x86_emulate: Disable writeback if BSF/BSR are passed zero input.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>

diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c b/xen/arch/x86/x86_emulate/x86_emulate.c
index 2b7ddeffbbac26baadfd29faacc2c9526d192e39..8d8db35820c6b6895cc827c8492ff01e1beb6d15 100644 (file)
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -3134,7 +3134,11 @@ x86_emulate(
               : "=r" (dst.val), "=q" (zf)
               : "r" (src.val), "1" (0) );
         _regs.eflags &= ~EFLG_ZF;
-        _regs.eflags |= zf ? EFLG_ZF : 0;
+        if ( zf )
+        {
+            _regs.eflags |= EFLG_ZF;
+            dst.type = OP_NONE;
+        }
         break;
     }
 
@@ -3144,7 +3148,11 @@ x86_emulate(
               : "=r" (dst.val), "=q" (zf)
               : "r" (src.val), "1" (0) );
         _regs.eflags &= ~EFLG_ZF;
-        _regs.eflags |= zf ? EFLG_ZF : 0;
+        if ( zf )
+        {
+            _regs.eflags |= EFLG_ZF;
+            dst.type = OP_NONE;
+        }
         break;
     }