linux-kernel - Re: [PATCH v4 05/10] KVM/x86: expose MSR_IA32_PERF

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Message-ID: <CALMp9eT2J=qFTEEpsym_aqeCRwyCquTYnY8=taZ3nY3rs8WeJA@mail.gmail.com>
Date:   Mon, 7 Jan 2019 10:48:38 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Wei Wang <wei.w.wang@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Kan Liang <kan.liang@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        like.xu@...el.com, Jann Horn <jannh@...gle.com>,
        arei.gonglei@...wei.com
Subject: Re: [PATCH v4 05/10] KVM/x86: expose MSR_IA32_PERF_CAPABILITIES to
 the guest

On Mon, Jan 7, 2019 at 10:20 AM Andi Kleen <ak@...ux.intel.com> wrote:
>
> > The issue is compatibility. Prior to your change, reading this MSR
> > from a VM would raise #GP. After your change, it won't. That means
> > that if you have a VM migrating between hosts with kernel versions
> > before and after this change, the results will be inconsistent. In the
>
> No it will not be. All Linux kernel uses of this MSR are guarded
> by a CPUID check.

Linux usage is irrelevant to the architected behavior of the virtual
CPU. According to volume 4 of the SDM, this MSR is only supported when
CPUID.01H:ECX.PDCM [bit 15] is set. Therefore, kvm should raise #GP
whenever a guest tries to read this MSR and the guest's
CPUID.01H:ECX.PDCM [bit 15] is clear.