Move lguest guest support to arch/x86.

Lguest has two sides: host support (to launch guests) and guest
support (replacement boot path and paravirt_ops).  This moves the
guest side to arch/x86/lguest where it's closer to related code.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Andi Kleen <ak@suse.de>

4 files changed, 1208 insertions, 0 deletions

diff --git a/arch/x86/lguest/Kconfig b/arch/x86/lguest/Kconfig
new file mode 100644
index 000000000000..0fabf87db998
--- /dev/null
+++ b/arch/x86/lguest/Kconfig
@@ -0,0 +1,8 @@
+config LGUEST_GUEST
+        bool "Lguest guest support"
+        select PARAVIRT
+        depends on !X86_PAE
+        help
+          Lguest is a tiny in-kernel hypervisor.  Selecting this will
+          allow your kernel to boot under lguest.  This option will increase
+          your kernel size by about 6k.  If in doubt, say N.
diff --git a/arch/x86/lguest/Makefile b/arch/x86/lguest/Makefile
new file mode 100644
index 000000000000..27f0c9ed7f60
--- /dev/null
+++ b/arch/x86/lguest/Makefile
@@ -0,0 +1 @@
+obj-y           := i386_head.o boot.o
diff --git a/arch/x86/lguest/boot.c b/arch/x86/lguest/boot.c
new file mode 100644
index 000000000000..8e9e485a5cfa
--- /dev/null
+++ b/arch/x86/lguest/boot.c
@@ -0,0 +1,1106 @@
+/*P:010
+ * A hypervisor allows multiple Operating Systems to run on a single machine.
+ * To quote David Wheeler: "Any problem in computer science can be solved with
+ * another layer of indirection."
+ *
+ * We keep things simple in two ways.  First, we start with a normal Linux
+ * kernel and insert a module (lg.ko) which allows us to run other Linux
+ * kernels the same way we'd run processes.  We call the first kernel the Host,
+ * and the others the Guests.  The program which sets up and configures Guests
+ * (such as the example in Documentation/lguest/lguest.c) is called the
+ * Launcher.
+ *
+ * Secondly, we only run specially modified Guests, not normal kernels.  When
+ * you set CONFIG_LGUEST to 'y' or 'm', this automatically sets
+ * CONFIG_LGUEST_GUEST=y, which compiles this file into the kernel so it knows
+ * how to be a Guest.  This means that you can use the same kernel you boot
+ * normally (ie. as a Host) as a Guest.
+ *
+ * These Guests know that they cannot do privileged operations, such as disable
+ * interrupts, and that they have to ask the Host to do such things explicitly.
+ * This file consists of all the replacements for such low-level native
+ * hardware operations: these special Guest versions call the Host.
+ *
+ * So how does the kernel know it's a Guest?  The Guest starts at a special
+ * entry point marked with a magic string, which sets up a few things then
+ * calls here.  We replace the native functions various "paravirt" structures
+ * with our Guest versions, then boot like normal. :*/
+
+/*
+ * Copyright (C) 2006, Rusty Russell <rusty@rustcorp.com.au> IBM Corporation.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, GOOD TITLE or
+ * NON INFRINGEMENT.  See the GNU General Public License for more
+ * details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+#include <linux/kernel.h>
+#include <linux/start_kernel.h>
+#include <linux/string.h>
+#include <linux/console.h>
+#include <linux/screen_info.h>
+#include <linux/irq.h>
+#include <linux/interrupt.h>
+#include <linux/clocksource.h>
+#include <linux/clockchips.h>
+#include <linux/lguest.h>
+#include <linux/lguest_launcher.h>
+#include <linux/lguest_bus.h>
+#include <asm/paravirt.h>
+#include <asm/param.h>
+#include <asm/page.h>
+#include <asm/pgtable.h>
+#include <asm/desc.h>
+#include <asm/setup.h>
+#include <asm/e820.h>
+#include <asm/mce.h>
+#include <asm/io.h>
+
+/*G:010 Welcome to the Guest!
+ *
+ * The Guest in our tale is a simple creature: identical to the Host but
+ * behaving in simplified but equivalent ways.  In particular, the Guest is the
+ * same kernel as the Host (or at least, built from the same source code). :*/
+
+/* Declarations for definitions in lguest_guest.S */
+extern char lguest_noirq_start[], lguest_noirq_end[];
+extern const char lgstart_cli[], lgend_cli[];
+extern const char lgstart_sti[], lgend_sti[];
+extern const char lgstart_popf[], lgend_popf[];
+extern const char lgstart_pushf[], lgend_pushf[];
+extern const char lgstart_iret[], lgend_iret[];
+extern void lguest_iret(void);
+
+struct lguest_data lguest_data = {
+        .hcall_status = { [0 ... LHCALL_RING_SIZE-1] = 0xFF },
+        .noirq_start = (u32)lguest_noirq_start,
+        .noirq_end = (u32)lguest_noirq_end,
+        .blocked_interrupts = { 1 }, /* Block timer interrupts */
+};
+static cycle_t clock_base;
+
+/*G:035 Notice the lazy_hcall() above, rather than hcall().  This is our first
+ * real optimization trick!
+ *
+ * When lazy_mode is set, it means we're allowed to defer all hypercalls and do
+ * them as a batch when lazy_mode is eventually turned off.  Because hypercalls
+ * are reasonably expensive, batching them up makes sense.  For example, a
+ * large mmap might update dozens of page table entries: that code calls
+ * paravirt_enter_lazy_mmu(), does the dozen updates, then calls
+ * lguest_leave_lazy_mode().
+ *
+ * So, when we're in lazy mode, we call async_hypercall() to store the call for
+ * future processing.  When lazy mode is turned off we issue a hypercall to
+ * flush the stored calls.
+ */
+static void lguest_leave_lazy_mode(void)
+{
+        paravirt_leave_lazy(paravirt_get_lazy_mode());
+        hcall(LHCALL_FLUSH_ASYNC, 0, 0, 0);
+}
+
+static void lazy_hcall(unsigned long call,
+                       unsigned long arg1,
+                       unsigned long arg2,
+                       unsigned long arg3)
+{
+        if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_NONE)
+                hcall(call, arg1, arg2, arg3);
+        else
+                async_hcall(call, arg1, arg2, arg3);
+}
+
+/* async_hcall() is pretty simple: I'm quite proud of it really.  We have a
+ * ring buffer of stored hypercalls which the Host will run though next time we
+ * do a normal hypercall.  Each entry in the ring has 4 slots for the hypercall
+ * arguments, and a "hcall_status" word which is 0 if the call is ready to go,
+ * and 255 once the Host has finished with it.
+ *
+ * If we come around to a slot which hasn't been finished, then the table is
+ * full and we just make the hypercall directly.  This has the nice side
+ * effect of causing the Host to run all the stored calls in the ring buffer
+ * which empties it for next time! */
+void async_hcall(unsigned long call,
+                 unsigned long arg1, unsigned long arg2, unsigned long arg3)
+{
+        /* Note: This code assumes we're uniprocessor. */
+        static unsigned int next_call;
+        unsigned long flags;
+
+        /* Disable interrupts if not already disabled: we don't want an
+         * interrupt handler making a hypercall while we're already doing
+         * one! */
+        local_irq_save(flags);
+        if (lguest_data.hcall_status[next_call] != 0xFF) {
+                /* Table full, so do normal hcall which will flush table. */
+                hcall(call, arg1, arg2, arg3);
+        } else {
+                lguest_data.hcalls[next_call].eax = call;
+                lguest_data.hcalls[next_call].edx = arg1;
+                lguest_data.hcalls[next_call].ebx = arg2;
+                lguest_data.hcalls[next_call].ecx = arg3;
+                /* Arguments must all be written before we mark it to go */
+                wmb();
+                lguest_data.hcall_status[next_call] = 0;
+                if (++next_call == LHCALL_RING_SIZE)
+                        next_call = 0;
+        }
+        local_irq_restore(flags);
+}
+/*:*/
+
+/* Wrappers for the SEND_DMA and BIND_DMA hypercalls.  This is mainly because
+ * Jeff Garzik complained that __pa() should never appear in drivers, and this
+ * helps remove most of them.   But also, it wraps some ugliness. */
+void lguest_send_dma(unsigned long key, struct lguest_dma *dma)
+{
+        /* The hcall might not write this if something goes wrong */
+        dma->used_len = 0;
+        hcall(LHCALL_SEND_DMA, key, __pa(dma), 0);
+}
+
+int lguest_bind_dma(unsigned long key, struct lguest_dma *dmas,
+                    unsigned int num, u8 irq)
+{
+        /* This is the only hypercall which actually wants 5 arguments, and we
+         * only support 4.  Fortunately the interrupt number is always less
+         * than 256, so we can pack it with the number of dmas in the final
+         * argument.  */
+        if (!hcall(LHCALL_BIND_DMA, key, __pa(dmas), (num << 8) | irq))
+                return -ENOMEM;
+        return 0;
+}
+
+/* Unbinding is the same hypercall as binding, but with 0 num & irq. */
+void lguest_unbind_dma(unsigned long key, struct lguest_dma *dmas)
+{
+        hcall(LHCALL_BIND_DMA, key, __pa(dmas), 0);
+}
+
+/* For guests, device memory can be used as normal memory, so we cast away the
+ * __iomem to quieten sparse. */
+void *lguest_map(unsigned long phys_addr, unsigned long pages)
+{
+        return (__force void *)ioremap(phys_addr, PAGE_SIZE*pages);
+}
+
+void lguest_unmap(void *addr)
+{
+        iounmap((__force void __iomem *)addr);
+}
+
+/*G:033
+ * Here are our first native-instruction replacements: four functions for
+ * interrupt control.
+ *
+ * The simplest way of implementing these would be to have "turn interrupts
+ * off" and "turn interrupts on" hypercalls.  Unfortunately, this is too slow:
+ * these are by far the most commonly called functions of those we override.
+ *
+ * So instead we keep an "irq_enabled" field inside our "struct lguest_data",
+ * which the Guest can update with a single instruction.  The Host knows to
+ * check there when it wants to deliver an interrupt.
+ */
+
+/* save_flags() is expected to return the processor state (ie. "eflags").  The
+ * eflags word contains all kind of stuff, but in practice Linux only cares
+ * about the interrupt flag.  Our "save_flags()" just returns that. */
+static unsigned long save_fl(void)
+{
+        return lguest_data.irq_enabled;
+}
+
+/* "restore_flags" just sets the flags back to the value given. */
+static void restore_fl(unsigned long flags)
+{
+        lguest_data.irq_enabled = flags;
+}
+
+/* Interrupts go off... */
+static void irq_disable(void)
+{
+        lguest_data.irq_enabled = 0;
+}
+
+/* Interrupts go on... */
+static void irq_enable(void)
+{
+        lguest_data.irq_enabled = X86_EFLAGS_IF;
+}
+/*:*/
+/*M:003 Note that we don't check for outstanding interrupts when we re-enable
+ * them (or when we unmask an interrupt).  This seems to work for the moment,
+ * since interrupts are rare and we'll just get the interrupt on the next timer
+ * tick, but when we turn on CONFIG_NO_HZ, we should revisit this.  One way
+ * would be to put the "irq_enabled" field in a page by itself, and have the
+ * Host write-protect it when an interrupt comes in when irqs are disabled.
+ * There will then be a page fault as soon as interrupts are re-enabled. :*/
+
+/*G:034
+ * The Interrupt Descriptor Table (IDT).
+ *
+ * The IDT tells the processor what to do when an interrupt comes in.  Each
+ * entry in the table is a 64-bit descriptor: this holds the privilege level,
+ * address of the handler, and... well, who cares?  The Guest just asks the
+ * Host to make the change anyway, because the Host controls the real IDT.
+ */
+static void lguest_write_idt_entry(struct desc_struct *dt,
+                                   int entrynum, u32 low, u32 high)
+{
+        /* Keep the local copy up to date. */
+        write_dt_entry(dt, entrynum, low, high);
+        /* Tell Host about this new entry. */
+        hcall(LHCALL_LOAD_IDT_ENTRY, entrynum, low, high);
+}
+
+/* Changing to a different IDT is very rare: we keep the IDT up-to-date every
+ * time it is written, so we can simply loop through all entries and tell the
+ * Host about them. */
+static void lguest_load_idt(const struct Xgt_desc_struct *desc)
+{
+        unsigned int i;
+        struct desc_struct *idt = (void *)desc->address;
+
+        for (i = 0; i < (desc->size+1)/8; i++)
+                hcall(LHCALL_LOAD_IDT_ENTRY, i, idt[i].a, idt[i].b);
+}
+
+/*
+ * The Global Descriptor Table.
+ *
+ * The Intel architecture defines another table, called the Global Descriptor
+ * Table (GDT).  You tell the CPU where it is (and its size) using the "lgdt"
+ * instruction, and then several other instructions refer to entries in the
+ * table.  There are three entries which the Switcher needs, so the Host simply
+ * controls the entire thing and the Guest asks it to make changes using the
+ * LOAD_GDT hypercall.
+ *
+ * This is the opposite of the IDT code where we have a LOAD_IDT_ENTRY
+ * hypercall and use that repeatedly to load a new IDT.  I don't think it
+ * really matters, but wouldn't it be nice if they were the same?
+ */
+static void lguest_load_gdt(const struct Xgt_desc_struct *desc)
+{
+        BUG_ON((desc->size+1)/8 != GDT_ENTRIES);
+        hcall(LHCALL_LOAD_GDT, __pa(desc->address), GDT_ENTRIES, 0);
+}
+
+/* For a single GDT entry which changes, we do the lazy thing: alter our GDT,
+ * then tell the Host to reload the entire thing.  This operation is so rare
+ * that this naive implementation is reasonable. */
+static void lguest_write_gdt_entry(struct desc_struct *dt,
+                                   int entrynum, u32 low, u32 high)
+{
+        write_dt_entry(dt, entrynum, low, high);
+        hcall(LHCALL_LOAD_GDT, __pa(dt), GDT_ENTRIES, 0);
+}
+
+/* OK, I lied.  There are three "thread local storage" GDT entries which change
+ * on every context switch (these three entries are how glibc implements
+ * __thread variables).  So we have a hypercall specifically for this case. */
+static void lguest_load_tls(struct thread_struct *t, unsigned int cpu)
+{
+        /* There's one problem which normal hardware doesn't have: the Host
+         * can't handle us removing entries we're currently using.  So we clear
+         * the GS register here: if it's needed it'll be reloaded anyway. */
+        loadsegment(gs, 0);
+        lazy_hcall(LHCALL_LOAD_TLS, __pa(&t->tls_array), cpu, 0);
+}
+
+/*G:038 That's enough excitement for now, back to ploughing through each of
+ * the different pv_ops structures (we're about 1/3 of the way through).
+ *
+ * This is the Local Descriptor Table, another weird Intel thingy.  Linux only
+ * uses this for some strange applications like Wine.  We don't do anything
+ * here, so they'll get an informative and friendly Segmentation Fault. */
+static void lguest_set_ldt(const void *addr, unsigned entries)
+{
+}
+
+/* This loads a GDT entry into the "Task Register": that entry points to a
+ * structure called the Task State Segment.  Some comments scattered though the
+ * kernel code indicate that this used for task switching in ages past, along
+ * with blood sacrifice and astrology.
+ *
+ * Now there's nothing interesting in here that we don't get told elsewhere.
+ * But the native version uses the "ltr" instruction, which makes the Host
+ * complain to the Guest about a Segmentation Fault and it'll oops.  So we
+ * override the native version with a do-nothing version. */
+static void lguest_load_tr_desc(void)
+{
+}
+
+/* The "cpuid" instruction is a way of querying both the CPU identity
+ * (manufacturer, model, etc) and its features.  It was introduced before the
+ * Pentium in 1993 and keeps getting extended by both Intel and AMD.  As you
+ * might imagine, after a decade and a half this treatment, it is now a giant
+ * ball of hair.  Its entry in the current Intel manual runs to 28 pages.
+ *
+ * This instruction even it has its own Wikipedia entry.  The Wikipedia entry
+ * has been translated into 4 languages.  I am not making this up!
+ *
+ * We could get funky here and identify ourselves as "GenuineLguest", but
+ * instead we just use the real "cpuid" instruction.  Then I pretty much turned
+ * off feature bits until the Guest booted.  (Don't say that: you'll damage
+ * lguest sales!)  Shut up, inner voice!  (Hey, just pointing out that this is
+ * hardly future proof.)  Noone's listening!  They don't like you anyway,
+ * parenthetic weirdo!
+ *
+ * Replacing the cpuid so we can turn features off is great for the kernel, but
+ * anyone (including userspace) can just use the raw "cpuid" instruction and
+ * the Host won't even notice since it isn't privileged.  So we try not to get
+ * too worked up about it. */
+static void lguest_cpuid(unsigned int *eax, unsigned int *ebx,
+                         unsigned int *ecx, unsigned int *edx)
+{
+        int function = *eax;
+
+        native_cpuid(eax, ebx, ecx, edx);
+        switch (function) {
+        case 1: /* Basic feature request. */
+                /* We only allow kernel to see SSE3, CMPXCHG16B and SSSE3 */
+                *ecx &= 0x00002201;
+                /* SSE, SSE2, FXSR, MMX, CMOV, CMPXCHG8B, FPU. */
+                *edx &= 0x07808101;
+                /* The Host can do a nice optimization if it knows that the
+                 * kernel mappings (addresses above 0xC0000000 or whatever
+                 * PAGE_OFFSET is set to) haven't changed.  But Linux calls
+                 * flush_tlb_user() for both user and kernel mappings unless
+                 * the Page Global Enable (PGE) feature bit is set. */
+                *edx |= 0x00002000;
+                break;
+        case 0x80000000:
+                /* Futureproof this a little: if they ask how much extended
+                 * processor information there is, limit it to known fields. */
+                if (*eax > 0x80000008)
+                        *eax = 0x80000008;
+                break;
+        }
+}
+
+/* Intel has four control registers, imaginatively named cr0, cr2, cr3 and cr4.
+ * I assume there's a cr1, but it hasn't bothered us yet, so we'll not bother
+ * it.  The Host needs to know when the Guest wants to change them, so we have
+ * a whole series of functions like read_cr0() and write_cr0().
+ *
+ * We start with CR0.  CR0 allows you to turn on and off all kinds of basic
+ * features, but Linux only really cares about one: the horrifically-named Task
+ * Switched (TS) bit at bit 3 (ie. 8)
+ *
+ * What does the TS bit do?  Well, it causes the CPU to trap (interrupt 7) if
+ * the floating point unit is used.  Which allows us to restore FPU state
+ * lazily after a task switch, and Linux uses that gratefully, but wouldn't a
+ * name like "FPUTRAP bit" be a little less cryptic?
+ *
+ * We store cr0 (and cr3) locally, because the Host never changes it.  The
+ * Guest sometimes wants to read it and we'd prefer not to bother the Host
+ * unnecessarily. */
+static unsigned long current_cr0, current_cr3;
+static void lguest_write_cr0(unsigned long val)
+{
+        /* 8 == TS bit. */
+        lazy_hcall(LHCALL_TS, val & 8, 0, 0);
+        current_cr0 = val;
+}
+
+static unsigned long lguest_read_cr0(void)
+{
+        return current_cr0;
+}
+
+/* Intel provided a special instruction to clear the TS bit for people too cool
+ * to use write_cr0() to do it.  This "clts" instruction is faster, because all
+ * the vowels have been optimized out. */
+static void lguest_clts(void)
+{
+        lazy_hcall(LHCALL_TS, 0, 0, 0);
+        current_cr0 &= ~8U;
+}
+
+/* CR2 is the virtual address of the last page fault, which the Guest only ever
+ * reads.  The Host kindly writes this into our "struct lguest_data", so we
+ * just read it out of there. */
+static unsigned long lguest_read_cr2(void)
+{
+        return lguest_data.cr2;
+}
+
+/* CR3 is the current toplevel pagetable page: the principle is the same as
+ * cr0.  Keep a local copy, and tell the Host when it changes. */
+static void lguest_write_cr3(unsigned long cr3)
+{
+        lazy_hcall(LHCALL_NEW_PGTABLE, cr3, 0, 0);
+        current_cr3 = cr3;
+}
+
+static unsigned long lguest_read_cr3(void)
+{
+        return current_cr3;
+}
+
+/* CR4 is used to enable and disable PGE, but we don't care. */
+static unsigned long lguest_read_cr4(void)
+{
+        return 0;
+}
+
+static void lguest_write_cr4(unsigned long val)
+{
+}
+
+/*
+ * Page Table Handling.
+ *
+ * Now would be a good time to take a rest and grab a coffee or similarly
+ * relaxing stimulant.  The easy parts are behind us, and the trek gradually
+ * winds uphill from here.
+ *
+ * Quick refresher: memory is divided into "pages" of 4096 bytes each.  The CPU
+ * maps virtual addresses to physical addresses using "page tables".  We could
+ * use one huge index of 1 million entries: each address is 4 bytes, so that's
+ * 1024 pages just to hold the page tables.   But since most virtual addresses
+ * are unused, we use a two level index which saves space.  The CR3 register
+ * contains the physical address of the top level "page directory" page, which
+ * contains physical addresses of up to 1024 second-level pages.  Each of these
+ * second level pages contains up to 1024 physical addresses of actual pages,
+ * or Page Table Entries (PTEs).
+ *
+ * Here's a diagram, where arrows indicate physical addresses:
+ *
+ * CR3 ---> +---------+
+ *          |      --------->+---------+
+ *          |         |      | PADDR1  |
+ *        Top-level   |      | PADDR2  |
+ *        (PMD) page  |      |         |
+ *          |         |    Lower-level |
+ *          |         |    (PTE) page  |
+ *          |         |      |         |
+ *            ....               ....
+ *
+ * So to convert a virtual address to a physical address, we look up the top
+ * level, which points us to the second level, which gives us the physical
+ * address of that page.  If the top level entry was not present, or the second
+ * level entry was not present, then the virtual address is invalid (we
+ * say "the page was not mapped").
+ *
+ * Put another way, a 32-bit virtual address is divided up like so:
+ *
+ *  1 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
+ * |<---- 10 bits ---->|<---- 10 bits ---->|<------ 12 bits ------>|
+ *    Index into top     Index into second      Offset within page
+ *  page directory page    pagetable page
+ *
+ * The kernel spends a lot of time changing both the top-level page directory
+ * and lower-level pagetable pages.  The Guest doesn't know physical addresses,
+ * so while it maintains these page tables exactly like normal, it also needs
+ * to keep the Host informed whenever it makes a change: the Host will create
+ * the real page tables based on the Guests'.
+ */
+
+/* The Guest calls this to set a second-level entry (pte), ie. to map a page
+ * into a process' address space.  We set the entry then tell the Host the
+ * toplevel and address this corresponds to.  The Guest uses one pagetable per
+ * process, so we need to tell the Host which one we're changing (mm->pgd). */
+static void lguest_set_pte_at(struct mm_struct *mm, unsigned long addr,
+                              pte_t *ptep, pte_t pteval)
+{
+        *ptep = pteval;
+        lazy_hcall(LHCALL_SET_PTE, __pa(mm->pgd), addr, pteval.pte_low);
+}
+
+/* The Guest calls this to set a top-level entry.  Again, we set the entry then
+ * tell the Host which top-level page we changed, and the index of the entry we
+ * changed. */
+static void lguest_set_pmd(pmd_t *pmdp, pmd_t pmdval)
+{
+        *pmdp = pmdval;
+        lazy_hcall(LHCALL_SET_PMD, __pa(pmdp)&PAGE_MASK,
+                   (__pa(pmdp)&(PAGE_SIZE-1))/4, 0);
+}
+
+/* There are a couple of legacy places where the kernel sets a PTE, but we
+ * don't know the top level any more.  This is useless for us, since we don't
+ * know which pagetable is changing or what address, so we just tell the Host
+ * to forget all of them.  Fortunately, this is very rare.
+ *
+ * ... except in early boot when the kernel sets up the initial pagetables,
+ * which makes booting astonishingly slow.  So we don't even tell the Host
+ * anything changed until we've done the first page table switch.
+ */
+static void lguest_set_pte(pte_t *ptep, pte_t pteval)
+{
+        *ptep = pteval;
+        /* Don't bother with hypercall before initial setup. */
+        if (current_cr3)
+                lazy_hcall(LHCALL_FLUSH_TLB, 1, 0, 0);
+}
+
+/* Unfortunately for Lguest, the pv_mmu_ops for page tables were based on
+ * native page table operations.  On native hardware you can set a new page
+ * table entry whenever you want, but if you want to remove one you have to do
+ * a TLB flush (a TLB is a little cache of page table entries kept by the CPU).
+ *
+ * So the lguest_set_pte_at() and lguest_set_pmd() functions above are only
+ * called when a valid entry is written, not when it's removed (ie. marked not
+ * present).  Instead, this is where we come when the Guest wants to remove a
+ * page table entry: we tell the Host to set that entry to 0 (ie. the present
+ * bit is zero). */
+static void lguest_flush_tlb_single(unsigned long addr)
+{
+        /* Simply set it to zero: if it was not, it will fault back in. */
+        lazy_hcall(LHCALL_SET_PTE, current_cr3, addr, 0);
+}
+
+/* This is what happens after the Guest has removed a large number of entries.
+ * This tells the Host that any of the page table entries for userspace might
+ * have changed, ie. virtual addresses below PAGE_OFFSET. */
+static void lguest_flush_tlb_user(void)
+{
+        lazy_hcall(LHCALL_FLUSH_TLB, 0, 0, 0);
+}
+
+/* This is called when the kernel page tables have changed.  That's not very
+ * common (unless the Guest is using highmem, which makes the Guest extremely
+ * slow), so it's worth separating this from the user flushing above. */
+static void lguest_flush_tlb_kernel(void)
+{
+        lazy_hcall(LHCALL_FLUSH_TLB, 1, 0, 0);
+}
+
+/*
+ * The Unadvanced Programmable Interrupt Controller.
+ *
+ * This is an attempt to implement the simplest possible interrupt controller.
+ * I spent some time looking though routines like set_irq_chip_and_handler,
+ * set_irq_chip_and_handler_name, set_irq_chip_data and set_phasers_to_stun and
+ * I *think* this is as simple as it gets.
+ *
+ * We can tell the Host what interrupts we want blocked ready for using the
+ * lguest_data.interrupts bitmap, so disabling (aka "masking") them is as
+ * simple as setting a bit.  We don't actually "ack" interrupts as such, we
+ * just mask and unmask them.  I wonder if we should be cleverer?
+ */
+static void disable_lguest_irq(unsigned int irq)
+{
+        set_bit(irq, lguest_data.blocked_interrupts);
+}
+
+static void enable_lguest_irq(unsigned int irq)
+{
+        clear_bit(irq, lguest_data.blocked_interrupts);
+}
+
+/* This structure describes the lguest IRQ controller. */
+static struct irq_chip lguest_irq_controller = {
+        .name           = "lguest",
+        .mask           = disable_lguest_irq,
+        .mask_ack       = disable_lguest_irq,
+        .unmask         = enable_lguest_irq,
+};
+
+/* This sets up the Interrupt Descriptor Table (IDT) entry for each hardware
+ * interrupt (except 128, which is used for system calls), and then tells the
+ * Linux infrastructure that each interrupt is controlled by our level-based
+ * lguest interrupt controller. */
+static void __init lguest_init_IRQ(void)
+{
+        unsigned int i;
+
+        for (i = 0; i < LGUEST_IRQS; i++) {
+                int vector = FIRST_EXTERNAL_VECTOR + i;
+                if (vector != SYSCALL_VECTOR) {
+                        set_intr_gate(vector, interrupt[i]);
+                        set_irq_chip_and_handler(i, &lguest_irq_controller,
+                                                 handle_level_irq);
+                }
+        }
+        /* This call is required to set up for 4k stacks, where we have
+         * separate stacks for hard and soft interrupts. */
+        irq_ctx_init(smp_processor_id());
+}
+
+/*
+ * Time.
+ *
+ * It would be far better for everyone if the Guest had its own clock, but
+ * until then the Host gives us the time on every interrupt.
+ */
+static unsigned long lguest_get_wallclock(void)
+{
+        return lguest_data.time.tv_sec;
+}
+
+static cycle_t lguest_clock_read(void)
+{
+        unsigned long sec, nsec;
+
+        /* If the Host tells the TSC speed, we can trust that. */
+        if (lguest_data.tsc_khz)
+                return native_read_tsc();
+
+        /* If we can't use the TSC, we read the time value written by the Host.
+         * Since it's in two parts (seconds and nanoseconds), we risk reading
+         * it just as it's changing from 99 & 0.999999999 to 100 and 0, and
+         * getting 99 and 0.  As Linux tends to come apart under the stress of
+         * time travel, we must be careful: */
+        do {
+                /* First we read the seconds part. */
+                sec = lguest_data.time.tv_sec;
+                /* This read memory barrier tells the compiler and the CPU that
+                 * this can't be reordered: we have to complete the above
+                 * before going on. */
+                rmb();
+                /* Now we read the nanoseconds part. */
+                nsec = lguest_data.time.tv_nsec;
+                /* Make sure we've done that. */
+                rmb();
+                /* Now if the seconds part has changed, try again. */
+        } while (unlikely(lguest_data.time.tv_sec != sec));
+
+        /* Our non-TSC clock is in real nanoseconds. */
+        return sec*1000000000ULL + nsec;
+}
+
+/* This is what we tell the kernel is our clocksource.  */
+static struct clocksource lguest_clock = {
+        .name           = "lguest",
+        .rating         = 400,
+        .read           = lguest_clock_read,
+        .mask           = CLOCKSOURCE_MASK(64),
+        .mult           = 1 << 22,
+        .shift          = 22,
+        .flags          = CLOCK_SOURCE_IS_CONTINUOUS,
+};
+
+/* The "scheduler clock" is just our real clock, adjusted to start at zero */
+static unsigned long long lguest_sched_clock(void)
+{
+        return cyc2ns(&lguest_clock, lguest_clock_read() - clock_base);
+}
+
+/* We also need a "struct clock_event_device": Linux asks us to set it to go
+ * off some time in the future.  Actually, James Morris figured all this out, I
+ * just applied the patch. */
+static int lguest_clockevent_set_next_event(unsigned long delta,
+                                           struct clock_event_device *evt)
+{
+        if (delta < LG_CLOCK_MIN_DELTA) {
+                if (printk_ratelimit())
+                        printk(KERN_DEBUG "%s: small delta %lu ns\n",
+                               __FUNCTION__, delta);
+                return -ETIME;
+        }
+        hcall(LHCALL_SET_CLOCKEVENT, delta, 0, 0);
+        return 0;
+}
+
+static void lguest_clockevent_set_mode(enum clock_event_mode mode,
+                                      struct clock_event_device *evt)
+{
+        switch (mode) {
+        case CLOCK_EVT_MODE_UNUSED:
+        case CLOCK_EVT_MODE_SHUTDOWN:
+                /* A 0 argument shuts the clock down. */
+                hcall(LHCALL_SET_CLOCKEVENT, 0, 0, 0);
+                break;
+        case CLOCK_EVT_MODE_ONESHOT:
+                /* This is what we expect. */
+                break;
+        case CLOCK_EVT_MODE_PERIODIC:
+                BUG();
+        case CLOCK_EVT_MODE_RESUME:
+                break;
+        }
+}
+
+/* This describes our primitive timer chip. */
+static struct clock_event_device lguest_clockevent = {
+        .name                   = "lguest",
+        .features               = CLOCK_EVT_FEAT_ONESHOT,
+        .set_next_event         = lguest_clockevent_set_next_event,
+        .set_mode               = lguest_clockevent_set_mode,
+        .rating                 = INT_MAX,
+        .mult                   = 1,
+        .shift                  = 0,
+        .min_delta_ns           = LG_CLOCK_MIN_DELTA,
+        .max_delta_ns           = LG_CLOCK_MAX_DELTA,
+};
+
+/* This is the Guest timer interrupt handler (hardware interrupt 0).  We just
+ * call the clockevent infrastructure and it does whatever needs doing. */
+static void lguest_time_irq(unsigned int irq, struct irq_desc *desc)
+{
+        unsigned long flags;
+
+        /* Don't interrupt us while this is running. */
+        local_irq_save(flags);
+        lguest_clockevent.event_handler(&lguest_clockevent);
+        local_irq_restore(flags);
+}
+
+/* At some point in the boot process, we get asked to set up our timing
+ * infrastructure.  The kernel doesn't expect timer interrupts before this, but
+ * we cleverly initialized the "blocked_interrupts" field of "struct
+ * lguest_data" so that timer interrupts were blocked until now. */
+static void lguest_time_init(void)
+{
+        /* Set up the timer interrupt (0) to go to our simple timer routine */
+        set_irq_handler(0, lguest_time_irq);
+
+        /* Our clock structure look like arch/i386/kernel/tsc.c if we can use
+         * the TSC, otherwise it's a dumb nanosecond-resolution clock.  Either
+         * way, the "rating" is initialized so high that it's always chosen
+         * over any other clocksource. */
+        if (lguest_data.tsc_khz)
+                lguest_clock.mult = clocksource_khz2mult(lguest_data.tsc_khz,
+                                                         lguest_clock.shift);
+        clock_base = lguest_clock_read();
+        clocksource_register(&lguest_clock);
+
+        /* Now we've set up our clock, we can use it as the scheduler clock */
+        pv_time_ops.sched_clock = lguest_sched_clock;
+
+        /* We can't set cpumask in the initializer: damn C limitations!  Set it
+         * here and register our timer device. */
+        lguest_clockevent.cpumask = cpumask_of_cpu(0);
+        clockevents_register_device(&lguest_clockevent);
+
+        /* Finally, we unblock the timer interrupt. */
+        enable_lguest_irq(0);
+}
+
+/*
+ * Miscellaneous bits and pieces.
+ *
+ * Here is an oddball collection of functions which the Guest needs for things
+ * to work.  They're pretty simple.
+ */
+
+/* The Guest needs to tell the host what stack it expects traps to use.  For
+ * native hardware, this is part of the Task State Segment mentioned above in
+ * lguest_load_tr_desc(), but to help hypervisors there's this special call.
+ *
+ * We tell the Host the segment we want to use (__KERNEL_DS is the kernel data
+ * segment), the privilege level (we're privilege level 1, the Host is 0 and
+ * will not tolerate us trying to use that), the stack pointer, and the number
+ * of pages in the stack. */
+static void lguest_load_esp0(struct tss_struct *tss,
+                                     struct thread_struct *thread)
+{
+        lazy_hcall(LHCALL_SET_STACK, __KERNEL_DS|0x1, thread->esp0,
+                   THREAD_SIZE/PAGE_SIZE);
+}
+
+/* Let's just say, I wouldn't do debugging under a Guest. */
+static void lguest_set_debugreg(int regno, unsigned long value)
+{
+        /* FIXME: Implement */
+}
+
+/* There are times when the kernel wants to make sure that no memory writes are
+ * caught in the cache (that they've all reached real hardware devices).  This
+ * doesn't matter for the Guest which has virtual hardware.
+ *
+ * On the Pentium 4 and above, cpuid() indicates that the Cache Line Flush
+ * (clflush) instruction is available and the kernel uses that.  Otherwise, it
+ * uses the older "Write Back and Invalidate Cache" (wbinvd) instruction.
+ * Unlike clflush, wbinvd can only be run at privilege level 0.  So we can
+ * ignore clflush, but replace wbinvd.
+ */
+static void lguest_wbinvd(void)
+{
+}
+
+/* If the Guest expects to have an Advanced Programmable Interrupt Controller,
+ * we play dumb by ignoring writes and returning 0 for reads.  So it's no
+ * longer Programmable nor Controlling anything, and I don't think 8 lines of
+ * code qualifies for Advanced.  It will also never interrupt anything.  It
+ * does, however, allow us to get through the Linux boot code. */
+#ifdef CONFIG_X86_LOCAL_APIC
+static void lguest_apic_write(unsigned long reg, unsigned long v)
+{
+}
+
+static unsigned long lguest_apic_read(unsigned long reg)
+{
+        return 0;
+}
+#endif
+
+/* STOP!  Until an interrupt comes in. */
+static void lguest_safe_halt(void)
+{
+        hcall(LHCALL_HALT, 0, 0, 0);
+}
+
+/* Perhaps CRASH isn't the best name for this hypercall, but we use it to get a
+ * message out when we're crashing as well as elegant termination like powering
+ * off.
+ *
+ * Note that the Host always prefers that the Guest speak in physical addresses
+ * rather than virtual addresses, so we use __pa() here. */
+static void lguest_power_off(void)
+{
+        hcall(LHCALL_CRASH, __pa("Power down"), 0, 0);
+}
+
+/*
+ * Panicing.
+ *
+ * Don't.  But if you did, this is what happens.
+ */
+static int lguest_panic(struct notifier_block *nb, unsigned long l, void *p)
+{
+        hcall(LHCALL_CRASH, __pa(p), 0, 0);
+        /* The hcall won't return, but to keep gcc happy, we're "done". */
+        return NOTIFY_DONE;
+}
+
+static struct notifier_block paniced = {
+        .notifier_call = lguest_panic
+};
+
+/* Setting up memory is fairly easy. */
+static __init char *lguest_memory_setup(void)
+{
+        /* We do this here and not earlier because lockcheck barfs if we do it
+         * before start_kernel() */
+        atomic_notifier_chain_register(&panic_notifier_list, &paniced);
+
+        /* The Linux bootloader header contains an "e820" memory map: the
+         * Launcher populated the first entry with our memory limit. */
+        add_memory_region(boot_params.e820_map[0].addr,
+                          boot_params.e820_map[0].size,
+                          boot_params.e820_map[0].type);
+
+        /* This string is for the boot messages. */
+        return "LGUEST";
+}
+
+/*G:050
+ * Patching (Powerfully Placating Performance Pedants)
+ *
+ * We have already seen that pv_ops structures let us replace simple
+ * native instructions with calls to the appropriate back end all throughout
+ * the kernel.  This allows the same kernel to run as a Guest and as a native
+ * kernel, but it's slow because of all the indirect branches.
+ *
+ * Remember that David Wheeler quote about "Any problem in computer science can
+ * be solved with another layer of indirection"?  The rest of that quote is
+ * "... But that usually will create another problem."  This is the first of
+ * those problems.
+ *
+ * Our current solution is to allow the paravirt back end to optionally patch
+ * over the indirect calls to replace them with something more efficient.  We
+ * patch the four most commonly called functions: disable interrupts, enable
+ * interrupts, restore interrupts and save interrupts.  We usually have 10
+ * bytes to patch into: the Guest versions of these operations are small enough
+ * that we can fit comfortably.
+ *
+ * First we need assembly templates of each of the patchable Guest operations,
+ * and these are in lguest_asm.S. */
+
+/*G:060 We construct a table from the assembler templates: */
+static const struct lguest_insns
+{
+        const char *start, *end;
+} lguest_insns[] = {
+        [PARAVIRT_PATCH(pv_irq_ops.irq_disable)] = { lgstart_cli, lgend_cli },
+        [PARAVIRT_PATCH(pv_irq_ops.irq_enable)] = { lgstart_sti, lgend_sti },
+        [PARAVIRT_PATCH(pv_irq_ops.restore_fl)] = { lgstart_popf, lgend_popf },
+        [PARAVIRT_PATCH(pv_irq_ops.save_fl)] = { lgstart_pushf, lgend_pushf },
+};
+
+/* Now our patch routine is fairly simple (based on the native one in
+ * paravirt.c).  If we have a replacement, we copy it in and return how much of
+ * the available space we used. */
+static unsigned lguest_patch(u8 type, u16 clobber, void *ibuf,
+                             unsigned long addr, unsigned len)
+{
+        unsigned int insn_len;
+
+        /* Don't do anything special if we don't have a replacement */
+        if (type >= ARRAY_SIZE(lguest_insns) || !lguest_insns[type].start)
+                return paravirt_patch_default(type, clobber, ibuf, addr, len);
+
+        insn_len = lguest_insns[type].end - lguest_insns[type].start;
+
+        /* Similarly if we can't fit replacement (shouldn't happen, but let's
+         * be thorough). */
+        if (len < insn_len)
+                return paravirt_patch_default(type, clobber, ibuf, addr, len);
+
+        /* Copy in our instructions. */
+        memcpy(ibuf, lguest_insns[type].start, insn_len);
+        return insn_len;
+}
+
+/*G:030 Once we get to lguest_init(), we know we're a Guest.  The pv_ops
+ * structures in the kernel provide points for (almost) every routine we have
+ * to override to avoid privileged instructions. */
+__init void lguest_init(void *boot)
+{
+        /* Copy boot parameters first: the Launcher put the physical location
+         * in %esi, and head.S converted that to a virtual address and handed
+         * it to us.  We use "__memcpy" because "memcpy" sometimes tries to do
+         * tricky things to go faster, and we're not ready for that. */
+        __memcpy(&boot_params, boot, PARAM_SIZE);
+        /* The boot parameters also tell us where the command-line is: save
+         * that, too. */
+        __memcpy(boot_command_line, __va(boot_params.hdr.cmd_line_ptr),
+               COMMAND_LINE_SIZE);
+
+        /* We're under lguest, paravirt is enabled, and we're running at
+         * privilege level 1, not 0 as normal. */
+        pv_info.name = "lguest";
+        pv_info.paravirt_enabled = 1;
+        pv_info.kernel_rpl = 1;
+
+        /* We set up all the lguest overrides for sensitive operations.  These
+         * are detailed with the operations themselves. */
+
+        /* interrupt-related operations */
+        pv_irq_ops.init_IRQ = lguest_init_IRQ;
+        pv_irq_ops.save_fl = save_fl;
+        pv_irq_ops.restore_fl = restore_fl;
+        pv_irq_ops.irq_disable = irq_disable;
+        pv_irq_ops.irq_enable = irq_enable;
+        pv_irq_ops.safe_halt = lguest_safe_halt;
+
+        /* init-time operations */
+        pv_init_ops.memory_setup = lguest_memory_setup;
+        pv_init_ops.patch = lguest_patch;
+
+        /* Intercepts of various cpu instructions */
+        pv_cpu_ops.load_gdt = lguest_load_gdt;
+        pv_cpu_ops.cpuid = lguest_cpuid;
+        pv_cpu_ops.load_idt = lguest_load_idt;
+        pv_cpu_ops.iret = lguest_iret;
+        pv_cpu_ops.load_esp0 = lguest_load_esp0;
+        pv_cpu_ops.load_tr_desc = lguest_load_tr_desc;
+        pv_cpu_ops.set_ldt = lguest_set_ldt;
+        pv_cpu_ops.load_tls = lguest_load_tls;
+        pv_cpu_ops.set_debugreg = lguest_set_debugreg;
+        pv_cpu_ops.clts = lguest_clts;
+        pv_cpu_ops.read_cr0 = lguest_read_cr0;
+        pv_cpu_ops.write_cr0 = lguest_write_cr0;
+        pv_cpu_ops.read_cr4 = lguest_read_cr4;
+        pv_cpu_ops.write_cr4 = lguest_write_cr4;
+        pv_cpu_ops.write_gdt_entry = lguest_write_gdt_entry;
+        pv_cpu_ops.write_idt_entry = lguest_write_idt_entry;
+        pv_cpu_ops.wbinvd = lguest_wbinvd;
+        pv_cpu_ops.lazy_mode.enter = paravirt_enter_lazy_cpu;
+        pv_cpu_ops.lazy_mode.leave = lguest_leave_lazy_mode;
+
+        /* pagetable management */
+        pv_mmu_ops.write_cr3 = lguest_write_cr3;
+        pv_mmu_ops.flush_tlb_user = lguest_flush_tlb_user;
+        pv_mmu_ops.flush_tlb_single = lguest_flush_tlb_single;
+        pv_mmu_ops.flush_tlb_kernel = lguest_flush_tlb_kernel;
+        pv_mmu_ops.set_pte = lguest_set_pte;
+        pv_mmu_ops.set_pte_at = lguest_set_pte_at;
+        pv_mmu_ops.set_pmd = lguest_set_pmd;
+        pv_mmu_ops.read_cr2 = lguest_read_cr2;
+        pv_mmu_ops.read_cr3 = lguest_read_cr3;
+        pv_mmu_ops.lazy_mode.enter = paravirt_enter_lazy_mmu;
+        pv_mmu_ops.lazy_mode.leave = lguest_leave_lazy_mode;
+
+#ifdef CONFIG_X86_LOCAL_APIC
+        /* apic read/write intercepts */
+        pv_apic_ops.apic_write = lguest_apic_write;
+        pv_apic_ops.apic_write_atomic = lguest_apic_write;
+        pv_apic_ops.apic_read = lguest_apic_read;
+#endif
+
+        /* time operations */
+        pv_time_ops.get_wallclock = lguest_get_wallclock;
+        pv_time_ops.time_init = lguest_time_init;
+
+        /* Now is a good time to look at the implementations of these functions
+         * before returning to the rest of lguest_init(). */
+
+        /*G:070 Now we've seen all the paravirt_ops, we return to
+         * lguest_init() where the rest of the fairly chaotic boot setup
+         * occurs.
+         *
+         * The Host expects our first hypercall to tell it where our "struct
+         * lguest_data" is, so we do that first. */
+        hcall(LHCALL_LGUEST_INIT, __pa(&lguest_data), 0, 0);
+
+        /* The native boot code sets up initial page tables immediately after
+         * the kernel itself, and sets init_pg_tables_end so they're not
+         * clobbered.  The Launcher places our initial pagetables somewhere at
+         * the top of our physical memory, so we don't need extra space: set
+         * init_pg_tables_end to the end of the kernel. */
+        init_pg_tables_end = __pa(pg0);
+
+        /* Load the %fs segment register (the per-cpu segment register) with
+         * the normal data segment to get through booting. */
+        asm volatile ("mov %0, %%fs" : : "r" (__KERNEL_DS) : "memory");
+
+        /* Clear the part of the kernel data which is expected to be zero.
+         * Normally it will be anyway, but if we're loading from a bzImage with
+         * CONFIG_RELOCATALE=y, the relocations will be sitting here. */
+        memset(__bss_start, 0, __bss_stop - __bss_start);
+
+        /* The Host uses the top of the Guest's virtual address space for the
+         * Host<->Guest Switcher, and it tells us how much it needs in
+         * lguest_data.reserve_mem, set up on the LGUEST_INIT hypercall. */
+        reserve_top_address(lguest_data.reserve_mem);
+
+        /* If we don't initialize the lock dependency checker now, it crashes
+         * paravirt_disable_iospace. */
+        lockdep_init();
+
+        /* The IDE code spends about 3 seconds probing for disks: if we reserve
+         * all the I/O ports up front it can't get them and so doesn't probe.
+         * Other device drivers are similar (but less severe).  This cuts the
+         * kernel boot time on my machine from 4.1 seconds to 0.45 seconds. */
+        paravirt_disable_iospace();
+
+        /* This is messy CPU setup stuff which the native boot code does before
+         * start_kernel, so we have to do, too: */
+        cpu_detect(&new_cpu_data);
+        /* head.S usually sets up the first capability word, so do it here. */
+        new_cpu_data.x86_capability[0] = cpuid_edx(1);
+
+        /* Math is always hard! */
+        new_cpu_data.hard_math = 1;
+
+#ifdef CONFIG_X86_MCE
+        mce_disabled = 1;
+#endif
+#ifdef CONFIG_ACPI
+        acpi_disabled = 1;
+        acpi_ht = 0;
+#endif
+
+        /* We set the perferred console to "hvc".  This is the "hypervisor
+         * virtual console" driver written by the PowerPC people, which we also
+         * adapted for lguest's use. */
+        add_preferred_console("hvc", 0, NULL);
+
+        /* Last of all, we set the power management poweroff hook to point to
+         * the Guest routine to power off. */
+        pm_power_off = lguest_power_off;
+
+        /* Now we're set up, call start_kernel() in init/main.c and we proceed
+         * to boot as normal.  It never returns. */
+        start_kernel();
+}
+/*
+ * This marks the end of stage II of our journey, The Guest.
+ *
+ * It is now time for us to explore the nooks and crannies of the three Guest
+ * devices and complete our understanding of the Guest in "make Drivers".
+ */
diff --git a/arch/x86/lguest/i386_head.S b/arch/x86/lguest/i386_head.S
new file mode 100644
index 000000000000..6d7a74f07c41
--- /dev/null
+++ b/arch/x86/lguest/i386_head.S
@@ -0,0 +1,93 @@
+#include <linux/linkage.h>
+#include <linux/lguest.h>
+#include <asm/asm-offsets.h>
+#include <asm/thread_info.h>
+#include <asm/processor-flags.h>
+
+/*G:020 This is where we begin: we have a magic signature which the launcher
+ * looks for.  The plan is that the Linux boot protocol will be extended with a
+ * "platform type" field which will guide us here from the normal entry point,
+ * but for the moment this suffices.  The normal boot code uses %esi for the
+ * boot header, so we do too.  We convert it to a virtual address by adding
+ * PAGE_OFFSET, and hand it to lguest_init() as its argument (ie. %eax).
+ *
+ * The .section line puts this code in .init.text so it will be discarded after
+ * boot. */
+.section .init.text, "ax", @progbits
+.ascii "GenuineLguest"
+        /* Set up initial stack. */
+        movl $(init_thread_union+THREAD_SIZE),%esp
+        movl %esi, %eax
+        addl $__PAGE_OFFSET, %eax
+        jmp lguest_init
+
+/*G:055 We create a macro which puts the assembler code between lgstart_ and
+ * lgend_ markers.  These templates are put in the .text section: they can't be
+ * discarded after boot as we may need to patch modules, too. */
+.text
+#define LGUEST_PATCH(name, insns...)                    \
+        lgstart_##name: insns; lgend_##name:;           \
+        .globl lgstart_##name; .globl lgend_##name
+
+LGUEST_PATCH(cli, movl $0, lguest_data+LGUEST_DATA_irq_enabled)
+LGUEST_PATCH(sti, movl $X86_EFLAGS_IF, lguest_data+LGUEST_DATA_irq_enabled)
+LGUEST_PATCH(popf, movl %eax, lguest_data+LGUEST_DATA_irq_enabled)
+LGUEST_PATCH(pushf, movl lguest_data+LGUEST_DATA_irq_enabled, %eax)
+/*:*/
+
+/* These demark the EIP range where host should never deliver interrupts. */
+.global lguest_noirq_start
+.global lguest_noirq_end
+
+/*M:004 When the Host reflects a trap or injects an interrupt into the Guest,
+ * it sets the eflags interrupt bit on the stack based on
+ * lguest_data.irq_enabled, so the Guest iret logic does the right thing when
+ * restoring it.  However, when the Host sets the Guest up for direct traps,
+ * such as system calls, the processor is the one to push eflags onto the
+ * stack, and the interrupt bit will be 1 (in reality, interrupts are always
+ * enabled in the Guest).
+ *
+ * This turns out to be harmless: the only trap which should happen under Linux
+ * with interrupts disabled is Page Fault (due to our lazy mapping of vmalloc
+ * regions), which has to be reflected through the Host anyway.  If another
+ * trap *does* go off when interrupts are disabled, the Guest will panic, and
+ * we'll never get to this iret! :*/
+
+/*G:045 There is one final paravirt_op that the Guest implements, and glancing
+ * at it you can see why I left it to last.  It's *cool*!  It's in *assembler*!
+ *
+ * The "iret" instruction is used to return from an interrupt or trap.  The
+ * stack looks like this:
+ *   old address
+ *   old code segment & privilege level
+ *   old processor flags ("eflags")
+ *
+ * The "iret" instruction pops those values off the stack and restores them all
+ * at once.  The only problem is that eflags includes the Interrupt Flag which
+ * the Guest can't change: the CPU will simply ignore it when we do an "iret".
+ * So we have to copy eflags from the stack to lguest_data.irq_enabled before
+ * we do the "iret".
+ *
+ * There are two problems with this: firstly, we need to use a register to do
+ * the copy and secondly, the whole thing needs to be atomic.  The first
+ * problem is easy to solve: push %eax on the stack so we can use it, and then
+ * restore it at the end just before the real "iret".
+ *
+ * The second is harder: copying eflags to lguest_data.irq_enabled will turn
+ * interrupts on before we're finished, so we could be interrupted before we
+ * return to userspace or wherever.  Our solution to this is to surround the
+ * code with lguest_noirq_start: and lguest_noirq_end: labels.  We tell the
+ * Host that it is *never* to interrupt us there, even if interrupts seem to be
+ * enabled. */
+ENTRY(lguest_iret)
+        pushl   %eax
+        movl    12(%esp), %eax
+lguest_noirq_start:
+        /* Note the %ss: segment prefix here.  Normal data accesses use the
+         * "ds" segment, but that will have already been restored for whatever
+         * we're returning to (such as userspace): we can't trust it.  The %ss:
+         * prefix makes sure we use the stack segment, which is still valid. */
+        movl    %eax,%ss:lguest_data+LGUEST_DATA_irq_enabled
+        popl    %eax
+        iret
+lguest_noirq_end: