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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 08/07/2026 11:24 am, Jan Beulich wrote:
> On 08.07.2026 11:59, Teddy Astie wrote:
>> Le 08/07/2026 à 11:37, Jan Beulich a écrit :
>>> On 08.07.2026 11:29, Teddy Astie wrote:
>>>> Le 07/07/2026 à 21:49, Andrew Cooper a écrit :
>>>>> --- 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.
>>> Why would we set the flag when hardware doesn't have it? Note the
>>> vcpu_has_cr8_legacy() check there - we only engage this behavior if
>>> the guest sees the feature as available.
>> Hmm, ok, though this bit is named as "AltMovCr8" in July 2025 APM; but 
>> we name it "cr8_legacy". Has the name of the feature bit changed in APM 
>> over time ?
> I don't know. What I do know is that imo AltMovCr8 isn't the greatest of
> all possible names. (Neither is cr8_legacy, first and foremost as it
> alludes to this being a compat-mode-only thing when it isn't.)

We inherited the name CR8_LEGACY from Linux.

commit c458f45d2eddcafed054651363ffb5c830e5df5b
Author: Keir Fraser <keir@xxxxxxx>
Date:   Sat Jan 8 10:48:09 2011
    libxc: Update AMD CPU feature flags 0x80000001:ECX for Xen tools
    
    This patch syncs-up AMD CPU feature flags 0x80000001:ECX in libxc with
    the latest Linux kernel.
    
    Signed-off-by: Wei Huang <wei.huang2@xxxxxxx>


renamed from ALTMOVCR to CR8_LEGACY

~Andrew