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

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Message-ID: <Y1lrGgyIcgweVGup@zn.tnic>
Date:   Wed, 26 Oct 2022 19:15:06 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Jiaxi Chen <jiaxi.chen@...ux.intel.com>
Cc:     kvm@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        pbonzini@...hat.com, ndesaulniers@...gle.com,
        alexandre.belloni@...tlin.com, peterz@...radead.org,
        jpoimboe@...nel.org, chang.seok.bae@...el.com,
        pawan.kumar.gupta@...ux.intel.com, babu.moger@....com,
        jmattson@...gle.com, sandipan.das@....com, tony.luck@...el.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, fenghua.yu@...el.com,
        keescook@...omium.org, jane.malalane@...rix.com, nathan@...nel.org,
        linux-kernel@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH 1/6] x86: KVM: Enable CMPccXADD CPUID and expose it to
 guest

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.

https://people.kernel.org/tglx/notes-about-netiquette