Re: [PATCH 1/6] x86: KVM: Enable CMPccXADD CPUID and expose it to guest

Jiaxi Chen
2022-11-03T02:35:25Z

On 11/1/2022 11:07 PM, Sean Christopherson wrote:
> On Tue, Nov 01, 2022, Jiaxi Chen wrote:
>>
>>
>> On 10/27/2022 1:15 AM, Borislav Petkov wrote:
>>> On Wed, Oct 26, 2022 at 11:40:31AM +0800, Jiaxi Chen wrote:
>>>>> What do you think about moving CPUID_7_1_EAX to be a KVM-only leaf too? AFAICT,
>>>>> KVM passthrough is the only reason the existing features are defined.
>>>
>>> Yap, looking at the patches which added those 2 feature flags upstream,
>>> they don't look like some particular use was the goal but rather to
>>> expose it to guests. Besides, AVX512 apps do their own CPUID detection.
>>>
>>>> Since CPUID_7_1_EAX has only 5 features now, it is a big waste,
>>>> should we move it to KVM-only leaf as Sean suggested. What's your
>>>> opinion about this?
>>>
>>> Yes, pls do.
>>>
>>> And when you do, make sure to undo what
>>>
>>> b302e4b176d0 ("x86/cpufeatures: Enumerate the new AVX512 BFLOAT16 instructions")
>>>
>>> added.
>>>
>>> Thx.
>>>
>> Hi Sean and Boris,
>>
>> Just realized moving CPUID_7_1_EAX to kvm-only leaf will not save space
>> in enum cpuid_leafs[]. CPUID_7_1_EAX is indeed removed, but someone
>> else, ie. CPUID_DUMMY needs to take the place, otherwise the cpuid_leafs
>> array would be deranged. Therefore, the length of x86 cpuid leaves is
>> not decreased.
>
> The order of "enum cpuid_leafs" is completely arbitrary.
>
> After replacing CPUID_7_1_EAX with CPUID_DUMMY, replace CPUID_DUMMY with the last
> leaf, which is currently CPUID_8000_001F_EAX, and update the #defines accordingly.
> Alternatively, Boris may prefer skipping the intermediate CPUID_DUMMY step and
> just replace CPUID_7_1_EAX with CPUID_8000_001F_EAX straightaway.

Yes, thanks for Sean's kind suggestion. I think use CPUID_DUMMY as the
transition leaf will make the code logic and commit message clearer.
Will change it in v2.
--
Regards,
Jiaxi