[xen staging-4.20] x86/emul: Adjust handling of CR8_LEGACY

[patchbot@xxxxxxx]

commit c87190b87934e984d94f71aa4d531a7eeabd0fec
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Jul 20 13:31:54 2026 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Jul 20 13:31:54 2026 +0200

    x86/emul: Adjust handling of CR8_LEGACY
    
    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>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Teddy Astie <teddy.astie@xxxxxxxxxx>
    master commit: 7d48ef45b6df0cf8985b84305e02b7440de8f50f
    master date: 2026-07-10 19:35:49 +0100
---
 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 7ce97c4726..52a4a76797 100644
--- a/xen/arch/x86/x86_emulate/decode.c
+++ b/xen/arch/x86/x86_emulate/decode.c
@@ -775,12 +775,12 @@ 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);
--
generated by git-patchbot for /home/xen/git/xen.git#staging-4.20