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.
Wonder if the intention of moving this leaf to kvm-only is for saving
space in x86_capability[], or just because there's no other use case in
the host kernel side and the cpuflags of this features can be removed.
Hope for your suggestions.
--
Regards,
Jiaxi
