[PATCH v2] x86/emul: Adjust handling of CR8_LEGACY

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

The CR8_LEGACY feature was introduced in the K8 Revision F.  It doesn't exist
in prior revisions of the K8.

Furthermore, from APM Vol2 3.1.5 CR8 (Task Priority Register, TPR):

  The AMD64 architecture introduces a new control register, CR8, defined as
  the task priority register (TPR).

Additionally, from APM Vol3 4 System Instructions MOV CRn:

  CR8 can be read and written in 64-bit mode, using a REX prefix.  CR8 can be
  read and written in all modes using a LOCK prefix instead of a REX prefix to
  specify the additional opcode bit.

i.e. the LOCK prefix serves as an alternative encoding for REX.R.

Switch decode_twobyte() from += 8 to |= 8 to better match the description
given.  Other indications that the encoding isn't additive are that the CR
intercepts stop at 15, and that LOCK MOV CR8 generates #UD rather than
becoming a CR0 access.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

v2:
 * No dependency on LM.

Also, designers never put an ADD into silicon if they can possibly avoid it,
because it's large and slow compared to the single OR gate needed in this
case.
---
 xen/arch/x86/x86_emulate/decode.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/x86_emulate/decode.c 
b/xen/arch/x86/x86_emulate/decode.c
index 2c13356c4d17..51c89ddfc893 100644
--- a/xen/arch/x86/x86_emulate/decode.c
+++ b/xen/arch/x86/x86_emulate/decode.c
@@ -782,10 +782,10 @@ decode_twobyte(struct x86_emulate_state *s,
     case 0x20: case 0x22: /* mov to/from cr */
         if ( s->lock_prefix && vcpu_has_cr8_legacy() )
         {
-            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