1 files changed, 181 insertions, 4 deletions

diff --git a/arch/i386/xen/xen-asm.S b/arch/i386/xen/xen-asm.S
index dc4d36d51bc1..1a43b60c0c62 100644
--- a/arch/i386/xen/xen-asm.S
+++ b/arch/i386/xen/xen-asm.S
@@ -12,15 +12,21 @@
  */
 
 #include <linux/linkage.h>
+
 #include <asm/asm-offsets.h>
 #include <asm/thread_info.h>
 #include <asm/percpu.h>
-#include <asm/asm-offsets.h>
 #include <asm/processor-flags.h>
+#include <asm/segment.h>
+
+#include <xen/interface/xen.h>
 
 #define RELOC(x, v)     .globl x##_reloc; x##_reloc=v
 #define ENDPATCH(x)     .globl x##_end; x##_end=.
 
+/* Pseudo-flag used for virtual NMI, which we don't implement yet */
+#define XEN_EFLAGS_NMI  0x80000000
+
 /*
         Enable events.  This clears the event mask and tests the pending
         event status with one and operation.  If there are pending
@@ -81,13 +87,12 @@ ENDPATCH(xen_save_fl_direct)
  */
 ENTRY(xen_restore_fl_direct)
         testb $X86_EFLAGS_IF>>8, %ah
-        setz %al
-        movb %al, PER_CPU_VAR(xen_vcpu_info)+XEN_vcpu_info_mask
+        setz PER_CPU_VAR(xen_vcpu_info)+XEN_vcpu_info_mask
         /* Preempt here doesn't matter because that will deal with
            any pending interrupts.  The pending check may end up being
            run on the wrong CPU, but that doesn't hurt. */
 
-        /* check for pending but unmasked */
+        /* check for unmasked and pending */
         cmpw $0x0001, PER_CPU_VAR(xen_vcpu_info)+XEN_vcpu_info_pending
         jz 1f
 2:      call check_events
@@ -97,6 +102,178 @@ ENDPATCH(xen_restore_fl_direct)
         ENDPROC(xen_restore_fl_direct)
         RELOC(xen_restore_fl_direct, 2b+1)
 
+/*
+        This is run where a normal iret would be run, with the same stack setup:
+              8: eflags
+              4: cs
+        esp-> 0: eip
+
+        This attempts to make sure that any pending events are dealt
+        with on return to usermode, but there is a small window in
+        which an event can happen just before entering usermode.  If
+        the nested interrupt ends up setting one of the TIF_WORK_MASK
+        pending work flags, they will not be tested again before
+        returning to usermode. This means that a process can end up
+        with pending work, which will be unprocessed until the process
+        enters and leaves the kernel again, which could be an
+        unbounded amount of time.  This means that a pending signal or
+        reschedule event could be indefinitely delayed.
+
+        The fix is to notice a nested interrupt in the critical
+        window, and if one occurs, then fold the nested interrupt into
+        the current interrupt stack frame, and re-process it
+        iteratively rather than recursively.  This means that it will
+        exit via the normal path, and all pending work will be dealt
+        with appropriately.
+
+        Because the nested interrupt handler needs to deal with the
+        current stack state in whatever form its in, we keep things
+        simple by only using a single register which is pushed/popped
+        on the stack.
+
+        Non-direct iret could be done in the same way, but it would
+        require an annoying amount of code duplication.  We'll assume
+        that direct mode will be the common case once the hypervisor
+        support becomes commonplace.
+ */
+ENTRY(xen_iret_direct)
+        /* test eflags for special cases */
+        testl $(X86_EFLAGS_VM | XEN_EFLAGS_NMI), 8(%esp)
+        jnz hyper_iret
+
+        push %eax
+        ESP_OFFSET=4    # bytes pushed onto stack
+
+        /* Store vcpu_info pointer for easy access.  Do it this
+           way to avoid having to reload %fs */
+#ifdef CONFIG_SMP
+        GET_THREAD_INFO(%eax)
+        movl TI_cpu(%eax),%eax
+        movl __per_cpu_offset(,%eax,4),%eax
+        lea per_cpu__xen_vcpu_info(%eax),%eax
+#else
+        movl $per_cpu__xen_vcpu_info, %eax
+#endif
+
+        /* check IF state we're restoring */
+        testb $X86_EFLAGS_IF>>8, 8+1+ESP_OFFSET(%esp)
+
+        /* Maybe enable events.  Once this happens we could get a
+           recursive event, so the critical region starts immediately
+           afterwards.  However, if that happens we don't end up
+           resuming the code, so we don't have to be worried about
+           being preempted to another CPU. */
+        setz XEN_vcpu_info_mask(%eax)
+xen_iret_start_crit:
+
+        /* check for unmasked and pending */
+        cmpw $0x0001, XEN_vcpu_info_pending(%eax)
+
+        /* If there's something pending, mask events again so we
+           can jump back into xen_hypervisor_callback */
+        sete XEN_vcpu_info_mask(%eax)
+
+        popl %eax
+
+        /* From this point on the registers are restored and the stack
+           updated, so we don't need to worry about it if we're preempted */
+iret_restore_end:
+
+        /* Jump to hypervisor_callback after fixing up the stack.
+           Events are masked, so jumping out of the critical
+           region is OK. */
+        je xen_hypervisor_callback
+
+        iret
+xen_iret_end_crit:
+
+hyper_iret:
+        /* put this out of line since its very rarely used */
+        jmp hypercall_page + __HYPERVISOR_iret * 32
+
+        .globl xen_iret_start_crit, xen_iret_end_crit
+
+/*
+   This is called by xen_hypervisor_callback in entry.S when it sees
+   that the EIP at the time of interrupt was between xen_iret_start_crit
+   and xen_iret_end_crit.  We're passed the EIP in %eax so we can do
+   a more refined determination of what to do.
+
+   The stack format at this point is:
+        ----------------
+         ss             : (ss/esp may be present if we came from usermode)
+         esp            :
+         eflags         }  outer exception info
+         cs             }
+         eip            }
+        ---------------- <- edi (copy dest)
+         eax            :  outer eax if it hasn't been restored
+        ----------------
+         eflags         }  nested exception info
+         cs             }   (no ss/esp because we're nested
+         eip            }    from the same ring)
+         orig_eax       }<- esi (copy src)
+         - - - - - - - -
+         fs             }
+         es             }
+         ds             }  SAVE_ALL state
+         eax            }
+          :             :
+         ebx            }
+        ----------------
+         return addr     <- esp
+        ----------------
+
+   In order to deliver the nested exception properly, we need to shift
+   everything from the return addr up to the error code so it
+   sits just under the outer exception info.  This means that when we
+   handle the exception, we do it in the context of the outer exception
+   rather than starting a new one.
+
+   The only caveat is that if the outer eax hasn't been
+   restored yet (ie, it's still on stack), we need to insert
+   its value into the SAVE_ALL state before going on, since
+   it's usermode state which we eventually need to restore.
+ */
+ENTRY(xen_iret_crit_fixup)
+        /* offsets +4 for return address */
+
+        /*
+           Paranoia: Make sure we're really coming from userspace.
+           One could imagine a case where userspace jumps into the
+           critical range address, but just before the CPU delivers a GP,
+           it decides to deliver an interrupt instead.  Unlikely?
+           Definitely.  Easy to avoid?  Yes.  The Intel documents
+           explicitly say that the reported EIP for a bad jump is the
+           jump instruction itself, not the destination, but some virtual
+           environments get this wrong.
+         */
+        movl PT_CS+4(%esp), %ecx
+        andl $SEGMENT_RPL_MASK, %ecx
+        cmpl $USER_RPL, %ecx
+        je 2f
+
+        lea PT_ORIG_EAX+4(%esp), %esi
+        lea PT_EFLAGS+4(%esp), %edi
+
+        /* If eip is before iret_restore_end then stack
+           hasn't been restored yet. */
+        cmp $iret_restore_end, %eax
+        jae 1f
+
+        movl 0+4(%edi),%eax             /* copy EAX */
+        movl %eax, PT_EAX+4(%esp)
+
+        lea ESP_OFFSET(%edi),%edi       /* move dest up over saved regs */
+
+        /* set up the copy */
+1:      std
+        mov $(PT_EIP+4) / 4, %ecx       /* copy ret+saved regs up to orig_eax */
+        rep movsl
+        cld
+
+        lea 4(%edi),%esp                /* point esp to new frame */
+2:      ret
 
 
 /*