diff -r 9fdcd3ab84b7 xen/arch/x86/hvm/viridian.c
--- a/xen/arch/x86/hvm/viridian.c	Mon Apr 27 15:40:09 2009 +0100
+++ b/xen/arch/x86/hvm/viridian.c	Mon Apr 27 19:59:05 2009 -0700
@@ -22,6 +22,7 @@
 #define VIRIDIAN_MSR_EOI         0x40000070
 #define VIRIDIAN_MSR_ICR         0x40000071
 #define VIRIDIAN_MSR_TPR         0x40000072
+#define VIRIDIAN_MSR_APIC_ASSIST 0x40000073
 
 /* Viridian Hypercall Status Codes. */
 #define HV_STATUS_SUCCESS                       0x0000
@@ -49,14 +50,14 @@ int cpuid_viridian_leaves(unsigned int l
         return 0;
 
     leaf -= 0x40000000;
-    if ( leaf > 5 )
+    if ( leaf > 6 )
         return 0;
 
     *eax = *ebx = *ecx = *edx = 0;
     switch ( leaf )
     {
     case 0:
-        *eax = 0x40000005; /* Maximum leaf */
+        *eax = 0x40000006; /* Maximum leaf */
         *ebx = 0x7263694d; /* Magic numbers  */
         *ecx = 0x666F736F;
         *edx = 0x76482074;
@@ -192,6 +193,36 @@ int wrmsr_viridian_regs(uint32_t idx, ui
         vlapic_set_reg(vcpu_vlapic(current), APIC_TASKPRI, eax & 0xff);
         break;
 
+    case VIRIDIAN_MSR_APIC_ASSIST: {
+        /* We don't support the APIC assist page, and that fact is
+           reflected in our CPUID flags.  However, Windows 7 build
+           7000 has a bug which means that it doesn't recognise that,
+           and tries to use the page anyway.  We therefore have to
+           fake up just enough to keep win7 happy.  Fortunately,
+           that's really easy: just setting the first four bytes in
+           the page to zero effectively disables the page again, so
+           that's what we do. */
+        /* Semantically, the first four bytes are supposed to be a
+           flag saying whether the guest really needs to issue an EOI.
+           Setting that flag to zero means that it must always issue
+           one, which is what we want.  Once a page has been
+           repurposed as an APIC assist page the guest isn't allowed
+           to set anything in it, so the flag remains zero and all is
+           fine.  The guest is allowed to clear flags in the page, but
+           that doesn't cause us any problems. */
+        uint32_t first_word;
+        paddr_t page_start;
+
+        if (!(val & 1)) {
+            /* APIC assist page is being disabled -> nothing to do */
+            return 1;
+        }
+        page_start = val & ~1ul;
+        first_word = 0;
+        hvm_copy_to_guest_phys(page_start, &first_word, sizeof(first_word));
+        break;
+    }
+
     default:
         return 0;
     }