Xen devel - Feb 2007 - [PATCH] x86 emulation: suppress rep prefix handling on non-repeatable instructions

Otherwise, pause or the recommended one-byte ret workaround on AMD
processors may have ill side effects.

Signed-off-by: Jan Beulich <jbeulich@novell.com>

Index: 2007-02-07/xen/arch/x86/x86_emulate.c
==================================================================---
2007-02-07.orig/xen/arch/x86/x86_emulate.c	2007-02-08 08:46:39.000000000 +0100
+++ 2007-02-07/xen/arch/x86/x86_emulate.c	2007-02-08 09:32:00.000000000 +0100
@@ -1579,7 +1579,9 @@ x86_emulate(
     if ( twobyte )
         goto twobyte_special_insn;
 
-    if ( rep_prefix )
+    if ( rep_prefix &&
+         (((b & ~0xf) == 0xa0 && ((0xfcf0 >> (b & 0xf))
& 1)) ||
+          (b & ~3) == 0x6c) )
     {
         if ( _regs.ecx == 0 )
         {


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