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

[Teddy Astie <teddy.astie@xxxxxxxxxx>]

Le 07/07/2026 à 21:49, Andrew Cooper a écrit :
> 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);
I'm fine with it; though we probably want to make sure we always 
advertise the AltMovCr8 feature bit as APM suggest users to always check 
for this before relying on this behavior.
Interestingly, I see this feature bit set on Intel hardware.
Though, it's a reserved bit in SDM, and lock mov cr0 is defined as being 
#UD.
Which CPU model introduced this feature bit ? And does Intel behavior 
matches AMD's spec ?
Teddy

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