file mode 100644
index 000000000000..bff0c554485d
--- /dev/null
+++ b/Documentation/virtual/lguest/lguest.txt
@@ -0,0 +1,129 @@
+      __
+ (___()'`;  Rusty's Remarkably Unreliable Guide to Lguest
+ /,    /`      - or, A Young Coder's Illustrated Hypervisor
+ \\"--\\    http://lguest.ozlabs.org
+
+Lguest is designed to be a minimal 32-bit x86 hypervisor for the Linux kernel,
+for Linux developers and users to experiment with virtualization with the
+minimum of complexity.  Nonetheless, it should have sufficient features to
+make it useful for specific tasks, and, of course, you are encouraged to fork
+and enhance it (see drivers/lguest/README).
+
+Features:
+
+- Kernel module which runs in a normal kernel.
+- Simple I/O model for communication.
+- Simple program to create new guests.
+- Logo contains cute puppies: http://lguest.ozlabs.org
+
+Developer features:
+
+- Fun to hack on.
+- No ABI: being tied to a specific kernel anyway, you can change anything.
+- Many opportunities for improvement or feature implementation.
+
+Running Lguest:
+
+- The easiest way to run lguest is to use same kernel as guest and host.
+  You can configure them differently, but usually it's easiest not to.
+
+  You will need to configure your kernel with the following options:
+
+  "General setup":
+     "Prompt for development and/or incomplete code/drivers" = Y
+        (CONFIG_EXPERIMENTAL=y)
+
+  "Processor type and features":
+     "Paravirtualized guest support" = Y
+        "Lguest guest support" = Y
+     "High Memory Support" = off/4GB
+     "Alignment value to which kernel should be aligned" = 0x100000
+        (CONFIG_PARAVIRT=y, CONFIG_LGUEST_GUEST=y, CONFIG_HIGHMEM64G=n and
+         CONFIG_PHYSICAL_ALIGN=0x100000)
+
+  "Device Drivers":
+     "Block devices"
+        "Virtio block driver (EXPERIMENTAL)" = M/Y
+     "Network device support"
+        "Universal TUN/TAP device driver support" = M/Y
+        "Virtio network driver (EXPERIMENTAL)" = M/Y
+           (CONFIG_VIRTIO_BLK=m, CONFIG_VIRTIO_NET=m and CONFIG_TUN=m)
+
+  "Virtualization"
+     "Linux hypervisor example code" = M/Y
+        (CONFIG_LGUEST=m)
+
+- A tool called "lguest" is available in this directory: type "make"
+  to build it.  If you didn't build your kernel in-tree, use "make
+  O=<builddir>".
+
+- Create or find a root disk image.  There are several useful ones
+  around, such as the xm-test tiny root image at
+          http://xm-test.xensource.com/ramdisks/initrd-1.1-i386.img
+
+  For more serious work, I usually use a distribution ISO image and
+  install it under qemu, then make multiple copies:
+
+          dd if=/dev/zero of=rootfile bs=1M count=2048
+          qemu -cdrom image.iso -hda rootfile -net user -net nic -boot d
+
+  Make sure that you install a getty on /dev/hvc0 if you want to log in on the
+  console!
+
+- "modprobe lg" if you built it as a module.
+
+- Run an lguest as root:
+
+      Documentation/virtual/lguest/lguest 64 vmlinux --tunnet=192.168.19.1 \
+        --block=rootfile root=/dev/vda
+
+   Explanation:
+    64: the amount of memory to use, in MB.
+
+    vmlinux: the kernel image found in the top of your build directory.  You
+       can also use a standard bzImage.
+
+    --tunnet=192.168.19.1: configures a "tap" device for networking with this
+       IP address.
+
+    --block=rootfile: a file or block device which becomes /dev/vda
+       inside the guest.
+
+    root=/dev/vda: this (and anything else on the command line) are
+       kernel boot parameters.
+
+- Configuring networking.  I usually have the host masquerade, using
+  "iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE" and "echo 1 >
+  /proc/sys/net/ipv4/ip_forward".  In this example, I would configure
+  eth0 inside the guest at 192.168.19.2.
+
+  Another method is to bridge the tap device to an external interface
+  using --tunnet=bridge:<bridgename>, and perhaps run dhcp on the guest
+  to obtain an IP address.  The bridge needs to be configured first:
+  this option simply adds the tap interface to it.
+
+  A simple example on my system:
+
+    ifconfig eth0 0.0.0.0
+    brctl addbr lg0
+    ifconfig lg0 up
+    brctl addif lg0 eth0
+    dhclient lg0
+
+  Then use --tunnet=bridge:lg0 when launching the guest.
+
+  See:
+  
+    http://www.linuxfoundation.org/collaborate/workgroups/networking/bridge
+    
+  for general information on how to get bridging to work.
+
+- Random number generation. Using the --rng option will provide a
+  /dev/hwrng in the guest that will read from the host's /dev/random.
+  Use this option in conjunction with rng-tools (see ../hw_random.txt)
+  to provide entropy to the guest kernel's /dev/random.
+
+There is a helpful mailing list at http://ozlabs.org/mailman/listinfo/lguest
+
+Good luck!
+Rusty Russell rusty@rustcorp.com.au.
diff --git a/Documentation/virtual/uml/UserModeLinux-HOWTO.txt b/Documentation/virtual/uml/UserModeLinux-HOWTO.txt
new file mode 100644
index 000000000000..9b7e1904db1c
--- /dev/null
+++ b/Documentation/virtual/uml/UserModeLinux-HOWTO.txt
@@ -0,0 +1,4579 @@
+  User Mode Linux HOWTO
+  User Mode Linux Core Team
+  Mon Nov 18 14:16:16 EST 2002
+
+  This document describes the use and abuse of Jeff Dike's User Mode
+  Linux: a port of the Linux kernel as a normal Intel Linux process.
+  ______________________________________________________________________
+
+  Table of Contents
+
+  1. Introduction
+
+     1.1 How is User Mode Linux Different?
+     1.2 Why Would I Want User Mode Linux?
+
+  2. Compiling the kernel and modules
+
+     2.1 Compiling the kernel
+     2.2 Compiling and installing kernel modules
+     2.3 Compiling and installing uml_utilities
+
+  3. Running UML and logging in
+
+     3.1 Running UML
+     3.2 Logging in
+     3.3 Examples
+
+  4. UML on 2G/2G hosts
+
+     4.1 Introduction
+     4.2 The problem
+     4.3 The solution
+
+  5. Setting up serial lines and consoles
+
+     5.1 Specifying the device
+     5.2 Specifying the channel
+     5.3 Examples
+
+  6. Setting up the network
+
+     6.1 General setup
+     6.2 Userspace daemons
+     6.3 Specifying ethernet addresses
+     6.4 UML interface setup
+     6.5 Multicast
+     6.6 TUN/TAP with the uml_net helper
+     6.7 TUN/TAP with a preconfigured tap device
+     6.8 Ethertap
+     6.9 The switch daemon
+     6.10 Slip
+     6.11 Slirp
+     6.12 pcap
+     6.13 Setting up the host yourself
+
+  7. Sharing Filesystems between Virtual Machines
+
+     7.1 A warning
+     7.2 Using layered block devices
+     7.3 Note!
+     7.4 Another warning
+     7.5 uml_moo : Merging a COW file with its backing file
+
+  8. Creating filesystems
+
+     8.1 Create the filesystem file
+     8.2 Assign the file to a UML device
+     8.3 Creating and mounting the filesystem
+
+  9. Host file access
+
+     9.1 Using hostfs
+     9.2 hostfs as the root filesystem
+     9.3 Building hostfs
+
+  10. The Management Console
+     10.1 version
+     10.2 halt and reboot
+     10.3 config
+     10.4 remove
+     10.5 sysrq
+     10.6 help
+     10.7 cad
+     10.8 stop
+     10.9 go
+
+  11. Kernel debugging
+
+     11.1 Starting the kernel under gdb
+     11.2 Examining sleeping processes
+     11.3 Running ddd on UML
+     11.4 Debugging modules
+     11.5 Attaching gdb to the kernel
+     11.6 Using alternate debuggers
+
+  12. Kernel debugging examples
+
+     12.1 The case of the hung fsck
+     12.2 Episode 2: The case of the hung fsck
+
+  13. What to do when UML doesn't work
+
+     13.1 Strange compilation errors when you build from source
+     13.2 (obsolete)
+     13.3 A variety of panics and hangs with /tmp on a reiserfs  filesystem
+     13.4 The compile fails with errors about conflicting types for 'open', 'dup', and 'waitpid'
+     13.5 UML doesn't work when /tmp is an NFS filesystem
+     13.6 UML hangs on boot when compiled with gprof support
+     13.7 syslogd dies with a SIGTERM on startup
+     13.8 TUN/TAP networking doesn't work on a 2.4 host
+     13.9 You can network to the host but not to other machines on the net
+     13.10 I have no root and I want to scream
+     13.11 UML build conflict between ptrace.h and ucontext.h
+     13.12 The UML BogoMips is exactly half the host's BogoMips
+     13.13 When you run UML, it immediately segfaults
+     13.14 xterms appear, then immediately disappear
+     13.15 Any other panic, hang, or strange behavior
+
+  14. Diagnosing Problems
+
+     14.1 Case 1 : Normal kernel panics
+     14.2 Case 2 : Tracing thread panics
+     14.3 Case 3 : Tracing thread panics caused by other threads
+     14.4 Case 4 : Hangs
+
+  15. Thanks
+
+     15.1 Code and Documentation
+     15.2 Flushing out bugs
+     15.3 Buglets and clean-ups
+     15.4 Case Studies
+     15.5 Other contributions
+
+
+  ______________________________________________________________________
+
+  11..  IInnttrroodduuccttiioonn
+
+  Welcome to User Mode Linux.  It's going to be fun.
+
+
+
+  11..11..  HHooww iiss UUsseerr MMooddee LLiinnuuxx DDiiffffeerreenntt??
+
+  Normally, the Linux Kernel talks straight to your hardware (video
+  card, keyboard, hard drives, etc), and any programs which run ask the
+  kernel to operate the hardware, like so:
+
+
+
+         +-----------+-----------+----+
+         | Process 1 | Process 2 | ...|
+         +-----------+-----------+----+
+         |       Linux Kernel         |
+         +----------------------------+
+         |         Hardware           |
+         +----------------------------+
+
+
+
+
+  The User Mode Linux Kernel is different; instead of talking to the
+  hardware, it talks to a `real' Linux kernel (called the `host kernel'
+  from now on), like any other program.  Programs can then run inside
+  User-Mode Linux as if they were running under a normal kernel, like
+  so:
+
+
+
+                     +----------------+
+                     | Process 2 | ...|
+         +-----------+----------------+
+         | Process 1 | User-Mode Linux|
+         +----------------------------+
+         |       Linux Kernel         |
+         +----------------------------+
+         |         Hardware           |
+         +----------------------------+
+
+
+
+
+
+  11..22..  WWhhyy WWoouulldd II WWaanntt UUsseerr MMooddee LLiinnuuxx??
+
+
+  1. If User Mode Linux crashes, your host kernel is still fine.
+
+  2. You can run a usermode kernel as a non-root user.
+
+  3. You can debug the User Mode Linux like any normal process.
+
+  4. You can run gprof (profiling) and gcov (coverage testing).
+
+  5. You can play with your kernel without breaking things.
+
+  6. You can use it as a sandbox for testing new apps.
+
+  7. You can try new development kernels safely.
+
+  8. You can run different distributions simultaneously.
+
+  9. It's extremely fun.
+
+
+
+
+
+  22..  CCoommppiilliinngg tthhee kkeerrnneell aanndd mmoodduulleess
+
+
+
+
+  22..11..  CCoommppiilliinngg tthhee kkeerrnneell
+
+
+  Compiling the user mode kernel is just like compiling any other
+  kernel.  Let's go through the steps, using 2.4.0-prerelease (current
+  as of this writing) as an example:
+
+
+  1. Download the latest UML patch from
+
+     the download page <http://user-mode-linux.sourceforge.net/
+
+     In this example, the file is uml-patch-2.4.0-prerelease.bz2.
+
+
+  2. Download the matching kernel from your favourite kernel mirror,
+     such as:
+
+     ftp://ftp.ca.kernel.org/pub/kernel/v2.4/linux-2.4.0-prerelease.tar.bz2
+     <ftp://ftp.ca.kernel.org/pub/kernel/v2.4/linux-2.4.0-prerelease.tar.bz2>
+     .
+
+
+  3. Make a directory and unpack the kernel into it.
+
+
+
+       host%
+       mkdir ~/uml
+
+
+
+
+
+
+       host%
+       cd ~/uml
+
+
+
+
+
+
+       host%
+       tar -xzvf linux-2.4.0-prerelease.tar.bz2
+
+
+
+
+
+
+  4. Apply the patch using
+
+
+
+       host%
+       cd ~/uml/linux
+
+
+
+       host%
+       bzcat uml-patch-2.4.0-prerelease.bz2 | patch -p1
+
+
+
+
+
+
+  5. Run your favorite config; `make xconfig ARCH=um' is the most
+     convenient.  `make config ARCH=um' and 'make menuconfig ARCH=um'
+     will work as well.  The defaults will give you a useful kernel.  If
+     you want to change something, go ahead, it probably won't hurt
+     anything.
+
+
+     Note:  If the host is configured with a 2G/2G address space split
+     rather than the usual 3G/1G split, then the packaged UML binaries
+     will not run.  They will immediately segfault.  See ``UML on 2G/2G
+     hosts''  for the scoop on running UML on your system.
+
+
+
+  6. Finish with `make linux ARCH=um': the result is a file called
+     `linux' in the top directory of your source tree.
+
+  Make sure that you don't build this kernel in /usr/src/linux.  On some
+  distributions, /usr/include/asm is a link into this pool.  The user-
+  mode build changes the other end of that link, and things that include
+  <asm/anything.h> stop compiling.
+
+  The sources are also available from cvs at the project's cvs page,
+  which has directions on getting the sources. You can also browse the
+  CVS pool from there.
+
+  If you get the CVS sources, you will have to check them out into an
+  empty directory. You will then have to copy each file into the
+  corresponding directory in the appropriate kernel pool.
+
+  If you don't have the latest kernel pool, you can get the
+  corresponding user-mode sources with
+
+
+       host% cvs co -r v_2_3_x linux
+
+
+
+
+  where 'x' is the version in your pool. Note that you will not get the
+  bug fixes and enhancements that have gone into subsequent releases.
+
+
+  22..22..  CCoommppiilliinngg aanndd iinnssttaalllliinngg kkeerrnneell mmoodduulleess
+
+  UML modules are built in the same way as the native kernel (with the
+  exception of the 'ARCH=um' that you always need for UML):
+
+
+       host% make modules ARCH=um
+
+
+
+
+  Any modules that you want to load into this kernel need to be built in
+  the user-mode pool.  Modules from the native kernel won't work.
+
+  You can install them by using ftp or something to copy them into the
+  virtual machine and dropping them into /lib/modules/`uname -r`.
+
+  You can also get the kernel build process to install them as follows:
+
+  1. with the kernel not booted, mount the root filesystem in the top
+     level of the kernel pool:
+
+
+       host% mount root_fs mnt -o loop
+
+
+
+
+
+
+  2. run
+
+
+       host%
+       make modules_install INSTALL_MOD_PATH=`pwd`/mnt ARCH=um
+
+
+
+
+
+
+  3. unmount the filesystem
+
+
+       host% umount mnt
+
+
+
+
+
+
+  4. boot the kernel on it
+
+
+  When the system is booted, you can use insmod as usual to get the
+  modules into the kernel.  A number of things have been loaded into UML
+  as modules, especially filesystems and network protocols and filters,
+  so most symbols which need to be exported probably already are.
+  However, if you do find symbols that need exporting, let  us
+  <http://user-mode-linux.sourceforge.net/>  know, and
+  they'll be "taken care of".
+
+
+
+  22..33..  CCoommppiilliinngg aanndd iinnssttaalllliinngg uummll__uuttiilliittiieess
+
+  Many features of the UML kernel require a user-space helper program,
+  so a uml_utilities package is distributed separately from the kernel
+  patch which provides these helpers. Included within this is:
+
+  +o  port-helper - Used by consoles which connect to xterms or ports
+
+  +o  tunctl - Configuration tool to create and delete tap devices
+
+  +o  uml_net - Setuid binary for automatic tap device configuration
+
+  +o  uml_switch - User-space virtual switch required for daemon
+     transport
+
+     The uml_utilities tree is compiled with:
+
+
+       host#
+       make && make install
+
+
+
+
+  Note that UML kernel patches may require a specific version of the
+  uml_utilities distribution. If you don't keep up with the mailing
+  lists, ensure that you have the latest release of uml_utilities if you
+  are experiencing problems with your UML kernel, particularly when
+  dealing with consoles or command-line switches to the helper programs
+
+
+
+
+
+
+
+
+  33..  RRuunnnniinngg UUMMLL aanndd llooggggiinngg iinn
+
+
+
+  33..11..  RRuunnnniinngg UUMMLL
+
+  It runs on 2.2.15 or later, and all 2.4 kernels.
+
+
+  Booting UML is straightforward.  Simply run 'linux': it will try to
+  mount the file `root_fs' in the current directory.  You do not need to
+  run it as root.  If your root filesystem is not named `root_fs', then
+  you need to put a `ubd0=root_fs_whatever' switch on the linux command
+  line.
+
+
+  You will need a filesystem to boot UML from.  There are a number
+  available for download from  here  <http://user-mode-
+  linux.sourceforge.net/> .  There are also  several tools
+  <http://user-mode-linux.sourceforge.net/>  which can be
+  used to generate UML-compatible filesystem images from media.
+  The kernel will boot up and present you with a login prompt.
+
+
+  Note:  If the host is configured with a 2G/2G address space split
+  rather than the usual 3G/1G split, then the packaged UML binaries will
+  not run.  They will immediately segfault.  See ``UML on 2G/2G hosts''
+  for the scoop on running UML on your system.
+
+
+
+  33..22..  LLooggggiinngg iinn
+
+
+
+  The prepackaged filesystems have a root account with password 'root'
+  and a user account with password 'user'.  The login banner will
+  generally tell you how to log in.  So, you log in and you will find
+  yourself inside a little virtual machine. Our filesystems have a
+  variety of commands and utilities installed (and it is fairly easy to
+  add more), so you will have a lot of tools with which to poke around
+  the system.
+
+  There are a couple of other ways to log in:
+
+  +o  On a virtual console
+
+
+
+     Each virtual console that is configured (i.e. the device exists in
+     /dev and /etc/inittab runs a getty on it) will come up in its own
+     xterm.  If you get tired of the xterms, read ``Setting up serial
+     lines and consoles''  to see how to attach the consoles to
+     something else, like host ptys.
+
+
+
+  +o  Over the serial line
+
+
+     In the boot output, find a line that looks like:
+
+
+
+       serial line 0 assigned pty /dev/ptyp1
+
+
+
+
+  Attach your favorite terminal program to the corresponding tty.  I.e.
+  for minicom, the command would be
+
+
+       host% minicom -o -p /dev/ttyp1
+
+
+
+
+
+
+  +o  Over the net
+
+
+     If the network is running, then you can telnet to the virtual
+     machine and log in to it.  See ``Setting up the network''  to learn
+     about setting up a virtual network.
+
+  When you're done using it, run halt, and the kernel will bring itself
+  down and the process will exit.
+
+
+  33..33..  EExxaammpplleess
+
+  Here are some examples of UML in action:
+
+  +o  A login session <http://user-mode-linux.sourceforge.net/login.html>
+
+  +o  A virtual network <http://user-mode-linux.sourceforge.net/net.html>
+
+
+
+
+
+
+
+  44..  UUMMLL oonn 22GG//22GG hhoossttss
+
+
+
+
+  44..11..  IInnttrroodduuccttiioonn
+
+
+  Most Linux machines are configured so that the kernel occupies the
+  upper 1G (0xc0000000 - 0xffffffff) of the 4G address space and
+  processes use the lower 3G (0x00000000 - 0xbfffffff).  However, some
+  machine are configured with a 2G/2G split, with the kernel occupying
+  the upper 2G (0x80000000 - 0xffffffff) and processes using the lower
+  2G (0x00000000 - 0x7fffffff).
+
+
+
+
+  44..22..  TThhee pprroobblleemm
+
+
+  The prebuilt UML binaries on this site will not run on 2G/2G hosts
+  because UML occupies the upper .5G of the 3G process address space
+  (0xa0000000 - 0xbfffffff).  Obviously, on 2G/2G hosts, this is right
+  in the middle of the kernel address space, so UML won't even load - it
+  will immediately segfault.
+
+
+
+
+  44..33..  TThhee ssoolluuttiioonn
+
+
+  The fix for this is to rebuild UML from source after enabling
+  CONFIG_HOST_2G_2G (under 'General Setup').  This will cause UML to
+  load itself in the top .5G of that smaller process address space,
+  where it will run fine.  See ``Compiling the kernel and modules''  if
+  you need help building UML from source.
+
+
+
+
+
+
+
+
+
+
+  55..  SSeettttiinngg uupp sseerriiaall lliinneess aanndd ccoonnssoolleess
+
+
+  It is possible to attach UML serial lines and consoles to many types
+  of host I/O channels by specifying them on the command line.
+
+
+  You can attach them to host ptys, ttys, file descriptors, and ports.
+  This allows you to do things like
+
+  +o  have a UML console appear on an unused host console,
+
+  +o  hook two virtual machines together by having one attach to a pty
+     and having the other attach to the corresponding tty
+
+  +o  make a virtual machine accessible from the net by attaching a
+     console to a port on the host.
+
+
+  The general format of the command line option is device=channel.
+
+
+
+  55..11..  SSppeecciiffyyiinngg tthhee ddeevviiccee
+
+  Devices are specified with "con" or "ssl" (console or serial line,
+  respectively), optionally with a device number if you are talking
+  about a specific device.
+
+
+  Using just "con" or "ssl" describes all of the consoles or serial
+  lines.  If you want to talk about console #3 or serial line #10, they
+  would be "con3" and "ssl10", respectively.
+
+
+  A specific device name will override a less general "con=" or "ssl=".
+  So, for example, you can assign a pty to each of the serial lines
+  except for the first two like this:
+
+
+        ssl=pty ssl0=tty:/dev/tty0 ssl1=tty:/dev/tty1
+
+
+
+
+  The specificity of the device name is all that matters; order on the
+  command line is irrelevant.
+
+
+
+  55..22..  SSppeecciiffyyiinngg tthhee cchhaannnneell
+
+  There are a number of different types of channels to attach a UML
+  device to, each with a different way of specifying exactly what to
+  attach to.
+
+  +o  pseudo-terminals - device=pty pts terminals - device=pts
+
+
+     This will cause UML to allocate a free host pseudo-terminal for the
+     device.  The terminal that it got will be announced in the boot
+     log.  You access it by attaching a terminal program to the
+     corresponding tty:
+
+  +o  screen /dev/pts/n
+
+  +o  screen /dev/ttyxx
+
+  +o  minicom -o -p /dev/ttyxx - minicom seems not able to handle pts
+     devices
+
+  +o  kermit - start it up, 'open' the device, then 'connect'
+
+
+
+
+
+  +o  terminals - device=tty:tty device file
+
+
+     This will make UML attach the device to the specified tty (i.e
+
+
+        con1=tty:/dev/tty3
+
+
+
+
+  will attach UML's console 1 to the host's /dev/tty3).  If the tty that
+  you specify is the slave end of a tty/pty pair, something else must
+  have already opened the corresponding pty in order for this to work.
+
+
+
+
+
+  +o  xterms - device=xterm
+
+
+     UML will run an xterm and the device will be attached to it.
+
+
+
+
+
+  +o  Port - device=port:port number
+
+
+     This will attach the UML devices to the specified host port.
+     Attaching console 1 to the host's port 9000 would be done like
+     this:
+
+
+        con1=port:9000
+
+
+
+
+  Attaching all the serial lines to that port would be done similarly:
+
+
+        ssl=port:9000
+
+
+
+
+  You access these devices by telnetting to that port.  Each active tel-
+  net session gets a different device.  If there are more telnets to a
+  port than UML devices attached to it, then the extra telnet sessions
+  will block until an existing telnet detaches, or until another device
+  becomes active (i.e. by being activated in /etc/inittab).
+
+  This channel has the advantage that you can both attach multiple UML
+  devices to it and know how to access them without reading the UML boot
+  log.  It is also unique in allowing access to a UML from remote
+  machines without requiring that the UML be networked.  This could be
+  useful in allowing public access to UMLs because they would be
+  accessible from the net, but wouldn't need any kind of network
+  filtering or access control because they would have no network access.
+
+
+  If you attach the main console to a portal, then the UML boot will
+  appear to hang.  In reality, it's waiting for a telnet to connect, at
+  which point the boot will proceed.
+
+
+
+
+
+  +o  already-existing file descriptors - device=file descriptor
+
+
+     If you set up a file descriptor on the UML command line, you can
+     attach a UML device to it.  This is most commonly used to put the
+     main console back on stdin and stdout after assigning all the other
+     consoles to something else:
+
+
+        con0=fd:0,fd:1 con=pts
+
+
+
+
+
+
+
+
+  +o  Nothing - device=null
+
+
+     This allows the device to be opened, in contrast to 'none', but
+     reads will block, and writes will succeed and the data will be
+     thrown out.
+
+
+
+
+
+  +o  None - device=none
+
+
+     This causes the device to disappear.
+
+
+
+  You can also specify different input and output channels for a device
+  by putting a comma between them:
+
+
+        ssl3=tty:/dev/tty2,xterm
+
+
+
+
+  will cause serial line 3 to accept input on the host's /dev/tty3 and
+  display output on an xterm.  That's a silly example - the most common
+  use of this syntax is to reattach the main console to stdin and stdout
+  as shown above.
+
+
+  If you decide to move the main console away from stdin/stdout, the
+  initial boot output will appear in the terminal that you're running
+  UML in.  However, once the console driver has been officially
+  initialized, then the boot output will start appearing wherever you
+  specified that console 0 should be.  That device will receive all
+  subsequent output.
+
+
+
+  55..33..  EExxaammpplleess
+
+  There are a number of interesting things you can do with this
+  capability.
+
+
+  First, this is how you get rid of those bleeding console xterms by
+  attaching them to host ptys:
+
+
+        con=pty con0=fd:0,fd:1
+
+
+
+
+  This will make a UML console take over an unused host virtual console,
+  so that when you switch to it, you will see the UML login prompt
+  rather than the host login prompt:
+
+
+        con1=tty:/dev/tty6
+
+
+
+
+  You can attach two virtual machines together with what amounts to a
+  serial line as follows:
+
+  Run one UML with a serial line attached to a pty -
+
+
+        ssl1=pty
+
+
+
+
+  Look at the boot log to see what pty it got (this example will assume
+  that it got /dev/ptyp1).
+
+  Boot the other UML with a serial line attached to the corresponding
+  tty -
+
+
+        ssl1=tty:/dev/ttyp1
+
+
+
+
+  Log in, make sure that it has no getty on that serial line, attach a
+  terminal program like minicom to it, and you should see the login
+  prompt of the other virtual machine.
+
+
+  66..  SSeettttiinngg uupp tthhee nneettwwoorrkk
+
+
+
+  This page describes how to set up the various transports and to
+  provide a UML instance with network access to the host, other machines
+  on the local net, and the rest of the net.
+
+
+  As of 2.4.5, UML networking has been completely redone to make it much
+  easier to set up, fix bugs, and add new features.
+
+
+  There is a new helper, uml_net, which does the host setup that
+  requires root privileges.
+
+
+  There are currently five transport types available for a UML virtual
+  machine to exchange packets with other hosts:
+
+  +o  ethertap
+
+  +o  TUN/TAP
+
+  +o  Multicast
+
+  +o  a switch daemon
+
+  +o  slip
+
+  +o  slirp
+
+  +o  pcap
+
+     The TUN/TAP, ethertap, slip, and slirp transports allow a UML
+     instance to exchange packets with the host.  They may be directed
+     to the host or the host may just act as a router to provide access
+     to other physical or virtual machines.
+
+
+  The pcap transport is a synthetic read-only interface, using the
+  libpcap binary to collect packets from interfaces on the host and
+  filter them.  This is useful for building preconfigured traffic
+  monitors or sniffers.
+
+
+  The daemon and multicast transports provide a completely virtual
+  network to other virtual machines.  This network is completely
+  disconnected from the physical network unless one of the virtual
+  machines on it is acting as a gateway.
+
+
+  With so many host transports, which one should you use?  Here's when
+  you should use each one:
+
+  +o  ethertap - if you want access to the host networking and it is
+     running 2.2
+
+  +o  TUN/TAP - if you want access to the host networking and it is
+     running 2.4.  Also, the TUN/TAP transport is able to use a
+     preconfigured device, allowing it to avoid using the setuid uml_net
+     helper, which is a security advantage.
+
+  +o  Multicast - if you want a purely virtual network and you don't want
+     to set up anything but the UML
+
+  +o  a switch daemon - if you want a purely virtual network and you
+     don't mind running the daemon in order to get somewhat better
+     performance
+
+  +o  slip - there is no particular reason to run the slip backend unless
+     ethertap and TUN/TAP are just not available for some reason
+
+  +o  slirp - if you don't have root access on the host to setup
+     networking, or if you don't want to allocate an IP to your UML
+
+  +o  pcap - not much use for actual network connectivity, but great for
+     monitoring traffic on the host
+
+     Ethertap is available on 2.4 and works fine.  TUN/TAP is preferred
+     to it because it has better performance and ethertap is officially
+     considered obsolete in 2.4.  Also, the root helper only needs to
+     run occasionally for TUN/TAP, rather than handling every packet, as
+     it does with ethertap.  This is a slight security advantage since
+     it provides fewer opportunities for a nasty UML user to somehow
+     exploit the helper's root privileges.
+
+
+  66..11..  GGeenneerraall sseettuupp
+
+  First, you must have the virtual network enabled in your UML.  If are
+  running a prebuilt kernel from this site, everything is already
+  enabled.  If you build the kernel yourself, under the "Network device
+  support" menu, enable "Network device support", and then the three
+  transports.
+
+
+  The next step is to provide a network device to the virtual machine.
+  This is done by describing it on the kernel command line.
+
+  The general format is
+
+
+       eth <n> = <transport> , <transport args>
+
+
+
+
+  For example, a virtual ethernet device may be attached to a host
+  ethertap device as follows:
+
+
+       eth0=ethertap,tap0,fe:fd:0:0:0:1,192.168.0.254
+
+
+
+
+  This sets up eth0 inside the virtual machine to attach itself to the
+  host /dev/tap0, assigns it an ethernet address, and assigns the host
+  tap0 interface an IP address.
+
+
+
+  Note that the IP address you assign to the host end of the tap device
+  must be different than the IP you assign to the eth device inside UML.
+  If you are short on IPs and don't want to consume two per UML, then
+  you can reuse the host's eth IP address for the host ends of the tap
+  devices.  Internally, the UMLs must still get unique IPs for their eth
+  devices.  You can also give the UMLs non-routable IPs (192.168.x.x or
+  10.x.x.x) and have the host masquerade them.  This will let outgoing
+  connections work, but incoming connections won't without more work,
+  such as port forwarding from the host.
+  Also note that when you configure the host side of an interface, it is
+  only acting as a gateway.  It will respond to pings sent to it
+  locally, but is not useful to do that since it's a host interface.
+  You are not talking to the UML when you ping that interface and get a
+  response.
+
+
+  You can also add devices to a UML and remove them at runtime.  See the
+  ``The Management Console''  page for details.
+
+
+  The sections below describe this in more detail.
+
+
+  Once you've decided how you're going to set up the devices, you boot
+  UML, log in, configure the UML side of the devices, and set up routes
+  to the outside world.  At that point, you will be able to talk to any
+  other machines, physical or virtual, on the net.
+
+
+  If ifconfig inside UML fails and the network refuses to come up, run
+  tell you what went wrong.
+
+
+
+  66..22..  UUsseerrssppaaccee ddaaeemmoonnss
+
+  You will likely need the setuid helper, or the switch daemon, or both.
+  They are both installed with the RPM and deb, so if you've installed
+  either, you can skip the rest of this section.
+
+
+  If not, then you need to check them out of CVS, build them, and
+  install them.  The helper is uml_net, in CVS /tools/uml_net, and the
+  daemon is uml_switch, in CVS /tools/uml_router.  They are both built
+  with a plain 'make'.  Both need to be installed in a directory that's
+  in your path - /usr/bin is recommend.  On top of that, uml_net needs
+  to be setuid root.
+
+
+
+  66..33..  SSppeecciiffyyiinngg eetthheerrnneett aaddddrreesssseess
+
+  Below, you will see that the TUN/TAP, ethertap, and daemon interfaces
+  allow you to specify hardware addresses for the virtual ethernet
+  devices.  This is generally not necessary.  If you don't have a
+  specific reason to do it, you probably shouldn't.  If one is not
+  specified on the command line, the driver will assign one based on the
+  device IP address.  It will provide the address fe:fd:nn:nn:nn:nn
+  where nn.nn.nn.nn is the device IP address.  This is nearly always
+  sufficient to guarantee a unique hardware address for the device.  A
+  couple of exceptions are:
+
+  +o  Another set of virtual ethernet devices are on the same network and
+     they are assigned hardware addresses using a different scheme which
+     may conflict with the UML IP address-based scheme
+
+  +o  You aren't going to use the device for IP networking, so you don't
+     assign the device an IP address
+
+     If you let the driver provide the hardware address, you should make
+     sure that the device IP address is known before the interface is
+     brought up.  So, inside UML, this will guarantee that:
+
+
+
+  UML#
+  ifconfig eth0 192.168.0.250 up
+
+
+
+
+  If you decide to assign the hardware address yourself, make sure that
+  the first byte of the address is even.  Addresses with an odd first
+  byte are broadcast addresses, which you don't want assigned to a
+  device.
+
+
+
+  66..44..  UUMMLL iinntteerrffaaccee sseettuupp
+
+  Once the network devices have been described on the command line, you
+  should boot UML and log in.
+
+
+  The first thing to do is bring the interface up:
+
+
+       UML# ifconfig ethn ip-address up
+
+
+
+
+  You should be able to ping the host at this point.
+
+
+  To reach the rest of the world, you should set a default route to the
+  host:
+
+
+       UML# route add default gw host ip
+
+
+
+
+  Again, with host ip of 192.168.0.4:
+
+
+       UML# route add default gw 192.168.0.4
+
+
+
+
+  This page used to recommend setting a network route to your local net.
+  This is wrong, because it will cause UML to try to figure out hardware
+  addresses of the local machines by arping on the interface to the
+  host.  Since that interface is basically a single strand of ethernet
+  with two nodes on it (UML and the host) and arp requests don't cross
+  networks, they will fail to elicit any responses.  So, what you want
+  is for UML to just blindly throw all packets at the host and let it
+  figure out what to do with them, which is what leaving out the network
+  route and adding the default route does.
+
+
+  Note: If you can't communicate with other hosts on your physical
+  ethernet, it's probably because of a network route that's
+  automatically set up.  If you run 'route -n' and see a route that
+  looks like this:
+
+
+
+
+  Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
+  192.168.0.0     0.0.0.0         255.255.255.0   U     0      0      0   eth0
+
+
+
+
+  with a mask that's not 255.255.255.255, then replace it with a route
+  to your host:
+
+
+       UML#
+       route del -net 192.168.0.0 dev eth0 netmask 255.255.255.0
+
+
+
+
+
+
+       UML#
+       route add -host 192.168.0.4 dev eth0
+
+
+
+
+  This, plus the default route to the host, will allow UML to exchange
+  packets with any machine on your ethernet.
+
+
+
+  66..55..  MMuullttiiccaasstt
+
+  The simplest way to set up a virtual network between multiple UMLs is
+  to use the mcast transport.  This was written by Harald Welte and is
+  present in UML version 2.4.5-5um and later.  Your system must have
+  multicast enabled in the kernel and there must be a multicast-capable
+  network device on the host.  Normally, this is eth0, but if there is
+  no ethernet card on the host, then you will likely get strange error
+  messages when you bring the device up inside UML.
+
+
+  To use it, run two UMLs with
+
+
+        eth0=mcast
+
+
+
+
+  on their command lines.  Log in, configure the ethernet device in each
+  machine with different IP addresses:
+
+
+       UML1# ifconfig eth0 192.168.0.254
+
+
+
+
+
+
+       UML2# ifconfig eth0 192.168.0.253
+
+
+
+
+  and they should be able to talk to each other.
+
+  The full set of command line options for this transport are
+
+
+
+       ethn=mcast,ethernet address,multicast
+       address,multicast port,ttl
+
+
+
+
+  Harald's original README is here <http://user-mode-linux.source-
+  forge.net/>  and explains these in detail, as well as
+  some other issues.
+
+
+
+  66..66..  TTUUNN//TTAAPP wwiitthh tthhee uummll__nneett hheellppeerr
+
+  TUN/TAP is the preferred mechanism on 2.4 to exchange packets with the
+  host.  The TUN/TAP backend has been in UML since 2.4.9-3um.
+
+
+  The easiest way to get up and running is to let the setuid uml_net
+  helper do the host setup for you.  This involves insmod-ing the tun.o
+  module if necessary, configuring the device, and setting up IP
+  forwarding, routing, and proxy arp.  If you are new to UML networking,
+  do this first.  If you're concerned about the security implications of
+  the setuid helper, use it to get up and running, then read the next
+  section to see how to have UML use a preconfigured tap device, which
+  avoids the use of uml_net.
+
+
+  If you specify an IP address for the host side of the device, the
+  uml_net helper will do all necessary setup on the host - the only
+  requirement is that TUN/TAP be available, either built in to the host
+  kernel or as the tun.o module.
+
+  The format of the command line switch to attach a device to a TUN/TAP
+  device is
+
+
+       eth <n> =tuntap,,, <IP address>
+
+
+
+
+  For example, this argument will attach the UML's eth0 to the next
+  available tap device and assign an ethernet address to it based on its
+  IP address
+
+
+       eth0=tuntap,,,192.168.0.254
+
+
+
+
+
+
+  Note that the IP address that must be used for the eth device inside
+  UML is fixed by the routing and proxy arp that is set up on the
+  TUN/TAP device on the host.  You can use a different one, but it won't
+  work because reply packets won't reach the UML.  This is a feature.
+  It prevents a nasty UML user from doing things like setting the UML IP
+  to the same as the network's nameserver or mail server.
+
+
+  There are a couple potential problems with running the TUN/TAP
+  transport on a 2.4 host kernel
+
+  +o  TUN/TAP seems not to work on 2.4.3 and earlier.  Upgrade the host
+     kernel or use the ethertap transport.
+
+  +o  With an upgraded kernel, TUN/TAP may fail with
+
+
+       File descriptor in bad state
+
+
+
+
+  This is due to a header mismatch between the upgraded kernel and the
+  kernel that was originally installed on the machine.  The fix is to
+  make sure that /usr/src/linux points to the headers for the running
+  kernel.
+
+  These were pointed out by Tim Robinson <timro at trkr dot net> in
+  <http://www.geocrawler.com/> name="this uml-
+  user post"> .
+
+
+
+  66..77..  TTUUNN//TTAAPP wwiitthh aa pprreeccoonnffiigguurreedd ttaapp ddeevviiccee
+
+  If you prefer not to have UML use uml_net (which is somewhat
+  insecure), with UML 2.4.17-11, you can set up a TUN/TAP device
+  beforehand.  The setup needs to be done as root, but once that's done,
+  there is no need for root assistance.  Setting up the device is done
+  as follows:
+
+  +o  Create the device with tunctl (available from the UML utilities
+     tarball)
+
+
+
+
+       host#  tunctl -u uid
+
+
+
+
+  where uid is the user id or username that UML will be run as.  This
+  will tell you what device was created.
+
+  +o  Configure the device IP (change IP addresses and device name to
+     suit)
+
+
+
+
+       host#  ifconfig tap0 192.168.0.254 up
+
+
+
+
+
+  +o  Set up routing and arping if desired - this is my recipe, there are
+     other ways of doing the same thing
+
+
+       host#
+       bash -c 'echo 1 > /proc/sys/net/ipv4/ip_forward'
+
+       host#
+       route add -host 192.168.0.253 dev tap0
+
+
+
+
+
+
+       host#
+       bash -c 'echo 1 > /proc/sys/net/ipv4/conf/tap0/proxy_arp'
+
+
+
+
+
+
+       host#
+       arp -Ds 192.168.0.253 eth0 pub
+
+
+
+
+  Note that this must be done every time the host boots - this configu-
+  ration is not stored across host reboots.  So, it's probably a good
+  idea to stick it in an rc file.  An even better idea would be a little
+  utility which reads the information from a config file and sets up
+  devices at boot time.
+
+  +o  Rather than using up two IPs and ARPing for one of them, you can
+     also provide direct access to your LAN by the UML by using a
+     bridge.
+
+
+       host#
+       brctl addbr br0
+
+
+
+
+
+
+       host#
+       ifconfig eth0 0.0.0.0 promisc up
+
+
+
+
+
+
+       host#
+       ifconfig tap0 0.0.0.0 promisc up
+
+
+
+
+
+
+       host#
+       ifconfig br0 192.168.0.1 netmask 255.255.255.0 up
+
+
+
+
+
+
+
+  host#
+  brctl stp br0 off
+
+
+
+
+
+
+       host#
+       brctl setfd br0 1
+
+
+
+
+
+
+       host#
+       brctl sethello br0 1
+
+
+
+
+
+
+       host#
+       brctl addif br0 eth0
+
+
+
+
+
+
+       host#
+       brctl addif br0 tap0
+
+
+
+
+  Note that 'br0' should be setup using ifconfig with the existing IP
+  address of eth0, as eth0 no longer has its own IP.
+
+  +o
+
+
+     Also, the /dev/net/tun device must be writable by the user running
+     UML in order for the UML to use the device that's been configured
+     for it.  The simplest thing to do is
+
+
+       host#  chmod 666 /dev/net/tun
+
+
+
+
+  Making it world-writable looks bad, but it seems not to be
+  exploitable as a security hole.  However, it does allow anyone to cre-
+  ate useless tap devices (useless because they can't configure them),
+  which is a DOS attack.  A somewhat more secure alternative would to be
+  to create a group containing all the users who have preconfigured tap
+  devices and chgrp /dev/net/tun to that group with mode 664 or 660.
+
+
+  +o  Once the device is set up, run UML with 'eth0=tuntap,device name'
+     (i.e. 'eth0=tuntap,tap0') on the command line (or do it with the
+     mconsole config command).
+
+  +o  Bring the eth device up in UML and you're in business.
+
+     If you don't want that tap device any more, you can make it non-
+     persistent with
+
+
+       host#  tunctl -d tap device
+
+
+
+
+  Finally, tunctl has a -b (for brief mode) switch which causes it to
+  output only the name of the tap device it created.  This makes it
+  suitable for capture by a script:
+
+
+       host#  TAP=`tunctl -u 1000 -b`
+
+
+
+
+
+
+  66..88..  EEtthheerrttaapp
+
+  Ethertap is the general mechanism on 2.2 for userspace processes to
+  exchange packets with the kernel.
+
+
+
+  To use this transport, you need to describe the virtual network device
+  on the UML command line.  The general format for this is
+
+
+       eth <n> =ethertap, <device> , <ethernet address> , <tap IP address>
+
+
+
+
+  So, the previous example
+
+
+       eth0=ethertap,tap0,fe:fd:0:0:0:1,192.168.0.254
+
+
+
+
+  attaches the UML eth0 device to the host /dev/tap0, assigns it the
+  ethernet address fe:fd:0:0:0:1, and assigns the IP address
+  192.168.0.254 to the tap device.
+
+
+
+  The tap device is mandatory, but the others are optional.  If the
+  ethernet address is omitted, one will be assigned to it.
+
+
+  The presence of the tap IP address will cause the helper to run and do
+  whatever host setup is needed to allow the virtual machine to
+  communicate with the outside world.  If you're not sure you know what
+  you're doing, this is the way to go.
+
+
+  If it is absent, then you must configure the tap device and whatever
+  arping and routing you will need on the host.  However, even in this
+  case, the uml_net helper still needs to be in your path and it must be
+  setuid root if you're not running UML as root.  This is because the
+  tap device doesn't support SIGIO, which UML needs in order to use
+  something as a source of input.  So, the helper is used as a
+  convenient asynchronous IO thread.
+
+  If you're using the uml_net helper, you can ignore the following host
+  setup - uml_net will do it for you.  You just need to make sure you
+  have ethertap available, either built in to the host kernel or
+  available as a module.
+
+
+  If you want to set things up yourself, you need to make sure that the
+  appropriate /dev entry exists.  If it doesn't, become root and create
+  it as follows:
+
+
+       mknod /dev/tap <minor>  c 36  <minor>  + 16
+
+
+
+
+  For example, this is how to create /dev/tap0:
+
+
+       mknod /dev/tap0 c 36 0 + 16
+
+
+
+
+  You also need to make sure that the host kernel has ethertap support.
+  If ethertap is enabled as a module, you apparently need to insmod
+  ethertap once for each ethertap device you want to enable.  So,
+
+
+       host#
+       insmod ethertap
+
+
+
+
+  will give you the tap0 interface.  To get the tap1 interface, you need
+  to run
+
+
+       host#
+       insmod ethertap unit=1 -o ethertap1
+
+
+
+
+
+
+
+  66..99..  TThhee sswwiittcchh ddaaeemmoonn
+
+  NNoottee: This is the daemon formerly known as uml_router, but which was
+  renamed so the network weenies of the world would stop growling at me.
+
+
+  The switch daemon, uml_switch, provides a mechanism for creating a
+  totally virtual network.  By default, it provides no connection to the
+  host network (but see -tap, below).
+
+
+  The first thing you need to do is run the daemon.  Running it with no
+  arguments will make it listen on a default pair of unix domain
+  sockets.
+
+
+  If you want it to listen on a different pair of sockets, use
+
+
+        -unix control socket data socket
+
+
+
+
+
+  If you want it to act as a hub rather than a switch, use
+
+
+        -hub
+
+
+
+
+
+  If you want the switch to be connected to host networking (allowing
+  the umls to get access to the outside world through the host), use
+
+
+        -tap tap0
+
+
+
+
+
+  Note that the tap device must be preconfigured (see "TUN/TAP with a
+  preconfigured tap device", above).  If you're using a different tap
+  device than tap0, specify that instead of tap0.
+
+
+  uml_switch can be backgrounded as follows
+
+
+       host%
+       uml_switch [ options ] < /dev/null > /dev/null
+
+
+
+
+  The reason it doesn't background by default is that it listens to
+  stdin for EOF.  When it sees that, it exits.
+
+
+  The general format of the kernel command line switch is
+
+
+
+       ethn=daemon,ethernet address,socket
+       type,control socket,data socket
+
+
+
+
+  You can leave off everything except the 'daemon'.  You only need to
+  specify the ethernet address if the one that will be assigned to it
+  isn't acceptable for some reason.  The rest of the arguments describe
+  how to communicate with the daemon.  You should only specify them if
+  you told the daemon to use different sockets than the default.  So, if
+  you ran the daemon with no arguments, running the UML on the same
+  machine with
+       eth0=daemon
+
+
+
+
+  will cause the eth0 driver to attach itself to the daemon correctly.
+
+
+
+  66..1100..  SSlliipp
+
+  Slip is another, less general, mechanism for a process to communicate
+  with the host networking.  In contrast to the ethertap interface,
+  which exchanges ethernet frames with the host and can be used to
+  transport any higher-level protocol, it can only be used to transport
+  IP.
+
+
+  The general format of the command line switch is
+
+
+
+       ethn=slip,slip IP
+
+
+
+
+  The slip IP argument is the IP address that will be assigned to the
+  host end of the slip device.  If it is specified, the helper will run
+  and will set up the host so that the virtual machine can reach it and
+  the rest of the network.
+
+
+  There are some oddities with this interface that you should be aware
+  of.  You should only specify one slip device on a given virtual
+  machine, and its name inside UML will be 'umn', not 'eth0' or whatever
+  you specified on the command line.  These problems will be fixed at
+  some point.
+
+
+
+  66..1111..  SSlliirrpp
+
+  slirp uses an external program, usually /usr/bin/slirp, to provide IP
+  only networking connectivity through the host. This is similar to IP
+  masquerading with a firewall, although the translation is performed in
+  user-space, rather than by the kernel.  As slirp does not set up any
+  interfaces on the host, or changes routing, slirp does not require
+  root access or setuid binaries on the host.
+
+
+  The general format of the command line switch for slirp is:
+
+
+
+       ethn=slirp,ethernet address,slirp path
+
+
+
+
+  The ethernet address is optional, as UML will set up the interface
+  with an ethernet address based upon the initial IP address of the
+  interface.  The slirp path is generally /usr/bin/slirp, although it
+  will depend on distribution.
+
+
+  The slirp program can have a number of options passed to the command
+  line and we can't add them to the UML command line, as they will be
+  parsed incorrectly.  Instead, a wrapper shell script can be written or
+  the options inserted into the  /.slirprc file.  More information on
+  all of the slirp options can be found in its man pages.
+
+
+  The eth0 interface on UML should be set up with the IP 10.2.0.15,
+  although you can use anything as long as it is not used by a network
+  you will be connecting to. The default route on UML should be set to
+  use
+
+
+       UML#
+       route add default dev eth0
+
+
+
+
+  slirp provides a number of useful IP addresses which can be used by
+  UML, such as 10.0.2.3 which is an alias for the DNS server specified
+  in /etc/resolv.conf on the host or the IP given in the 'dns' option
+  for slirp.
+
+
+  Even with a baudrate setting higher than 115200, the slirp connection
+  is limited to 115200. If you need it to go faster, the slirp binary
+  needs to be compiled with FULL_BOLT defined in config.h.
+
+
+
+  66..1122..  ppccaapp
+
+  The pcap transport is attached to a UML ethernet device on the command
+  line or with uml_mconsole with the following syntax:
+
+
+
+       ethn=pcap,host interface,filter
+       expression,option1,option2
+
+
+
+
+  The expression and options are optional.
+
+
+  The interface is whatever network device on the host you want to
+  sniff.  The expression is a pcap filter expression, which is also what
+  tcpdump uses, so if you know how to specify tcpdump filters, you will
+  use the same expressions here.  The options are up to two of
+  'promisc', control whether pcap puts the host interface into
+  promiscuous mode. 'optimize' and 'nooptimize' control whether the pcap
+  expression optimizer is used.
+
+
+  Example:
+
+
+
+       eth0=pcap,eth0,tcp
+
+       eth1=pcap,eth0,!tcp
+
+
+
+  will cause the UML eth0 to emit all tcp packets on the host eth0 and
+  the UML eth1 to emit all non-tcp packets on the host eth0.
+
+
+
+  66..1133..  SSeettttiinngg uupp tthhee hhoosstt yyoouurrsseellff
+
+  If you don't specify an address for the host side of the ethertap or
+  slip device, UML won't do any setup on the host.  So this is what is
+  needed to get things working (the examples use a host-side IP of
+  192.168.0.251 and a UML-side IP of 192.168.0.250 - adjust to suit your
+  own network):
+
+  +o  The device needs to be configured with its IP address.  Tap devices
+     are also configured with an mtu of 1484.  Slip devices are
+     configured with a point-to-point address pointing at the UML ip
+     address.
+
+
+       host#  ifconfig tap0 arp mtu 1484 192.168.0.251 up
+
+
+
+
+
+
+       host#
+       ifconfig sl0 192.168.0.251 pointopoint 192.168.0.250 up
+
+
+
+
+
+  +o  If a tap device is being set up, a route is set to the UML IP.
+
+
+       UML# route add -host 192.168.0.250 gw 192.168.0.251
+
+
+
+
+
+  +o  To allow other hosts on your network to see the virtual machine,
+     proxy arp is set up for it.
+
+
+       host#  arp -Ds 192.168.0.250 eth0 pub
+
+
+
+
+
+  +o  Finally, the host is set up to route packets.
+
+
+       host#  echo 1 > /proc/sys/net/ipv4/ip_forward
+
+
+
+
+
+
+
+
+
+
+  77..  SShhaarriinngg FFiilleessyysstteemmss bbeettwweeeenn VViirrttuuaall MMaacchhiinneess
+
+
+
+
+  77..11..  AA wwaarrnniinngg
+
+  Don't attempt to share filesystems simply by booting two UMLs from the
+  same file.  That's the same thing as booting two physical machines
+  from a shared disk.  It will result in filesystem corruption.
+
+
+
+  77..22..  UUssiinngg llaayyeerreedd bblloocckk ddeevviicceess
+
+  The way to share a filesystem between two virtual machines is to use
+  the copy-on-write (COW) layering capability of the ubd block driver.
+  As of 2.4.6-2um, the driver supports layering a read-write private
+  device over a read-only shared device.  A machine's writes are stored
+  in the private device, while reads come from either device - the
+  private one if the requested block is valid in it, the shared one if
+  not.  Using this scheme, the majority of data which is unchanged is
+  shared between an arbitrary number of virtual machines, each of which
+  has a much smaller file containing the changes that it has made.  With
+  a large number of UMLs booting from a large root filesystem, this
+  leads to a huge disk space saving.  It will also help performance,
+  since the host will be able to cache the shared data using a much
+  smaller amount of memory, so UML disk requests will be served from the
+  host's memory rather than its disks.
+
+
+
+
+  To add a copy-on-write layer to an existing block device file, simply
+  add the name of the COW file to the appropriate ubd switch:
+
+
+        ubd0=root_fs_cow,root_fs_debian_22
+
+
+
+
+  where 'root_fs_cow' is the private COW file and 'root_fs_debian_22' is
+  the existing shared filesystem.  The COW file need not exist.  If it
+  doesn't, the driver will create and initialize it.  Once the COW file
+  has been initialized, it can be used on its own on the command line:
+
+
+        ubd0=root_fs_cow
+
+
+
+
+  The name of the backing file is stored in the COW file header, so it
+  would be redundant to continue specifying it on the command line.
+
+
+
+  77..33..  NNoottee!!
+
+  When checking the size of the COW file in order to see the gobs of
+  space that you're saving, make sure you use 'ls -ls' to see the actual
+  disk consumption rather than the length of the file.  The COW file is
+  sparse, so the length will be very different from the disk usage.
+  Here is a 'ls -l' of a COW file and backing file from one boot and
+  shutdown:
+       host% ls -l cow.debian debian2.2
+       -rw-r--r--    1 jdike    jdike    492504064 Aug  6 21:16 cow.debian
+       -rwxrw-rw-    1 jdike    jdike    537919488 Aug  6 20:42 debian2.2
+
+
+
+
+  Doesn't look like much saved space, does it?  Well, here's 'ls -ls':
+
+
+       host% ls -ls cow.debian debian2.2
+          880 -rw-r--r--    1 jdike    jdike    492504064 Aug  6 21:16 cow.debian
+       525832 -rwxrw-rw-    1 jdike    jdike    537919488 Aug  6 20:42 debian2.2
+
+
+
+
+  Now, you can see that the COW file has less than a meg of disk, rather
+  than 492 meg.
+
+
+
+  77..44..  AAnnootthheerr wwaarrnniinngg
+
+  Once a filesystem is being used as a readonly backing file for a COW
+  file, do not boot directly from it or modify it in any way.  Doing so
+  will invalidate any COW files that are using it.  The mtime and size
+  of the backing file are stored in the COW file header at its creation,
+  and they must continue to match.  If they don't, the driver will
+  refuse to use the COW file.
+
+
+
+
+  If you attempt to evade this restriction by changing either the
+  backing file or the COW header by hand, you will get a corrupted
+  filesystem.
+
+
+
+
+  Among other things, this means that upgrading the distribution in a
+  backing file and expecting that all of the COW files using it will see
+  the upgrade will not work.
+
+
+
+
+  77..55..  uummll__mmoooo :: MMeerrggiinngg aa CCOOWW ffiillee wwiitthh iittss bbaacckkiinngg ffiillee
+
+  Depending on how you use UML and COW devices, it may be advisable to
+  merge the changes in the COW file into the backing file every once in
+  a while.
+
+
+
+
+  The utility that does this is uml_moo.  Its usage is
+
+
+       host% uml_moo COW file new backing file
+
+
+
+
+  There's no need to specify the backing file since that information is
+  already in the COW file header.  If you're paranoid, boot the new
+  merged file, and if you're happy with it, move it over the old backing
+  file.
+
+
+
+
+  uml_moo creates a new backing file by default as a safety measure.  It
+  also has a destructive merge option which will merge the COW file
+  directly into its current backing file.  This is really only usable
+  when the backing file only has one COW file associated with it.  If
+  there are multiple COWs associated with a backing file, a -d merge of
+  one of them will invalidate all of the others.  However, it is
+  convenient if you're short of disk space, and it should also be
+  noticeably faster than a non-destructive merge.
+
+
+
+
+  uml_moo is installed with the UML deb and RPM.  If you didn't install
+  UML from one of those packages, you can also get it from the UML
+  utilities <http://user-mode-linux.sourceforge.net/
+  utilities>  tar file in tools/moo.
+
+
+
+
+
+
+
+
+  88..  CCrreeaattiinngg ffiilleessyysstteemmss
+
+
+  You may want to create and mount new UML filesystems, either because
+  your root filesystem isn't large enough or because you want to use a
+  filesystem other than ext2.
+
+
+  This was written on the occasion of reiserfs being included in the
+  2.4.1 kernel pool, and therefore the 2.4.1 UML, so the examples will
+  talk about reiserfs.  This information is generic, and the examples
+  should be easy to translate to the filesystem of your choice.
+
+
+  88..11..  CCrreeaattee tthhee ffiilleessyysstteemm ffiillee
+
+  dd is your friend.  All you need to do is tell dd to create an empty
+  file of the appropriate size.  I usually make it sparse to save time
+  and to avoid allocating disk space until it's actually used.  For
+  example, the following command will create a sparse 100 meg file full
+  of zeroes.
+
+
+       host%
+       dd if=/dev/zero of=new_filesystem seek=100 count=1 bs=1M
+
+
+
+
+
+
+  88..22..  AAssssiiggnn tthhee ffiillee ttoo aa UUMMLL ddeevviiccee
+
+  Add an argument like the following to the UML command line:
+
+  ubd4=new_filesystem
+
+
+
+
+  making sure that you use an unassigned ubd device number.
+
+
+
+  88..33..  CCrreeaattiinngg aanndd mmoouunnttiinngg tthhee ffiilleessyysstteemm
+
+  Make sure that the filesystem is available, either by being built into
+  the kernel, or available as a module, then boot up UML and log in.  If
+  the root filesystem doesn't have the filesystem utilities (mkfs, fsck,
+  etc), then get them into UML by way of the net or hostfs.
+
+
+  Make the new filesystem on the device assigned to the new file:
+
+
+       host#  mkreiserfs /dev/ubd/4
+
+
+       <----------- MKREISERFSv2 ----------->
+
+       ReiserFS version 3.6.25
+       Block size 4096 bytes
+       Block count 25856
+       Used blocks 8212
+               Journal - 8192 blocks (18-8209), journal header is in block 8210
+               Bitmaps: 17
+               Root block 8211
+       Hash function "r5"
+       ATTENTION: ALL DATA WILL BE LOST ON '/dev/ubd/4'! (y/n)y
+       journal size 8192 (from 18)
+       Initializing journal - 0%....20%....40%....60%....80%....100%
+       Syncing..done.
+
+
+
+
+  Now, mount it:
+
+
+       UML#
+       mount /dev/ubd/4 /mnt
+
+
+
+
+  and you're in business.
+
+
+
+
+
+
+
+
+
+  99..  HHoosstt ffiillee aacccceessss
+
+
+  If you want to access files on the host machine from inside UML, you
+  can treat it as a separate machine and either nfs mount directories
+  from the host or copy files into the virtual machine with scp or rcp.
+  However, since UML is running on the host, it can access those
+  files just like any other process and make them available inside the
+  virtual machine without needing to use the network.
+
+
+  This is now possible with the hostfs virtual filesystem.  With it, you
+  can mount a host directory into the UML filesystem and access the
+  files contained in it just as you would on the host.
+
+
+  99..11..  UUssiinngg hhoossttffss
+
+  To begin with, make sure that hostfs is available inside the virtual
+  machine with
+
+
+       UML# cat /proc/filesystems
+
+
+
+  .  hostfs should be listed.  If it's not, either rebuild the kernel
+  with hostfs configured into it or make sure that hostfs is built as a
+  module and available inside the virtual machine, and insmod it.
+
+
+  Now all you need to do is run mount:
+
+
+       UML# mount none /mnt/host -t hostfs
+
+
+
+
+  will mount the host's / on the virtual machine's /mnt/host.
+
+
+  If you don't want to mount the host root directory, then you can
+  specify a subdirectory to mount with the -o switch to mount:
+
+
+       UML# mount none /mnt/home -t hostfs -o /home
+
+
+
+
+  will mount the hosts's /home on the virtual machine's /mnt/home.
+
+
+
+  99..22..  hhoossttffss aass tthhee rroooott ffiilleessyysstteemm
+
+  It's possible to boot from a directory hierarchy on the host using
+  hostfs rather than using the standard filesystem in a file.
+
+  To start, you need that hierarchy.  The easiest way is to loop mount
+  an existing root_fs file:
+
+
+       host#  mount root_fs uml_root_dir -o loop
+
+
+
+
+  You need to change the filesystem type of / in etc/fstab to be
+  'hostfs', so that line looks like this:
+
+  /dev/ubd/0       /        hostfs      defaults          1   1
+
+
+
+
+  Then you need to chown to yourself all the files in that directory
+  that are owned by root.  This worked for me:
+
+
+       host#  find . -uid 0 -exec chown jdike {} \;
+
+
+
+
+  Next, make sure that your UML kernel has hostfs compiled in, not as a
+  module.  Then run UML with the boot device pointing at that directory:
+
+
+        ubd0=/path/to/uml/root/directory
+
+
+
+
+  UML should then boot as it does normally.
+
+
+  99..33..  BBuuiillddiinngg hhoossttffss
+
+  If you need to build hostfs because it's not in your kernel, you have
+  two choices:
+
+
+
+  +o  Compiling hostfs into the kernel:
+
+
+     Reconfigure the kernel and set the 'Host filesystem' option under
+
+
+  +o  Compiling hostfs as a module:
+
+
+     Reconfigure the kernel and set the 'Host filesystem' option under
+     be in arch/um/fs/hostfs/hostfs.o.  Install that in
+     /lib/modules/`uname -r`/fs in the virtual machine, boot it up, and
+
+
+       UML# insmod hostfs
+
+
+
+
+
+
+
+
+
+
+
+
+  1100..  TThhee MMaannaaggeemmeenntt CCoonnssoollee
+
+
+
+  The UML management console is a low-level interface to the kernel,
+  somewhat like the i386 SysRq interface.  Since there is a full-blown
+  operating system under UML, there is much greater flexibility possible
+  than with the SysRq mechanism.
+
+
+  There are a number of things you can do with the mconsole interface:
+
+  +o  get the kernel version
+
+  +o  add and remove devices
+
+  +o  halt or reboot the machine
+
+  +o  Send SysRq commands
+
+  +o  Pause and resume the UML
+
+
+  You need the mconsole client (uml_mconsole) which is present in CVS
+  (/tools/mconsole) in 2.4.5-9um and later, and will be in the RPM in
+  2.4.6.
+
+
+  You also need CONFIG_MCONSOLE (under 'General Setup') enabled in UML.
+  When you boot UML, you'll see a line like:
+
+
+       mconsole initialized on /home/jdike/.uml/umlNJ32yL/mconsole
+
+
+
+
+  If you specify a unique machine id one the UML command line, i.e.
+
+
+        umid=debian
+
+
+
+
+  you'll see this
+
+
+       mconsole initialized on /home/jdike/.uml/debian/mconsole
+
+
+
+
+  That file is the socket that uml_mconsole will use to communicate with
+  UML.  Run it with either the umid or the full path as its argument:
+
+
+       host% uml_mconsole debian
+
+
+
+
+  or
+
+
+       host% uml_mconsole /home/jdike/.uml/debian/mconsole
+
+
+
+
+  You'll get a prompt, at which you can run one of these commands:
+
+  +o  version
+
+  +o  halt
+
+  +o  reboot
+
+  +o  config
+
+  +o  remove
+
+  +o  sysrq
+
+  +o  help
+
+  +o  cad
+
+  +o  stop
+
+  +o  go
+
+
+  1100..11..  vveerrssiioonn
+
+  This takes no arguments.  It prints the UML version.
+
+
+       (mconsole)  version
+       OK Linux usermode 2.4.5-9um #1 Wed Jun 20 22:47:08 EDT 2001 i686
+
+
+
+
+  There are a couple actual uses for this.  It's a simple no-op which
+  can be used to check that a UML is running.  It's also a way of
+  sending an interrupt to the UML.  This is sometimes useful on SMP
+  hosts, where there's a bug which causes signals to UML to be lost,
+  often causing it to appear to hang.  Sending such a UML the mconsole
+  version command is a good way to 'wake it up' before networking has
+  been enabled, as it does not do anything to the function of the UML.
+
+
+
+  1100..22..  hhaalltt aanndd rreebboooott
+
+  These take no arguments.  They shut the machine down immediately, with
+  no syncing of disks and no clean shutdown of userspace.  So, they are
+  pretty close to crashing the machine.
+
+
+       (mconsole)  halt
+       OK
+
+
+
+
+
+
+  1100..33..  ccoonnffiigg
+
+  "config" adds a new device to the virtual machine.  Currently the ubd
+  and network drivers support this.  It takes one argument, which is the
+  device to add, with the same syntax as the kernel command line.
+
+
+
+
+  (mconsole)
+  config ubd3=/home/jdike/incoming/roots/root_fs_debian22
+
+  OK
+  (mconsole)  config eth1=mcast
+  OK
+
+
+
+
+
+
+  1100..44..  rreemmoovvee
+
+  "remove" deletes a device from the system.  Its argument is just the
+  name of the device to be removed. The device must be idle in whatever
+  sense the driver considers necessary.  In the case of the ubd driver,
+  the removed block device must not be mounted, swapped on, or otherwise
+  open, and in the case of the network driver, the device must be down.
+
+
+       (mconsole)  remove ubd3
+       OK
+       (mconsole)  remove eth1
+       OK
+
+
+
+
+
+
+  1100..55..  ssyyssrrqq
+
+  This takes one argument, which is a single letter.  It calls the
+  generic kernel's SysRq driver, which does whatever is called for by
+  that argument.  See the SysRq documentation in Documentation/sysrq.txt
+  in your favorite kernel tree to see what letters are valid and what
+  they do.
+
+
+
+  1100..66..  hheellpp
+
+  "help" returns a string listing the valid commands and what each one
+  does.
+
+
+
+  1100..77..  ccaadd
+
+  This invokes the Ctl-Alt-Del action on init.  What exactly this ends
+  up doing is up to /etc/inittab.  Normally, it reboots the machine.
+  With UML, this is usually not desired, so if a halt would be better,
+  then find the section of inittab that looks like this
+
+
+       # What to do when CTRL-ALT-DEL is pressed.
+       ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now
+
+
+
+
+  and change the command to halt.
+
+
+
+  1100..88..  ssttoopp
+
+  This puts the UML in a loop reading mconsole requests until a 'go'
+  mconsole command is received. This is very useful for making backups
+  of UML filesystems, as the UML can be stopped, then synced via 'sysrq
+  s', so that everything is written to the filesystem. You can then copy
+  the filesystem and then send the UML 'go' via mconsole.
+
+
+  Note that a UML running with more than one CPU will have problems
+  after you send the 'stop' command, as only one CPU will be held in a
+  mconsole loop and all others will continue as normal.  This is a bug,
+  and will be fixed.
+
+
+
+  1100..99..  ggoo
+
+  This resumes a UML after being paused by a 'stop' command. Note that
+  when the UML has resumed, TCP connections may have timed out and if
+  the UML is paused for a long period of time, crond might go a little
+  crazy, running all the jobs it didn't do earlier.
+
+
+
+
+
+
+
+
+  1111..  KKeerrnneell ddeebbuuggggiinngg
+
+
+  NNoottee:: The interface that makes debugging, as described here, possible
+  is present in 2.4.0-test6 kernels and later.
+
+
+  Since the user-mode kernel runs as a normal Linux process, it is
+  possible to debug it with gdb almost like any other process.  It is
+  slightly different because the kernel's threads are already being
+  ptraced for system call interception, so gdb can't ptrace them.
+  However, a mechanism has been added to work around that problem.
+
+
+  In order to debug the kernel, you need build it from source.  See
+  ``Compiling the kernel and modules''  for information on doing that.
+  Make sure that you enable CONFIG_DEBUGSYM and CONFIG_PT_PROXY during
+  the config.  These will compile the kernel with -g, and enable the
+  ptrace proxy so that gdb works with UML, respectively.
+
+
+
+
+  1111..11..  SSttaarrttiinngg tthhee kkeerrnneell uunnddeerr ggddbb
+
+  You can have the kernel running under the control of gdb from the
+  beginning by putting 'debug' on the command line.  You will get an
+  xterm with gdb running inside it.  The kernel will send some commands
+  to gdb which will leave it stopped at the beginning of start_kernel.
+  At this point, you can get things going with 'next', 'step', or
+  'cont'.
+
+
+  There is a transcript of a debugging session  here <debug-
+  session.html> , with breakpoints being set in the scheduler and in an
+  interrupt handler.
+  1111..22..  EExxaammiinniinngg sslleeeeppiinngg pprroocceesssseess
+
+  Not every bug is evident in the currently running process.  Sometimes,
+  processes hang in the kernel when they shouldn't because they've
+  deadlocked on a semaphore or something similar.  In this case, when
+  you ^C gdb and get a backtrace, you will see the idle thread, which
+  isn't very relevant.
+
+
+  What you want is the stack of whatever process is sleeping when it
+  shouldn't be.  You need to figure out which process that is, which is
+  generally fairly easy.  Then you need to get its host process id,
+  which you can do either by looking at ps on the host or at
+  task.thread.extern_pid in gdb.
+
+
+  Now what you do is this:
+
+  +o  detach from the current thread
+
+
+       (UML gdb)  det
+
+
+
+
+
+  +o  attach to the thread you are interested in
+
+
+       (UML gdb)  att <host pid>
+
+
+
+
+
+  +o  look at its stack and anything else of interest
+
+
+       (UML gdb)  bt
+
+
+
+
+  Note that you can't do anything at this point that requires that a
+  process execute, e.g. calling a function
+
+  +o  when you're done looking at that process, reattach to the current
+     thread and continue it
+
+
+       (UML gdb)
+       att 1
+
+
+
+
+
+
+       (UML gdb)
+       c
+
+
+
+
+  Here, specifying any pid which is not the process id of a UML thread
+  will cause gdb to reattach to the current thread.  I commonly use 1,
+  but any other invalid pid would work.
+
+
+
+  1111..33..  RRuunnnniinngg dddddd oonn UUMMLL
+
+  ddd works on UML, but requires a special kludge.  The process goes
+  like this:
+
+  +o  Start ddd
+
+
+       host% ddd linux
+
+
+
+
+
+  +o  With ps, get the pid of the gdb that ddd started.  You can ask the
+     gdb to tell you, but for some reason that confuses things and
+     causes a hang.
+
+  +o  run UML with 'debug=parent gdb-pid=<pid>' added to the command line
+     - it will just sit there after you hit return
+
+  +o  type 'att 1' to the ddd gdb and you will see something like
+
+
+       0xa013dc51 in __kill ()
+
+
+       (gdb)
+
+
+
+
+
+  +o  At this point, type 'c', UML will boot up, and you can use ddd just
+     as you do on any other process.
+
+
+
+  1111..44..  DDeebbuuggggiinngg mmoodduulleess
+
+  gdb has support for debugging code which is dynamically loaded into
+  the process.  This support is what is needed to debug kernel modules
+  under UML.
+
+
+  Using that support is somewhat complicated.  You have to tell gdb what
+  object file you just loaded into UML and where in memory it is.  Then,
+  it can read the symbol table, and figure out where all the symbols are
+  from the load address that you provided.  It gets more interesting
+  when you load the module again (i.e. after an rmmod).  You have to
+  tell gdb to forget about all its symbols, including the main UML ones
+  for some reason, then load then all back in again.
+
+
+  There's an easy way and a hard way to do this.  The easy way is to use
+  the umlgdb expect script written by Chandan Kudige.  It basically
+  automates the process for you.
+
+
+  First, you must tell it where your modules are.  There is a list in
+  the script that looks like this:
+       set MODULE_PATHS {
+       "fat" "/usr/src/uml/linux-2.4.18/fs/fat/fat.o"
+       "isofs" "/usr/src/uml/linux-2.4.18/fs/isofs/isofs.o"
+       "minix" "/usr/src/uml/linux-2.4.18/fs/minix/minix.o"
+       }
+
+
+
+
+  You change that to list the names and paths of the modules that you
+  are going to debug.  Then you run it from the toplevel directory of
+  your UML pool and it basically tells you what to do:
+
+
+
+
+                   ******** GDB pid is 21903 ********
+       Start UML as: ./linux <kernel switches> debug gdb-pid=21903
+
+
+
+       GNU gdb 5.0rh-5 Red Hat Linux 7.1
+       Copyright 2001 Free Software Foundation, Inc.
+       GDB is free software, covered by the GNU General Public License, and you are
+       welcome to change it and/or distribute copies of it under certain conditions.
+       Type "show copying" to see the conditions.
+       There is absolutely no warranty for GDB.  Type "show warranty" for details.
+       This GDB was configured as "i386-redhat-linux"...
+       (gdb) b sys_init_module
+       Breakpoint 1 at 0xa0011923: file module.c, line 349.
+       (gdb) att 1
+
+
+
+
+  After you run UML and it sits there doing nothing, you hit return at
+  the 'att 1' and continue it:
+
+
+       Attaching to program: /home/jdike/linux/2.4/um/./linux, process 1
+       0xa00f4221 in __kill ()
+       (UML gdb)  c
+       Continuing.
+
+
+
+
+  At this point, you debug normally.  When you insmod something, the
+  expect magic will kick in and you'll see something like:
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+   *** Module hostfs loaded ***
+  Breakpoint 1, sys_init_module (name_user=0x805abb0 "hostfs",
+      mod_user=0x8070e00) at module.c:349
+  349             char *name, *n_name, *name_tmp = NULL;
+  (UML gdb)  finish
+  Run till exit from #0  sys_init_module (name_user=0x805abb0 "hostfs",
+      mod_user=0x8070e00) at module.c:349
+  0xa00e2e23 in execute_syscall (r=0xa8140284) at syscall_kern.c:411
+  411             else res = EXECUTE_SYSCALL(syscall, regs);
+  Value returned is $1 = 0
+  (UML gdb)
+  p/x (int)module_list + module_list->size_of_struct
+
+  $2 = 0xa9021054
+  (UML gdb)  symbol-file ./linux
+  Load new symbol table from "./linux"? (y or n) y
+  Reading symbols from ./linux...
+  done.
+  (UML gdb)
+  add-symbol-file /home/jdike/linux/2.4/um/arch/um/fs/hostfs/hostfs.o 0xa9021054
+
+  add symbol table from file "/home/jdike/linux/2.4/um/arch/um/fs/hostfs/hostfs.o" at
+          .text_addr = 0xa9021054
+   (y or n) y
+
+  Reading symbols from /home/jdike/linux/2.4/um/arch/um/fs/hostfs/hostfs.o...
+  done.
+  (UML gdb)  p *module_list
+  $1 = {size_of_struct = 84, next = 0xa0178720, name = 0xa9022de0 "hostfs",
+    size = 9016, uc = {usecount = {counter = 0}, pad = 0}, flags = 1,
+    nsyms = 57, ndeps = 0, syms = 0xa9023170, deps = 0x0, refs = 0x0,
+    init = 0xa90221f0 <init_hostfs>, cleanup = 0xa902222c <exit_hostfs>,
+    ex_table_start = 0x0, ex_table_end = 0x0, persist_start = 0x0,
+    persist_end = 0x0, can_unload = 0, runsize = 0, kallsyms_start = 0x0,
+    kallsyms_end = 0x0,
+    archdata_start = 0x1b855 <Address 0x1b855 out of bounds>,
+    archdata_end = 0xe5890000 <Address 0xe5890000 out of bounds>,
+    kernel_data = 0xf689c35d <Address 0xf689c35d out of bounds>}
+  >> Finished loading symbols for hostfs ...
+
+
+
+
+  That's the easy way.  It's highly recommended.  The hard way is
+  described below in case you're interested in what's going on.
+
+
+  Boot the kernel under the debugger and load the module with insmod or
+  modprobe.  With gdb, do:
+
+
+       (UML gdb)  p module_list
+
+
+
+
+  This is a list of modules that have been loaded into the kernel, with
+  the most recently loaded module first.  Normally, the module you want
+  is at module_list.  If it's not, walk down the next links, looking at
+  the name fields until find the module you want to debug.  Take the
+  address of that structure, and add module.size_of_struct (which in
+  2.4.10 kernels is 96 (0x60)) to it.  Gdb can make this hard addition
+  for you :-):
+
+
+
+  (UML gdb)
+  printf "%#x\n", (int)module_list module_list->size_of_struct
+
+
+
+
+  The offset from the module start occasionally changes (before 2.4.0,
+  it was module.size_of_struct + 4), so it's a good idea to check the
+  init and cleanup addresses once in a while, as describe below.  Now
+  do:
+
+
+       (UML gdb)
+       add-symbol-file /path/to/module/on/host that_address
+
+
+
+
+  Tell gdb you really want to do it, and you're in business.
+
+
+  If there's any doubt that you got the offset right, like breakpoints
+  appear not to work, or they're appearing in the wrong place, you can
+  check it by looking at the module structure.  The init and cleanup
+  fields should look like:
+
+
+       init = 0x588066b0 <init_hostfs>, cleanup = 0x588066c0 <exit_hostfs>
+
+
+
+
+  with no offsets on the symbol names.  If the names are right, but they
+  are offset, then the offset tells you how much you need to add to the
+  address you gave to add-symbol-file.
+
+
+  When you want to load in a new version of the module, you need to get
+  gdb to forget about the old one.  The only way I've found to do that
+  is to tell gdb to forget about all symbols that it knows about:
+
+
+       (UML gdb)  symbol-file
+
+
+
+
+  Then reload the symbols from the kernel binary:
+
+
+       (UML gdb)  symbol-file /path/to/kernel
+
+
+
+
+  and repeat the process above.  You'll also need to re-enable break-
+  points.  They were disabled when you dumped all the symbols because
+  gdb couldn't figure out where they should go.
+
+
+
+  1111..55..  AAttttaacchhiinngg ggddbb ttoo tthhee kkeerrnneell
+
+  If you don't have the kernel running under gdb, you can attach gdb to
+  it later by sending the tracing thread a SIGUSR1.  The first line of
+  the console output identifies its pid:
+       tracing thread pid = 20093
+
+
+
+
+  When you send it the signal:
+
+
+       host% kill -USR1 20093
+
+
+
+
+  you will get an xterm with gdb running in it.
+
+
+  If you have the mconsole compiled into UML, then the mconsole client
+  can be used to start gdb:
+
+
+       (mconsole)  (mconsole) config gdb=xterm
+
+
+
+
+  will fire up an xterm with gdb running in it.
+
+
+
+  1111..66..  UUssiinngg aalltteerrnnaattee ddeebbuuggggeerrss
+
+  UML has support for attaching to an already running debugger rather
+  than starting gdb itself.  This is present in CVS as of 17 Apr 2001.
+  I sent it to Alan for inclusion in the ac tree, and it will be in my
+  2.4.4 release.
+
+
+  This is useful when gdb is a subprocess of some UI, such as emacs or
+  ddd.  It can also be used to run debuggers other than gdb on UML.
+  Below is an example of using strace as an alternate debugger.
+
+
+  To do this, you need to get the pid of the debugger and pass it in
+  with the
+
+
+  If you are using gdb under some UI, then tell it to 'att 1', and
+  you'll find yourself attached to UML.
+
+
+  If you are using something other than gdb as your debugger, then
+  you'll need to get it to do the equivalent of 'att 1' if it doesn't do
+  it automatically.
+
+
+  An example of an alternate debugger is strace.  You can strace the
+  actual kernel as follows:
+
+  +o  Run the following in a shell
+
+
+       host%
+       sh -c 'echo pid=$$; echo -n hit return; read x; exec strace -p 1 -o strace.out'
+
+
+
+  +o  Run UML with 'debug' and 'gdb-pid=<pid>' with the pid printed out
+     by the previous command
+
+  +o  Hit return in the shell, and UML will start running, and strace
+     output will start accumulating in the output file.
+
+     Note that this is different from running
+
+
+       host% strace ./linux
+
+
+
+
+  That will strace only the main UML thread, the tracing thread, which
+  doesn't do any of the actual kernel work.  It just oversees the vir-
+  tual machine.  In contrast, using strace as described above will show
+  you the low-level activity of the virtual machine.
+
+
+
+
+
+  1122..  KKeerrnneell ddeebbuuggggiinngg eexxaammpplleess
+
+  1122..11..  TThhee ccaassee ooff tthhee hhuunngg ffsscckk
+
+  When booting up the kernel, fsck failed, and dropped me into a shell
+  to fix things up.  I ran fsck -y, which hung:
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+  Setting hostname uml                    [ OK ]
+  Checking root filesystem
+  /dev/fhd0 was not cleanly unmounted, check forced.
+  Error reading block 86894 (Attempt to read block from filesystem resulted in short read) while reading indirect blocks of inode 19780.
+
+  /dev/fhd0: UNEXPECTED INCONSISTENCY; RUN fsck MANUALLY.
+          (i.e., without -a or -p options)
+  [ FAILED ]
+
+  *** An error occurred during the file system check.
+  *** Dropping you to a shell; the system will reboot
+  *** when you leave the shell.
+  Give root password for maintenance
+  (or type Control-D for normal startup):
+
+  [root@uml /root]# fsck -y /dev/fhd0
+  fsck -y /dev/fhd0
+  Parallelizing fsck version 1.14 (9-Jan-1999)
+  e2fsck 1.14, 9-Jan-1999 for EXT2 FS 0.5b, 95/08/09
+  /dev/fhd0 contains a file system with errors, check forced.
+  Pass 1: Checking inodes, blocks, and sizes
+  Error reading block 86894 (Attempt to read block from filesystem resulted in short read) while reading indirect blocks of inode 19780.  Ignore error? yes
+
+  Inode 19780, i_blocks is 1548, should be 540.  Fix? yes
+
+  Pass 2: Checking directory structure
+  Error reading block 49405 (Attempt to read block from filesystem resulted in short read).  Ignore error? yes
+
+  Directory inode 11858, block 0, offset 0: directory corrupted
+  Salvage? yes
+
+  Missing '.' in directory inode 11858.
+  Fix? yes
+
+  Missing '..' in directory inode 11858.
+  Fix? yes
+
+
+
+
+
+  The standard drill in this sort of situation is to fire up gdb on the
+  signal thread, which, in this case, was pid 1935.  In another window,
+  I run gdb and attach pid 1935.
+
+
+
+
+       ~/linux/2.3.26/um 1016: gdb linux
+       GNU gdb 4.17.0.11 with Linux support
+       Copyright 1998 Free Software Foundation, Inc.
+       GDB is free software, covered by the GNU General Public License, and you are
+       welcome to change it and/or distribute copies of it under certain conditions.
+       Type "show copying" to see the conditions.
+       There is absolutely no warranty for GDB.  Type "show warranty" for details.
+       This GDB was configured as "i386-redhat-linux"...
+
+       (gdb) att 1935
+       Attaching to program `/home/dike/linux/2.3.26/um/linux', Pid 1935
+       0x100756d9 in __wait4 ()
+
+
+
+
+
+
+  Let's see what's currently running:
+
+
+
+       (gdb) p current_task.pid
+       $1 = 0
+
+
+
+
+
+  It's the idle thread, which means that fsck went to sleep for some
+  reason and never woke up.
+
+
+  Let's guess that the last process in the process list is fsck:
+
+
+
+       (gdb) p current_task.prev_task.comm
+       $13 = "fsck.ext2\000\000\000\000\000\000"
+
+
+
+
+
+  It is, so let's see what it thinks it's up to:
+
+
+
+       (gdb) p current_task.prev_task.thread
+       $14 = {extern_pid = 1980, tracing = 0, want_tracing = 0, forking = 0,
+         kernel_stack_page = 0, signal_stack = 1342627840, syscall = {id = 4, args = {
+             3, 134973440, 1024, 0, 1024}, have_result = 0, result = 50590720},
+         request = {op = 2, u = {exec = {ip = 1350467584, sp = 2952789424}, fork = {
+               regs = {1350467584, 2952789424, 0 <repeats 15 times>}, sigstack = 0,
+               pid = 0}, switch_to = 0x507e8000, thread = {proc = 0x507e8000,
+               arg = 0xaffffdb0, flags = 0, new_pid = 0}, input_request = {
+               op = 1350467584, fd = -1342177872, proc = 0, pid = 0}}}}
+
+
+
+
+
+  The interesting things here are the fact that its .thread.syscall.id
+  is __NR_write (see the big switch in arch/um/kernel/syscall_kern.c or
+  the defines in include/asm-um/arch/unistd.h), and that it never
+  returned.  Also, its .request.op is OP_SWITCH (see
+  arch/um/include/user_util.h).  These mean that it went into a write,
+  and, for some reason, called schedule().
+
+
+  The fact that it never returned from write means that its stack should
+  be fairly interesting.  Its pid is 1980 (.thread.extern_pid).  That
+  process is being ptraced by the signal thread, so it must be detached
+  before gdb can attach it:
+
+
+
+
+
+
+
+
+
+
+  (gdb) call detach(1980)
+
+  Program received signal SIGSEGV, Segmentation fault.
+  <function called from gdb>
+  The program being debugged stopped while in a function called from GDB.
+  When the function (detach) is done executing, GDB will silently
+  stop (instead of continuing to evaluate the expression containing
+  the function call).
+  (gdb) call detach(1980)
+  $15 = 0
+
+
+
+
+
+  The first detach segfaults for some reason, and the second one
+  succeeds.
+
+
+  Now I detach from the signal thread, attach to the fsck thread, and
+  look at its stack:
+
+
+       (gdb) det
+       Detaching from program: /home/dike/linux/2.3.26/um/linux Pid 1935
+       (gdb) att 1980
+       Attaching to program `/home/dike/linux/2.3.26/um/linux', Pid 1980
+       0x10070451 in __kill ()
+       (gdb) bt
+       #0  0x10070451 in __kill ()
+       #1  0x10068ccd in usr1_pid (pid=1980) at process.c:30
+       #2  0x1006a03f in _switch_to (prev=0x50072000, next=0x507e8000)
+           at process_kern.c:156
+       #3  0x1006a052 in switch_to (prev=0x50072000, next=0x507e8000, last=0x50072000)
+           at process_kern.c:161
+       #4  0x10001d12 in schedule () at sched.c:777
+       #5  0x1006a744 in __down (sem=0x507d241c) at semaphore.c:71
+       #6  0x1006aa10 in __down_failed () at semaphore.c:157
+       #7  0x1006c5d8 in segv_handler (sc=0x5006e940) at trap_user.c:174
+       #8  0x1006c5ec in kern_segv_handler (sig=11) at trap_user.c:182
+       #9  <signal handler called>
+       #10 0x10155404 in errno ()
+       #11 0x1006c0aa in segv (address=1342179328, is_write=2) at trap_kern.c:50
+       #12 0x1006c5d8 in segv_handler (sc=0x5006eaf8) at trap_user.c:174
+       #13 0x1006c5ec in kern_segv_handler (sig=11) at trap_user.c:182
+       #14 <signal handler called>
+       #15 0xc0fd in ?? ()
+       #16 0x10016647 in sys_write (fd=3,
+           buf=0x80b8800 <Address 0x80b8800 out of bounds>, count=1024)
+           at read_write.c:159
+       #17 0x1006d5b3 in execute_syscall (syscall=4, args=0x5006ef08)
+           at syscall_kern.c:254
+       #18 0x1006af87 in really_do_syscall (sig=12) at syscall_user.c:35
+       #19 <signal handler called>
+       #20 0x400dc8b0 in ?? ()
+
+
+
+
+
+  The interesting things here are :
+
+  +o  There are two segfaults on this stack (frames 9 and 14)
+
+  +o  The first faulting address (frame 11) is 0x50000800
+
+  (gdb) p (void *)1342179328
+  $16 = (void *) 0x50000800
+
+
+
+
+
+  The initial faulting address is interesting because it is on the idle
+  thread's stack.  I had been seeing the idle thread segfault for no
+  apparent reason, and the cause looked like stack corruption.  In hopes
+  of catching the culprit in the act, I had turned off all protections
+  to that stack while the idle thread wasn't running.  This apparently
+  tripped that trap.
+
+
+  However, the more immediate problem is that second segfault and I'm
+  going to concentrate on that.  First, I want to see where the fault
+  happened, so I have to go look at the sigcontent struct in frame 8:
+
+
+
+       (gdb) up
+       #1  0x10068ccd in usr1_pid (pid=1980) at process.c:30
+       30        kill(pid, SIGUSR1);
+       (gdb)
+       #2  0x1006a03f in _switch_to (prev=0x50072000, next=0x507e8000)
+           at process_kern.c:156
+       156       usr1_pid(getpid());
+       (gdb)
+       #3  0x1006a052 in switch_to (prev=0x50072000, next=0x507e8000, last=0x50072000)
+           at process_kern.c:161
+       161       _switch_to(prev, next);
+       (gdb)
+       #4  0x10001d12 in schedule () at sched.c:777
+       777             switch_to(prev, next, prev);
+       (gdb)
+       #5  0x1006a744 in __down (sem=0x507d241c) at semaphore.c:71
+       71                      schedule();
+       (gdb)
+       #6  0x1006aa10 in __down_failed () at semaphore.c:157
+       157     }
+       (gdb)
+       #7  0x1006c5d8 in segv_handler (sc=0x5006e940) at trap_user.c:174
+       174       segv(sc->cr2, sc->err & 2);
+       (gdb)
+       #8  0x1006c5ec in kern_segv_handler (sig=11) at trap_user.c:182
+       182       segv_handler(sc);
+       (gdb) p *sc
+       Cannot access memory at address 0x0.
+
+
+
+
+  That's not very useful, so I'll try a more manual method:
+
+
+       (gdb) p *((struct sigcontext *) (&sig + 1))
+       $19 = {gs = 0, __gsh = 0, fs = 0, __fsh = 0, es = 43, __esh = 0, ds = 43,
+         __dsh = 0, edi = 1342179328, esi = 1350378548, ebp = 1342630440,
+         esp = 1342630420, ebx = 1348150624, edx = 1280, ecx = 0, eax = 0,
+         trapno = 14, err = 4, eip = 268480945, cs = 35, __csh = 0, eflags = 66118,
+         esp_at_signal = 1342630420, ss = 43, __ssh = 0, fpstate = 0x0, oldmask = 0,
+         cr2 = 1280}
+
+
+
+  The ip is in handle_mm_fault:
+
+
+       (gdb) p (void *)268480945
+       $20 = (void *) 0x1000b1b1
+       (gdb) i sym $20
+       handle_mm_fault + 57 in section .text
+
+
+
+
+
+  Specifically, it's in pte_alloc:
+
+
+       (gdb) i line *$20
+       Line 124 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
+          starts at address 0x1000b1b1 <handle_mm_fault+57>
+          and ends at 0x1000b1b7 <handle_mm_fault+63>.
+
+
+
+
+
+  To find where in handle_mm_fault this is, I'll jump forward in the
+  code until I see an address in that procedure:
+
+
+
+       (gdb) i line *0x1000b1c0
+       Line 126 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
+          starts at address 0x1000b1b7 <handle_mm_fault+63>
+          and ends at 0x1000b1c3 <handle_mm_fault+75>.
+       (gdb) i line *0x1000b1d0
+       Line 131 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
+          starts at address 0x1000b1d0 <handle_mm_fault+88>
+          and ends at 0x1000b1da <handle_mm_fault+98>.
+       (gdb) i line *0x1000b1e0
+       Line 61 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
+          starts at address 0x1000b1da <handle_mm_fault+98>
+          and ends at 0x1000b1e1 <handle_mm_fault+105>.
+       (gdb) i line *0x1000b1f0
+       Line 134 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
+          starts at address 0x1000b1f0 <handle_mm_fault+120>
+          and ends at 0x1000b200 <handle_mm_fault+136>.
+       (gdb) i line *0x1000b200
+       Line 135 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
+          starts at address 0x1000b200 <handle_mm_fault+136>
+          and ends at 0x1000b208 <handle_mm_fault+144>.
+       (gdb) i line *0x1000b210
+       Line 139 of "/home/dike/linux/2.3.26/um/include/asm/pgalloc.h"
+          starts at address 0x1000b210 <handle_mm_fault+152>
+          and ends at 0x1000b219 <handle_mm_fault+161>.
+       (gdb) i line *0x1000b220
+       Line 1168 of "memory.c" starts at address 0x1000b21e <handle_mm_fault+166>
+          and ends at 0x1000b222 <handle_mm_fault+170>.
+
+
+
+
+
+  Something is apparently wrong with the page tables or vma_structs, so
+  lets go back to frame 11 and have a look at them:
+
+
+
+  #11 0x1006c0aa in segv (address=1342179328, is_write=2) at trap_kern.c:50
+  50        handle_mm_fault(current, vma, address, is_write);
+  (gdb) call pgd_offset_proc(vma->vm_mm, address)
+  $22 = (pgd_t *) 0x80a548c
+
+
+
+
+
+  That's pretty bogus.  Page tables aren't supposed to be in process
+  text or data areas.  Let's see what's in the vma:
+
+
+       (gdb) p *vma
+       $23 = {vm_mm = 0x507d2434, vm_start = 0, vm_end = 134512640,
+         vm_next = 0x80a4f8c, vm_page_prot = {pgprot = 0}, vm_flags = 31200,
+         vm_avl_height = 2058, vm_avl_left = 0x80a8c94, vm_avl_right = 0x80d1000,
+         vm_next_share = 0xaffffdb0, vm_pprev_share = 0xaffffe63,
+         vm_ops = 0xaffffe7a, vm_pgoff = 2952789626, vm_file = 0xafffffec,
+         vm_private_data = 0x62}
+       (gdb) p *vma.vm_mm
+       $24 = {mmap = 0x507d2434, mmap_avl = 0x0, mmap_cache = 0x8048000,
+         pgd = 0x80a4f8c, mm_users = {counter = 0}, mm_count = {counter = 134904288},
+         map_count = 134909076, mmap_sem = {count = {counter = 135073792},
+           sleepers = -1342177872, wait = {lock = <optimized out or zero length>,
+             task_list = {next = 0xaffffe63, prev = 0xaffffe7a},
+             __magic = -1342177670, __creator = -1342177300}, __magic = 98},
+         page_table_lock = {}, context = 138, start_code = 0, end_code = 0,
+         start_data = 0, end_data = 0, start_brk = 0, brk = 0, start_stack = 0,
+         arg_start = 0, arg_end = 0, env_start = 0, env_end = 0, rss = 1350381536,
+         total_vm = 0, locked_vm = 0, def_flags = 0, cpu_vm_mask = 0, swap_cnt = 0,
+         swap_address = 0, segments = 0x0}
+
+
+
+
+
+  This also pretty bogus.  With all of the 0x80xxxxx and 0xaffffxxx
+  addresses, this is looking like a stack was plonked down on top of
+  these structures.  Maybe it's a stack overflow from the next page:
+
+
+
+       (gdb) p vma
+       $25 = (struct vm_area_struct *) 0x507d2434
+
+
+
+
+
+  That's towards the lower quarter of the page, so that would have to
+  have been pretty heavy stack overflow:
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+  (gdb) x/100x $25
+  0x507d2434:     0x507d2434      0x00000000      0x08048000      0x080a4f8c
+  0x507d2444:     0x00000000      0x080a79e0      0x080a8c94      0x080d1000
+  0x507d2454:     0xaffffdb0      0xaffffe63      0xaffffe7a      0xaffffe7a
+  0x507d2464:     0xafffffec      0x00000062      0x0000008a      0x00000000
+  0x507d2474:     0x00000000      0x00000000      0x00000000      0x00000000
+  0x507d2484:     0x00000000      0x00000000      0x00000000      0x00000000
+  0x507d2494:     0x00000000      0x00000000      0x507d2fe0      0x00000000
+  0x507d24a4:     0x00000000      0x00000000      0x00000000      0x00000000
+  0x507d24b4:     0x00000000      0x00000000      0x00000000      0x00000000
+  0x507d24c4:     0x00000000      0x00000000      0x00000000      0x00000000
+  0x507d24d4:     0x00000000      0x00000000      0x00000000      0x00000000
+  0x507d24e4:     0x00000000      0x00000000      0x00000000      0x00000000
+  0x507d24f4:     0x00000000      0x00000000      0x00000000      0x00000000
+  0x507d2504:     0x00000000      0x00000000      0x00000000      0x00000000
+  0x507d2514:     0x00000000      0x00000000      0x00000000      0x00000000
+  0x507d2524:     0x00000000      0x00000000      0x00000000      0x00000000
+  0x507d2534:     0x00000000      0x00000000      0x507d25dc      0x00000000
+  0x507d2544:     0x00000000      0x00000000      0x00000000      0x00000000
+  0x507d2554:     0x00000000      0x00000000      0x00000000      0x00000000
+  0x507d2564:     0x00000000      0x00000000      0x00000000      0x00000000
+  0x507d2574:     0x00000000      0x00000000      0x00000000      0x00000000
+  0x507d2584:     0x00000000      0x00000000      0x00000000      0x00000000
+  0x507d2594:     0x00000000      0x00000000      0x00000000      0x00000000
+  0x507d25a4:     0x00000000      0x00000000      0x00000000      0x00000000
+  0x507d25b4:     0x00000000      0x00000000      0x00000000      0x00000000
+
+
+
+
+
+  It's not stack overflow.  The only "stack-like" piece of this data is
+  the vma_struct itself.
+
+
+  At this point, I don't see any avenues to pursue, so I just have to
+  admit that I have no idea what's going on.  What I will do, though, is
+  stick a trap on the segfault handler which will stop if it sees any
+  writes to the idle thread's stack.  That was the thing that happened
+  first, and it may be that if I can catch it immediately, what's going
+  on will be somewhat clearer.
+
+
+  1122..22..  EEppiissooddee 22:: TThhee ccaassee ooff tthhee hhuunngg ffsscckk
+
+  After setting a trap in the SEGV handler for accesses to the signal
+  thread's stack, I reran the kernel.
+
+
+  fsck hung again, this time by hitting the trap:
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+  Setting hostname uml                            [ OK ]
+  Checking root filesystem
+  /dev/fhd0 contains a file system with errors, check forced.
+  Error reading block 86894 (Attempt to read block from filesystem resulted in short read) while reading indirect blocks of inode 19780.
+
+  /dev/fhd0: UNEXPECTED INCONSISTENCY; RUN fsck MANUALLY.
+          (i.e., without -a or -p options)
+  [ FAILED ]
+
+  *** An error occurred during the file system check.
+  *** Dropping you to a shell; the system will reboot
+  *** when you leave the shell.
+  Give root password for maintenance
+  (or type Control-D for normal startup):
+
+  [root@uml /root]# fsck -y /dev/fhd0
+  fsck -y /dev/fhd0
+  Parallelizing fsck version 1.14 (9-Jan-1999)
+  e2fsck 1.14, 9-Jan-1999 for EXT2 FS 0.5b, 95/08/09
+  /dev/fhd0 contains a file system with errors, check forced.
+  Pass 1: Checking inodes, blocks, and sizes
+  Error reading block 86894 (Attempt to read block from filesystem resulted in short read) while reading indirect blocks of inode 19780.  Ignore error? yes
+
+  Pass 2: Checking directory structure
+  Error reading block 49405 (Attempt to read block from filesystem resulted in short read).  Ignore error? yes
+
+  Directory inode 11858, block 0, offset 0: directory corrupted
+  Salvage? yes
+
+  Missing '.' in directory inode 11858.
+  Fix? yes
+
+  Missing '..' in directory inode 11858.
+  Fix? yes
+
+  Untested (4127) [100fe44c]: trap_kern.c line 31
+
+
+
+
+
+  I need to get the signal thread to detach from pid 4127 so that I can
+  attach to it with gdb.  This is done by sending it a SIGUSR1, which is
+  caught by the signal thread, which detaches the process:
+
+
+       kill -USR1 4127
+
+
+
+
+
+  Now I can run gdb on it:
+
+
+
+
+
+
+
+
+
+
+
+
+
+  ~/linux/2.3.26/um 1034: gdb linux
+  GNU gdb 4.17.0.11 with Linux support
+  Copyright 1998 Free Software Foundation, Inc.
+  GDB is free software, covered by the GNU General Public License, and you are
+  welcome to change it and/or distribute copies of it under certain conditions.
+  Type "show copying" to see the conditions.
+  There is absolutely no warranty for GDB.  Type "show warranty" for details.
+  This GDB was configured as "i386-redhat-linux"...
+  (gdb) att 4127
+  Attaching to program `/home/dike/linux/2.3.26/um/linux', Pid 4127
+  0x10075891 in __libc_nanosleep ()
+
+
+
+
+
+  The backtrace shows that it was in a write and that the fault address
+  (address in frame 3) is 0x50000800, which is right in the middle of
+  the signal thread's stack page:
+
+
+       (gdb) bt
+       #0  0x10075891 in __libc_nanosleep ()
+       #1  0x1007584d in __sleep (seconds=1000000)
+           at ../sysdeps/unix/sysv/linux/sleep.c:78
+       #2  0x1006ce9a in stop () at user_util.c:191
+       #3  0x1006bf88 in segv (address=1342179328, is_write=2) at trap_kern.c:31
+       #4  0x1006c628 in segv_handler (sc=0x5006eaf8) at trap_user.c:174
+       #5  0x1006c63c in kern_segv_handler (sig=11) at trap_user.c:182
+       #6  <signal handler called>
+       #7  0xc0fd in ?? ()
+       #8  0x10016647 in sys_write (fd=3, buf=0x80b8800 "R.", count=1024)
+           at read_write.c:159
+       #9  0x1006d603 in execute_syscall (syscall=4, args=0x5006ef08)
+           at syscall_kern.c:254
+       #10 0x1006af87 in really_do_syscall (sig=12) at syscall_user.c:35
+       #11 <signal handler called>
+       #12 0x400dc8b0 in ?? ()
+       #13 <signal handler called>
+       #14 0x400dc8b0 in ?? ()
+       #15 0x80545fd in ?? ()
+       #16 0x804daae in ?? ()
+       #17 0x8054334 in ?? ()
+       #18 0x804d23e in ?? ()
+       #19 0x8049632 in ?? ()
+       #20 0x80491d2 in ?? ()
+       #21 0x80596b5 in ?? ()
+       (gdb) p (void *)1342179328
+       $3 = (void *) 0x50000800
+
+
+
+
+
+  Going up the stack to the segv_handler frame and looking at where in
+  the code the access happened shows that it happened near line 110 of
+  block_dev.c:
+
+
+
+
+
+
+
+
+
+  (gdb) up
+  #1  0x1007584d in __sleep (seconds=1000000)
+      at ../sysdeps/unix/sysv/linux/sleep.c:78
+  ../sysdeps/unix/sysv/linux/sleep.c:78: No such file or directory.
+  (gdb)
+  #2  0x1006ce9a in stop () at user_util.c:191
+  191       while(1) sleep(1000000);
+  (gdb)
+  #3  0x1006bf88 in segv (address=1342179328, is_write=2) at trap_kern.c:31
+  31          KERN_UNTESTED();
+  (gdb)
+  #4  0x1006c628 in segv_handler (sc=0x5006eaf8) at trap_user.c:174
+  174       segv(sc->cr2, sc->err & 2);
+  (gdb) p *sc
+  $1 = {gs = 0, __gsh = 0, fs = 0, __fsh = 0, es = 43, __esh = 0, ds = 43,
+    __dsh = 0, edi = 1342179328, esi = 134973440, ebp = 1342631484,
+    esp = 1342630864, ebx = 256, edx = 0, ecx = 256, eax = 1024, trapno = 14,
+    err = 6, eip = 268550834, cs = 35, __csh = 0, eflags = 66070,
+    esp_at_signal = 1342630864, ss = 43, __ssh = 0, fpstate = 0x0, oldmask = 0,
+    cr2 = 1342179328}
+  (gdb) p (void *)268550834
+  $2 = (void *) 0x1001c2b2
+  (gdb) i sym $2
+  block_write + 1090 in section .text
+  (gdb) i line *$2
+  Line 209 of "/home/dike/linux/2.3.26/um/include/asm/arch/string.h"
+     starts at address 0x1001c2a1 <block_write+1073>
+     and ends at 0x1001c2bf <block_write+1103>.
+  (gdb) i line *0x1001c2c0
+  Line 110 of "block_dev.c" starts at address 0x1001c2bf <block_write+1103>
+     and ends at 0x1001c2e3 <block_write+1139>.
+
+
+
+
+
+  Looking at the source shows that the fault happened during a call to
+  copy_to_user to copy the data into the kernel:
+
+
+       107             count -= chars;
+       108             copy_from_user(p,buf,chars);
+       109             p += chars;
+       110             buf += chars;
+
+
+
+
+
+  p is the pointer which must contain 0x50000800, since buf contains
+  0x80b8800 (frame 8 above).  It is defined as:
+
+
+                       p = offset + bh->b_data;
+
+
+
+
+
+  I need to figure out what bh is, and it just so happens that bh is
+  passed as an argument to mark_buffer_uptodate and mark_buffer_dirty a
+  few lines later, so I do a little disassembly:
+
+
+
+
+  (gdb) disas 0x1001c2bf 0x1001c2e0
+  Dump of assembler code from 0x1001c2bf to 0x1001c2d0:
+  0x1001c2bf <block_write+1103>:  addl   %eax,0xc(%ebp)
+  0x1001c2c2 <block_write+1106>:  movl   0xfffffdd4(%ebp),%edx
+  0x1001c2c8 <block_write+1112>:  btsl   $0x0,0x18(%edx)
+  0x1001c2cd <block_write+1117>:  btsl   $0x1,0x18(%edx)
+  0x1001c2d2 <block_write+1122>:  sbbl   %ecx,%ecx
+  0x1001c2d4 <block_write+1124>:  testl  %ecx,%ecx
+  0x1001c2d6 <block_write+1126>:  jne    0x1001c2e3 <block_write+1139>
+  0x1001c2d8 <block_write+1128>:  pushl  $0x0
+  0x1001c2da <block_write+1130>:  pushl  %edx
+  0x1001c2db <block_write+1131>:  call   0x1001819c <__mark_buffer_dirty>
+  End of assembler dump.
+
+
+
+
+
+  At that point, bh is in %edx (address 0x1001c2da), which is calculated
+  at 0x1001c2c2 as %ebp + 0xfffffdd4, so I figure exactly what that is,
+  taking %ebp from the sigcontext_struct above:
+
+
+       (gdb) p (void *)1342631484
+       $5 = (void *) 0x5006ee3c
+       (gdb) p 0x5006ee3c+0xfffffdd4
+       $6 = 1342630928
+       (gdb) p (void *)$6
+       $7 = (void *) 0x5006ec10
+       (gdb) p *((void **)$7)
+       $8 = (void *) 0x50100200
+
+
+
+
+
+  Now, I look at the structure to see what's in it, and particularly,
+  what its b_data field contains:
+
+
+       (gdb) p *((struct buffer_head *)0x50100200)
+       $13 = {b_next = 0x50289380, b_blocknr = 49405, b_size = 1024, b_list = 0,
+         b_dev = 15872, b_count = {counter = 1}, b_rdev = 15872, b_state = 24,
+         b_flushtime = 0, b_next_free = 0x501001a0, b_prev_free = 0x50100260,
+         b_this_page = 0x501001a0, b_reqnext = 0x0, b_pprev = 0x507fcf58,
+         b_data = 0x50000800 "", b_page = 0x50004000,
+         b_end_io = 0x10017f60 <end_buffer_io_sync>, b_dev_id = 0x0,
+         b_rsector = 98810, b_wait = {lock = <optimized out or zero length>,
+           task_list = {next = 0x50100248, prev = 0x50100248}, __magic = 1343226448,
+           __creator = 0}, b_kiobuf = 0x0}
+
+
+
+
+
+  The b_data field is indeed 0x50000800, so the question becomes how
+  that happened.  The rest of the structure looks fine, so this probably
+  is not a case of data corruption.  It happened on purpose somehow.
+
+
+  The b_page field is a pointer to the page_struct representing the
+  0x50000000 page.  Looking at it shows the kernel's idea of the state
+  of that page:
+
+
+
+  (gdb) p *$13.b_page
+  $17 = {list = {next = 0x50004a5c, prev = 0x100c5174}, mapping = 0x0,
+    index = 0, next_hash = 0x0, count = {counter = 1}, flags = 132, lru = {
+      next = 0x50008460, prev = 0x50019350}, wait = {
+      lock = <optimized out or zero length>, task_list = {next = 0x50004024,
+        prev = 0x50004024}, __magic = 1342193708, __creator = 0},
+    pprev_hash = 0x0, buffers = 0x501002c0, virtual = 1342177280,
+    zone = 0x100c5160}
+
+
+
+
+
+  Some sanity-checking: the virtual field shows the "virtual" address of
+  this page, which in this kernel is the same as its "physical" address,
+  and the page_struct itself should be mem_map[0], since it represents
+  the first page of memory:
+
+
+
+       (gdb) p (void *)1342177280
+       $18 = (void *) 0x50000000
+       (gdb) p mem_map
+       $19 = (mem_map_t *) 0x50004000
+
+
+
+
+
+  These check out fine.
+
+
+  Now to check out the page_struct itself.  In particular, the flags
+  field shows whether the page is considered free or not:
+
+
+       (gdb) p (void *)132
+       $21 = (void *) 0x84
+
+
+
+
+
+  The "reserved" bit is the high bit, which is definitely not set, so
+  the kernel considers the signal stack page to be free and available to
+  be used.
+
+
+  At this point, I jump to conclusions and start looking at my early
+  boot code, because that's where that page is supposed to be reserved.
+
+
+  In my setup_arch procedure, I have the following code which looks just
+  fine:
+
+
+
+       bootmap_size = init_bootmem(start_pfn, end_pfn - start_pfn);
+       free_bootmem(__pa(low_physmem) + bootmap_size, high_physmem - low_physmem);
+
+
+
+
+
+  Two stack pages have already been allocated, and low_physmem points to
+  the third page, which is the beginning of free memory.
+  The init_bootmem call declares the entire memory to the boot memory
+  manager, which marks it all reserved.  The free_bootmem call frees up
+  all of it, except for the first two pages.  This looks correct to me.
+
+
+  So, I decide to see init_bootmem run and make sure that it is marking
+  those first two pages as reserved.  I never get that far.
+
+
+  Stepping into init_bootmem, and looking at bootmem_map before looking
+  at what it contains shows the following:
+
+
+
+       (gdb) p bootmem_map
+       $3 = (void *) 0x50000000
+
+
+
+
+
+  Aha!  The light dawns.  That first page is doing double duty as a
+  stack and as the boot memory map.  The last thing that the boot memory
+  manager does is to free the pages used by its memory map, so this page
+  is getting freed even its marked as reserved.
+
+
+  The fix was to initialize the boot memory manager before allocating
+  those two stack pages, and then allocate them through the boot memory
+  manager.  After doing this, and fixing a couple of subsequent buglets,
+  the stack corruption problem disappeared.
+
+
+
+
+
+  1133..  WWhhaatt ttoo ddoo wwhheenn UUMMLL ddooeessnn''tt wwoorrkk
+
+
+
+
+  1133..11..  SSttrraannggee ccoommppiillaattiioonn eerrrroorrss wwhheenn yyoouu bbuuiilldd ffrroomm ssoouurrccee
+
+  As of test11, it is necessary to have "ARCH=um" in the environment or
+  on the make command line for all steps in building UML, including
+  clean, distclean, or mrproper, config, menuconfig, or xconfig, dep,
+  and linux.  If you forget for any of them, the i386 build seems to
+  contaminate the UML build.  If this happens, start from scratch with
+
+
+       host%
+       make mrproper ARCH=um
+
+
+
+
+  and repeat the build process with ARCH=um on all the steps.
+
+
+  See ``Compiling the kernel and modules''  for more details.
+
+
+  Another cause of strange compilation errors is building UML in
+  /usr/src/linux.  If you do this, the first thing you need to do is
+  clean up the mess you made.  The /usr/src/linux/asm link will now
+  point to /usr/src/linux/asm-um.  Make it point back to
+  /usr/src/linux/asm-i386.  Then, move your UML pool someplace else and
+  build it there.  Also see below, where a more specific set of symptoms
+  is described.
+
+
+
+  1133..33..  AA vvaarriieettyy ooff ppaanniiccss aanndd hhaannggss wwiitthh //ttmmpp oonn aa rreeiisseerrffss  ffiilleessyyss--
+  tteemm
+
+  I saw this on reiserfs 3.5.21 and it seems to be fixed in 3.5.27.
+  Panics preceded by
+
+
+       Detaching pid nnnn
+
+
+
+  are diagnostic of this problem.  This is a reiserfs bug which causes a
+  thread to occasionally read stale data from a mmapped page shared with
+  another thread.  The fix is to upgrade the filesystem or to have /tmp
+  be an ext2 filesystem.
+
+
+
+  1133..44..  TThhee ccoommppiillee ffaaiillss wwiitthh eerrrroorrss aabboouutt ccoonnfflliiccttiinngg ttyyppeess ffoorr
+  ''ooppeenn'',, ''dduupp'',, aanndd ''wwaaiittppiidd''
+
+  This happens when you build in /usr/src/linux.  The UML build makes
+  the include/asm link point to include/asm-um.  /usr/include/asm points
+  to /usr/src/linux/include/asm, so when that link gets moved, files
+  which need to include the asm-i386 versions of headers get the
+  incompatible asm-um versions.  The fix is to move the include/asm link
+  back to include/asm-i386 and to do UML builds someplace else.
+
+
+
+  1133..55..  UUMMLL ddooeessnn''tt wwoorrkk wwhheenn //ttmmpp iiss aann NNFFSS ffiilleessyysstteemm
+
+  This seems to be a similar situation with the ReiserFS problem above.
+  Some versions of NFS seems not to handle mmap correctly, which UML
+  depends on.  The workaround is have /tmp be a non-NFS directory.
+
+
+  1133..66..  UUMMLL hhaannggss oonn bboooott wwhheenn ccoommppiilleedd wwiitthh ggpprrooff ssuuppppoorrtt
+
+  If you build UML with gprof support and, early in the boot, it does
+  this
+
+
+       kernel BUG at page_alloc.c:100!
+
+
+
+
+  you have a buggy gcc.  You can work around the problem by removing
+  UM_FASTCALL from CFLAGS in arch/um/Makefile-i386.  This will open up
+  another bug, but that one is fairly hard to reproduce.
+
+
+
+  1133..77..  ssyyssllooggdd ddiieess wwiitthh aa SSIIGGTTEERRMM oonn ssttaarrttuupp
+
+  The exact boot error depends on the distribution that you're booting,
+  but Debian produces this:
+
+
+       /etc/rc2.d/S10sysklogd: line 49:    93 Terminated
+       start-stop-daemon --start --quiet --exec /sbin/syslogd -- $SYSLOGD
+
+
+
+
+  This is a syslogd bug.  There's a race between a parent process
+  installing a signal handler and its child sending the signal.  See
+  this uml-devel post <http://www.geocrawler.com/lists/3/Source-
+  Forge/709/0/6612801>  for the details.
+
+
+
+  1133..88..  TTUUNN//TTAAPP nneettwwoorrkkiinngg ddooeessnn''tt wwoorrkk oonn aa 22..44 hhoosstt
+
+  There are a couple of problems which were
+  <http://www.geocrawler.com/lists/3/SourceForge/597/0/> name="pointed
+  out">  by Tim Robinson <timro at trkr dot net>
+
+  +o  It doesn't work on hosts running 2.4.7 (or thereabouts) or earlier.
+     The fix is to upgrade to something more recent and then read the
+     next item.
+
+  +o  If you see
+
+
+       File descriptor in bad state
+
+
+
+  when you bring up the device inside UML, you have a header mismatch
+  between the original kernel and the upgraded one.  Make /usr/src/linux
+  point at the new headers.  This will only be a problem if you build
+  uml_net yourself.
+
+
+
+  1133..99..  YYoouu ccaann nneettwwoorrkk ttoo tthhee hhoosstt bbuutt nnoott ttoo ootthheerr mmaacchhiinneess oonn tthhee
+  nneett
+
+  If you can connect to the host, and the host can connect to UML, but
+  you cannot connect to any other machines, then you may need to enable
+  IP Masquerading on the host.  Usually this is only experienced when
+  using private IP addresses (192.168.x.x or 10.x.x.x) for host/UML
+  networking, rather than the public address space that your host is
+  connected to.  UML does not enable IP Masquerading, so you will need
+  to create a static rule to enable it:
+
+
+       host%
+       iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
+
+
+
+
+  Replace eth0 with the interface that you use to talk to the rest of
+  the world.
+
+
+  Documentation on IP Masquerading, and SNAT, can be found at
+  www.netfilter.org  <http://www.netfilter.org> .
+
+
+  If you can reach the local net, but not the outside Internet, then
+  that is usually a routing problem.  The UML needs a default route:
+
+
+       UML#
+       route add default gw gateway IP
+
+
+
+
+  The gateway IP can be any machine on the local net that knows how to
+  reach the outside world.  Usually, this is the host or the local net-
+  work's gateway.
+
+
+  Occasionally, we hear from someone who can reach some machines, but
+  not others on the same net, or who can reach some ports on other
+  machines, but not others.  These are usually caused by strange
+  firewalling somewhere between the UML and the other box.  You track
+  this down by running tcpdump on every interface the packets travel
+  over and see where they disappear.  When you find a machine that takes
+  the packets in, but does not send them onward, that's the culprit.
+
+
+
+  1133..1100..  II hhaavvee nnoo rroooott aanndd II wwaanntt ttoo ssccrreeaamm
+
+  Thanks to Birgit Wahlich for telling me about this strange one.  It
+  turns out that there's a limit of six environment variables on the
+  kernel command line.  When that limit is reached or exceeded, argument
+  processing stops, which means that the 'root=' argument that UML
+  usually adds is not seen.  So, the filesystem has no idea what the
+  root device is, so it panics.
+
+
+  The fix is to put less stuff on the command line.  Glomming all your
+  setup variables into one is probably the best way to go.
+
+
+
+  1133..1111..  UUMMLL bbuuiilldd ccoonnfflliicctt bbeettwweeeenn ppttrraaccee..hh aanndd uuccoonntteexxtt..hh
+
+  On some older systems, /usr/include/asm/ptrace.h and
+  /usr/include/sys/ucontext.h define the same names.  So, when they're
+  included together, the defines from one completely mess up the parsing
+  of the other, producing errors like:
+       /usr/include/sys/ucontext.h:47: parse error before
+       `10'
+
+
+
+
+  plus a pile of warnings.
+
+
+  This is a libc botch, which has since been fixed, and I don't see any
+  way around it besides upgrading.
+
+
+
+  1133..1122..  TThhee UUMMLL BBooggooMMiippss iiss eexxaaccttllyy hhaallff tthhee hhoosstt''ss BBooggooMMiippss
+
+  On i386 kernels, there are two ways of running the loop that is used
+  to calculate the BogoMips rating, using the TSC if it's there or using
+  a one-instruction loop.  The TSC produces twice the BogoMips as the
+  loop.  UML uses the loop, since it has nothing resembling a TSC, and
+  will get almost exactly the same BogoMips as a host using the loop.
+  However, on a host with a TSC, its BogoMips will be double the loop
+  BogoMips, and therefore double the UML BogoMips.
+
+
+
+  1133..1133..  WWhheenn yyoouu rruunn UUMMLL,, iitt iimmmmeeddiiaatteellyy sseeggffaauullttss
+
+  If the host is configured with the 2G/2G address space split, that's
+  why.  See ``UML on 2G/2G hosts''  for the details on getting UML to
+  run on your host.
+
+
+
+  1133..1144..  xxtteerrmmss aappppeeaarr,, tthheenn iimmmmeeddiiaatteellyy ddiissaappppeeaarr
+
+  If you're running an up to date kernel with an old release of
+  uml_utilities, the port-helper program will not work properly, so
+  xterms will exit straight after they appear. The solution is to
+  upgrade to the latest release of uml_utilities.  Usually this problem
+  occurs when you have installed a packaged release of UML then compiled
+  your own development kernel without upgrading the uml_utilities from
+  the source distribution.
+
+
+
+  1133..1155..  AAnnyy ootthheerr ppaanniicc,, hhaanngg,, oorr ssttrraannggee bbeehhaavviioorr
+
+  If you're seeing truly strange behavior, such as hangs or panics that
+  happen in random places, or you try running the debugger to see what's
+  happening and it acts strangely, then it could be a problem in the
+  host kernel.  If you're not running a stock Linus or -ac kernel, then
+  try that.  An early version of the preemption patch and a 2.4.10 SuSE
+  kernel have caused very strange problems in UML.
+
+
+  Otherwise, let me know about it.  Send a message to one of the UML
+  mailing lists - either the developer list - user-mode-linux-devel at
+  lists dot sourceforge dot net (subscription info) or the user list -
+  user-mode-linux-user at lists dot sourceforge do net (subscription
+  info), whichever you prefer.  Don't assume that everyone knows about
+  it and that a fix is imminent.
+
+
+  If you want to be super-helpful, read ``Diagnosing Problems'' and
+  follow the instructions contained therein.
+  1144..  DDiiaaggnnoossiinngg PPrroobblleemmss
+
+
+  If you get UML to crash, hang, or otherwise misbehave, you should
+  report this on one of the project mailing lists, either the developer
+  list - user-mode-linux-devel at lists dot sourceforge dot net
+  (subscription info) or the user list - user-mode-linux-user at lists
+  dot sourceforge dot net (subscription info).  When you do, it is
+  likely that I will want more information.  So, it would be helpful to
+  read the stuff below, do whatever is applicable in your case, and
+  report the results to the list.
+
+
+  For any diagnosis, you're going to need to build a debugging kernel.
+  The binaries from this site aren't debuggable.  If you haven't done
+  this before, read about ``Compiling the kernel and modules''  and
+  ``Kernel debugging''  UML first.
+
+
+  1144..11..  CCaassee 11 :: NNoorrmmaall kkeerrnneell ppaanniiccss
+
+  The most common case is for a normal thread to panic.  To debug this,
+  you will need to run it under the debugger (add 'debug' to the command
+  line).  An xterm will start up with gdb running inside it.  Continue
+  it when it stops in start_kernel and make it crash.  Now ^C gdb and
+
+
+  If the panic was a "Kernel mode fault", then there will be a segv
+  frame on the stack and I'm going to want some more information.  The
+  stack might look something like this:
+
+
+       (UML gdb)  backtrace
+       #0  0x1009bf76 in __sigprocmask (how=1, set=0x5f347940, oset=0x0)
+           at ../sysdeps/unix/sysv/linux/sigprocmask.c:49
+       #1  0x10091411 in change_sig (signal=10, on=1) at process.c:218
+       #2  0x10094785 in timer_handler (sig=26) at time_kern.c:32
+       #3  0x1009bf38 in __restore ()
+           at ../sysdeps/unix/sysv/linux/i386/sigaction.c:125
+       #4  0x1009534c in segv (address=8, ip=268849158, is_write=2, is_user=0)
+           at trap_kern.c:66
+       #5  0x10095c04 in segv_handler (sig=11) at trap_user.c:285
+       #6  0x1009bf38 in __restore ()
+
+
+
+
+  I'm going to want to see the symbol and line information for the value
+  of ip in the segv frame.  In this case, you would do the following:
+
+
+       (UML gdb)  i sym 268849158
+
+
+
+
+  and
+
+
+       (UML gdb)  i line *268849158
+
+
+
+
+  The reason for this is the __restore frame right above the segv_han-
+  dler frame is hiding the frame that actually segfaulted.  So, I have
+  to get that information from the faulting ip.
+
+
+  1144..22..  CCaassee 22 :: TTrraacciinngg tthhrreeaadd ppaanniiccss
+
+  The less common and more painful case is when the tracing thread
+  panics.  In this case, the kernel debugger will be useless because it
+  needs a healthy tracing thread in order to work.  The first thing to
+  do is get a backtrace from the tracing thread.  This is done by
+  figuring out what its pid is, firing up gdb, and attaching it to that
+  pid.  You can figure out the tracing thread pid by looking at the
+  first line of the console output, which will look like this:
+
+
+       tracing thread pid = 15851
+
+
+
+
+  or by running ps on the host and finding the line that looks like
+  this:
+
+
+       jdike 15851 4.5 0.4 132568 1104 pts/0 S 21:34 0:05 ./linux [(tracing thread)]
+
+
+
+
+  If the panic was 'segfault in signals', then follow the instructions
+  above for collecting information about the location of the seg fault.
+
+
+  If the tracing thread flaked out all by itself, then send that
+  backtrace in and wait for our crack debugging team to fix the problem.
+
+
+  1144..33..  CCaassee 33 :: TTrraacciinngg tthhrreeaadd ppaanniiccss ccaauusseedd bbyy ootthheerr tthhrreeaaddss
+
+  However, there are cases where the misbehavior of another thread
+  caused the problem.  The most common panic of this type is:
+
+
+       wait_for_stop failed to wait for  <pid>  to stop with  <signal number>
+
+
+
+
+  In this case, you'll need to get a backtrace from the process men-
+  tioned in the panic, which is complicated by the fact that the kernel
+  debugger is defunct and without some fancy footwork, another gdb can't
+  attach to it.  So, this is how the fancy footwork goes:
+
+  In a shell:
+
+
+       host% kill -STOP pid
+
+
+
+
+  Run gdb on the tracing thread as described in case 2 and do:
+
+
+       (host gdb)  call detach(pid)
+
+
+  If you get a segfault, do it again.  It always works the second time.
+
+  Detach from the tracing thread and attach to that other thread:
+
+
+       (host gdb)  detach
+
+
+
+
+
+
+       (host gdb)  attach pid
+
+
+
+
+  If gdb hangs when attaching to that process, go back to a shell and
+  do:
+
+
+       host%
+       kill -CONT pid
+
+
+
+
+  And then get the backtrace:
+
+
+       (host gdb)  backtrace
+
+
+
+
+
+  1144..44..  CCaassee 44 :: HHaannggss
+
+  Hangs seem to be fairly rare, but they sometimes happen.  When a hang
+  happens, we need a backtrace from the offending process.  Run the
+  kernel debugger as described in case 1 and get a backtrace.  If the
+  current process is not the idle thread, then send in the backtrace.
+  You can tell that it's the idle thread if the stack looks like this:
+
+
+       #0  0x100b1401 in __libc_nanosleep ()
+       #1  0x100a2885 in idle_sleep (secs=10) at time.c:122
+       #2  0x100a546f in do_idle () at process_kern.c:445
+       #3  0x100a5508 in cpu_idle () at process_kern.c:471
+       #4  0x100ec18f in start_kernel () at init/main.c:592
+       #5  0x100a3e10 in start_kernel_proc (unused=0x0) at um_arch.c:71
+       #6  0x100a383f in signal_tramp (arg=0x100a3dd8) at trap_user.c:50
+
+
+
+
+  If this is the case, then some other process is at fault, and went to
+  sleep when it shouldn't have.  Run ps on the host and figure out which
+  process should not have gone to sleep and stayed asleep.  Then attach
+  to it with gdb and get a backtrace as described in case 3.
+
+
+
+
+
+
+  1155..  TThhaannkkss
+
+
+  A number of people have helped this project in various ways, and this
+  page gives recognition where recognition is due.
+
+
+  If you're listed here and you would prefer a real link on your name,
+  or no link at all, instead of the despammed email address pseudo-link,
+  let me know.
+
+
+  If you're not listed here and you think maybe you should be, please
+  let me know that as well.  I try to get everyone, but sometimes my
+  bookkeeping lapses and I forget about contributions.
+
+
+  1155..11..  CCooddee aanndd DDooccuummeennttaattiioonn
+
+  Rusty Russell <rusty at linuxcare.com.au>  -
+
+  +o  wrote the  HOWTO <http://user-mode-
+     linux.sourceforge.net/UserModeLinux-HOWTO.html>
+
+  +o  prodded me into making this project official and putting it on
+     SourceForge
+
+  +o  came up with the way cool UML logo <http://user-mode-
+     linux.sourceforge.net/uml-small.png>
+
+  +o  redid the config process
+
+
+  Peter Moulder <reiter at netspace.net.au>  - Fixed my config and build
+  processes, and added some useful code to the block driver
+
+
+  Bill Stearns <wstearns at pobox.com>  -
+
+  +o  HOWTO updates
+
+  +o  lots of bug reports
+
+  +o  lots of testing
+
+  +o  dedicated a box (uml.ists.dartmouth.edu) to support UML development
+
+  +o  wrote the mkrootfs script, which allows bootable filesystems of
+     RPM-based distributions to be cranked out
+
+  +o  cranked out a large number of filesystems with said script
+
+
+  Jim Leu <jleu at mindspring.com>  - Wrote the virtual ethernet driver
+  and associated usermode tools
+
+  Lars Brinkhoff <http://lars.nocrew.org/>  - Contributed the ptrace
+  proxy from his own  project <http://a386.nocrew.org/> to allow easier
+  kernel debugging
+
+
+  Andrea Arcangeli <andrea at suse.de>  - Redid some of the early boot
+  code so that it would work on machines with Large File Support
+
+
+  Chris Emerson <http://www.chiark.greenend.org.uk/~cemerson/>  - Did
+  the first UML port to Linux/ppc
+
+
+  Harald Welte <laforge at gnumonks.org>  - Wrote the multicast
+  transport for the network driver
+
+
+  Jorgen Cederlof - Added special file support to hostfs
+
+
+  Greg Lonnon  <glonnon at ridgerun dot com>  - Changed the ubd driver
+  to allow it to layer a COW file on a shared read-only filesystem and
+  wrote the iomem emulation support
+
+
+  Henrik Nordstrom <http://hem.passagen.se/hno/>  - Provided a variety
+  of patches, fixes, and clues
+
+
+  Lennert Buytenhek - Contributed various patches, a rewrite of the
+  network driver, the first implementation of the mconsole driver, and
+  did the bulk of the work needed to get SMP working again.
+
+
+  Yon Uriarte - Fixed the TUN/TAP network backend while I slept.
+
+
+  Adam Heath - Made a bunch of nice cleanups to the initialization code,
+  plus various other small patches.
+
+
+  Matt Zimmerman - Matt volunteered to be the UML Debian maintainer and
+  is doing a real nice job of it.  He also noticed and fixed a number of
+  actually and potentially exploitable security holes in uml_net.  Plus
+  the occasional patch.  I like patches.
+
+
+  James McMechan - James seems to have taken over maintenance of the ubd
+  driver and is doing a nice job of it.
+
+
+  Chandan Kudige - wrote the umlgdb script which automates the reloading
+  of module symbols.
+
+
+  Steve Schmidtke - wrote the UML slirp transport and hostaudio drivers,
+  enabling UML processes to access audio devices on the host. He also
+  submitted patches for the slip transport and lots of other things.
+
+
+  David Coulson <http://davidcoulson.net>  -
+
+  +o  Set up the usermodelinux.org <http://usermodelinux.org>  site,
+     which is a great way of keeping the UML user community on top of
+     UML goings-on.
+
+  +o  Site documentation and updates
+
+  +o  Nifty little UML management daemon  UMLd
+     <http://uml.openconsultancy.com/umld/>
+
+  +o  Lots of testing and bug reports
+
+
+
+
+  1155..22..  FFlluusshhiinngg oouutt bbuuggss
+
+
+
+  +o  Yuri Pudgorodsky
+
+  +o  Gerald Britton
+
+  +o  Ian Wehrman
+
+  +o  Gord Lamb
+
+  +o  Eugene Koontz
+
+  +o  John H. Hartman
+
+  +o  Anders Karlsson
+
+  +o  Daniel Phillips
+
+  +o  John Fremlin
+
+  +o  Rainer Burgstaller
+
+  +o  James Stevenson
+
+  +o  Matt Clay
+
+  +o  Cliff Jefferies
+
+  +o  Geoff Hoff
+
+  +o  Lennert Buytenhek
+
+  +o  Al Viro
+
+  +o  Frank Klingenhoefer
+
+  +o  Livio Baldini Soares
+
+  +o  Jon Burgess
+
+  +o  Petru Paler
+
+  +o  Paul
+
+  +o  Chris Reahard
+
+  +o  Sverker Nilsson
+
+  +o  Gong Su
+
+  +o  johan verrept
+
+  +o  Bjorn Eriksson
+
+  +o  Lorenzo Allegrucci
+
+  +o  Muli Ben-Yehuda
+
+  +o  David Mansfield
+
+  +o  Howard Goff
+
+  +o  Mike Anderson
+
+  +o  John Byrne
+
+  +o  Sapan J. Batia
+
+  +o  Iris Huang
+
+  +o  Jan Hudec
+
+  +o  Voluspa
+
+
+
+
+  1155..33..  BBuugglleettss aanndd cclleeaann--uuppss
+
+
+
+  +o  Dave Zarzycki
+
+  +o  Adam Lazur
+
+  +o  Boria Feigin
+
+  +o  Brian J. Murrell
+
+  +o  JS
+
+  +o  Roman Zippel
+
+  +o  Wil Cooley
+
+  +o  Ayelet Shemesh
+
+  +o  Will Dyson
+
+  +o  Sverker Nilsson
+
+  +o  dvorak
+
+  +o  v.naga srinivas
+
+  +o  Shlomi Fish
+
+  +o  Roger Binns
+
+  +o  johan verrept
+
+  +o  MrChuoi
+
+  +o  Peter Cleve
+
+  +o  Vincent Guffens
+
+  +o  Nathan Scott
+
+  +o  Patrick Caulfield
+
+  +o  jbearce
+
+  +o  Catalin Marinas
+
+  +o  Shane Spencer
+
+  +o  Zou Min
+
+
+  +o  Ryan Boder
+
+  +o  Lorenzo Colitti
+
+  +o  Gwendal Grignou
+
+  +o  Andre' Breiler
+
+  +o  Tsutomu Yasuda
+
+
+
+  1155..44..  CCaassee SSttuuddiieess
+
+
+  +o  Jon Wright
+
+  +o  William McEwan
+
+  +o  Michael Richardson
+
+
+
+  1155..55..  OOtthheerr ccoonnttrriibbuuttiioonnss
+
+
+  Bill Carr <Bill.Carr at compaq.com>  made the Red Hat mkrootfs script
+  work with RH 6.2.
+
+  Michael Jennings <mikejen at hevanet.com>  sent in some material which
+  is now gracing the top of the  index  page <http://user-mode-
+  linux.sourceforge.net/>  of this site.
+
+  SGI <http://www.sgi.com>  (and more specifically Ralf Baechle <ralf at
+  uni-koblenz.de> ) gave me an account on oss.sgi.com
+  <http://www.oss.sgi.com> .  The bandwidth there made it possible to
+  produce most of the filesystems available on the project download
+  page.
+
+  Laurent Bonnaud <Laurent.Bonnaud at inpg.fr>  took the old grotty
+  Debian filesystem that I've been distributing and updated it to 2.2.
+  It is now available by itself here.
+
+  Rik van Riel gave me some ftp space on ftp.nl.linux.org so I can make
+  releases even when Sourceforge is broken.
+
+  Rodrigo de Castro looked at my broken pte code and told me what was
+  wrong with it, letting me fix a long-standing (several weeks) and
+  serious set of bugs.
+
+  Chris Reahard built a specialized root filesystem for running a DNS
+  server jailed inside UML.  It's available from the download
+  <http://user-mode-linux.sourceforge.net/dl-sf.html>  page in the Jail
+  Filesystems section.
+
+
+
+
+
+
+
+
+
+
+
+