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

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Date:   Tue, 1 Nov 2022 17:07:22 +0800
From:   Jiaxi Chen <jiaxi.chen@...ux.intel.com>
To:     Borislav Petkov <bp@...en8.de>,
        Sean Christopherson <seanjc@...gle.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
Subject: Re: [PATCH 1/6] x86: KVM: Enable CMPccXADD CPUID and expose it to
 guest



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