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

Borislav Petkov
2022-10-26T17:15:30Z

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.

--
Regards/Gruss,
Boris.