linux-kernel - Re: [PATCH v4 1/6] x86: KVM: Advertise CMPccXADD CPUID to user space

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Message-ID: <Y3ue4PoJD7EGC5dV@zn.tnic>
Date:   Mon, 21 Nov 2022 16:53:04 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Jiaxi Chen <jiaxi.chen@...ux.intel.com>, 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, nathan@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/6] x86: KVM: Advertise CMPccXADD CPUID to user space

On Mon, Nov 21, 2022 at 03:48:06PM +0000, Sean Christopherson wrote:
> Actually, for these features that don't require additional KVM enabling, KVM isn't
> making the feature avaiable to the guest.  KVM is just advertising to userspace
> that KVM "supports" these features.  Userspace ultimately controls guest CPUID;
> outside of a few special cases, KVM neither rejects nor filters unsupported bits
> in CPUID.

So is there any point to those "enable it in KVM" patches streaming constantly?

AFAICT, the only reason is to "pass through" the CPUID bit to the guest
in case KVM is not showing it in the intercepted CPUID...

-- 
Regards/Gruss,
    Boris.

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