[PATCH v3] x86/emul: Adjust handling of CR8_LEGACY

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

The APM description of the AltMovCR8 feature bit is:

  "LOCK MOV CR0 means MOV CR8"

Adjust the decode logic to behave like this.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <jbeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Teddy Astie <teddy.astie@xxxxxxxxxx>

v3:
 * Change yet again.  A contact with information on the matter confirmed that
   it is a special case for Reg = 0, and not a general modifier to all Reg
   values.
---
 xen/arch/x86/x86_emulate/decode.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/x86_emulate/decode.c 
b/xen/arch/x86/x86_emulate/decode.c
index 2c13356c4d17..57f6baffb066 100644
--- a/xen/arch/x86/x86_emulate/decode.c
+++ b/xen/arch/x86/x86_emulate/decode.c
@@ -780,12 +780,12 @@ decode_twobyte(struct x86_emulate_state *s,
         break;
 
     case 0x20: case 0x22: /* mov to/from cr */
-        if ( s->lock_prefix && vcpu_has_cr8_legacy() )
+        if ( s->lock_prefix && vcpu_has_cr8_legacy() && s->modrm_reg == 0 )
         {
-            s->modrm_reg += 8;
+            s->modrm_reg = 8;
             s->lock_prefix = false;
         }
-        /* fall through */
+        fallthrough;
     case 0x21: case 0x23: /* mov to/from dr */
         ASSERT(s->ea.type == OP_REG); /* Early operand adjustment ensures 
this. */
         generate_exception_if(s->lock_prefix, X86_EXC_UD);
-- 
2.39.5