linux-kernel - Re: [PATCH v4] KVM: x86: Fix KVM_GET_CPUID2 ioctl to return cpuid entries count

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Message-ID: <CALMp9eSzNBv-EQoGryyx_eFGmYyUBQwYkB5ndmwuE1SP0Wu6_A@mail.gmail.com>
Date:   Tue, 4 May 2021 09:52:46 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Alexander Graf <graf@...zon.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        "Denis V. Lunev" <den@...nvz.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Valeriy Vdovin <valeriy.vdovin@...tuozzo.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Shuah Khan <shuah@...nel.org>,
        Aaron Lewis <aaronlewis@...gle.com>,
        Andrew Jones <drjones@...hat.com>,
        Oliver Upton <oupton@...gle.com>,
        Like Xu <like.xu@...ux.intel.com>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v4] KVM: x86: Fix KVM_GET_CPUID2 ioctl to return cpuid
 entries count

On Tue, May 4, 2021 at 2:26 AM Alexander Graf <graf@...zon.com> wrote:
>
>
>
> On 04.05.21 10:21, Paolo Bonzini wrote:
> >
> > On 04/05/21 10:15, Denis V. Lunev wrote:
> >> As far as I understand only some testing within kernel now.
> >> Though we have plans to expose it for QAPI as the series
> >> in QEMU
> >>    [PATCH 1/2] qapi: fix error handling for x-vz-query-cpu-model-cpuid
> >>    [PATCH 2/2] qapi: blacklisted x-vz-query-cpu-model-cpuid in tests
> >> is not coming in a good way.
> >> The idea was to avoid manual code rework in QEMU and
> >> expose collected model at least for debug.
> >
> > KVM_GET_CPUID2 as a VM ioctl cannot expose the whole truth about CPUID
> > either, since it doesn't handle the TSX_CTRL_CPUID_CLEAR bit.  Given
> > that QEMU doesn't need KVM_GET_CPUID2; it only needs to save whatever it
> > passed to KVM_SET_CPUID2.
>
> What if we instead deflect CPUID into user space so it can emulate it in
> whatever way it likes? Is the performance difference going to be
> relevant? Are people still using cpuid as barrier these days?

What else would they use (in ring 3 code)? Sure, serialize is coming
in Sapphire Rapids, but it will be 20+ years before kvm drops support
for CPUs without serialize.

>
> Alex
>
>
>
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