diff -r e9156d9d996b xen/arch/x86/acpi/cpu_idle.c
--- a/xen/arch/x86/acpi/cpu_idle.c	Tue Nov 16 11:54:48 2010 +0000
+++ b/xen/arch/x86/acpi/cpu_idle.c	Mon Nov 22 15:12:15 2010 +0000
@@ -717,7 +717,8 @@
     {
     case ACPI_ADR_SPACE_FIXED_HARDWARE:
         if ( xen_cx->reg.bit_width == VENDOR_INTEL &&
-             xen_cx->reg.bit_offset == NATIVE_CSTATE_BEYOND_HALT )
+             xen_cx->reg.bit_offset == NATIVE_CSTATE_BEYOND_HALT &&
+             boot_cpu_has(X86_FEATURE_MWAIT) )
             cx->entry_method = ACPI_CSTATE_EM_FFH;
         else
             cx->entry_method = ACPI_CSTATE_EM_HALT;