linux-kernel - Re: [PATCH 5/5] KVM: x86/pmu: Hide guest counter updates from the VMRUN instruction

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Message-ID: <ZDAvDhV/bpPyt3oX@google.com>
Date:   Fri, 7 Apr 2023 07:56:14 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Ravi Bangoria <ravi.bangoria@....com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/5] KVM: x86/pmu: Hide guest counter updates from the
 VMRUN instruction

On Fri, Apr 07, 2023, Like Xu wrote:
> On 7/4/2023 10:18 am, Sean Christopherson wrote:
> > Wait, really?  VMRUN is counted if and only if it enters to a CPL0 guest?  Can
> > someone from AMD confirm this?  I was going to say we should just treat this as
> > "normal" behavior, but counting CPL0 but not CPL>0 is definitely quirky.
> 
> VMRUN is only counted on a CPL0-target (branch) instruction counter.

Yes or no question: if KVM does VMRUN and a PMC is programmed to count _all_ taken
branches, will the PMC count VMRUN as a branch if guest CPL>0 according to the VMCB?

> This issue makes a guest CPL0-target instruction counter inexplicably
> increase, as if it would have been under-counted before the virtualization
> instructions were counted.

Heh, it's very much explicable, it's just not desirable, and you and I would argue
that it's also incorrect.

AMD folks, are there plans to document this as an erratum?  I agree with Like that
counting VMRUN as a taken branch in guest context is a CPU bug, even if the behavior
is known/expected.