linux-kernel - Re: [PATCH 1/1] KVM: x86/vPMU: Check PMU is enabled for vCPU before searching for PMC

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Date:   Thu, 9 Nov 2023 15:05:36 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Konstantin Khorenko <khorenko@...tuozzo.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, "Denis V. Lunev" <den@...tuozzo.com>
Subject: Re: [PATCH 1/1] KVM: x86/vPMU: Check PMU is enabled for vCPU before
 searching for PMC

On Thu, Nov 09, 2023, Jim Mattson wrote:
> On Thu, Nov 9, 2023 at 10:24 AM Konstantin Khorenko <khorenko@...tuozzo.com> wrote:
> > ---
> >  arch/x86/kvm/pmu.c | 26 ++++++++++++++++++++++++++
> >  1 file changed, 26 insertions(+)
> >
> > diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> > index 9ae07db6f0f6..290d407f339b 100644
> > --- a/arch/x86/kvm/pmu.c
> > +++ b/arch/x86/kvm/pmu.c
> > @@ -731,12 +731,38 @@ static inline bool cpl_is_matched(struct kvm_pmc *pmc)
> >         return (static_call(kvm_x86_get_cpl)(pmc->vcpu) == 0) ? select_os : select_user;
> >  }
> >
> > +static inline bool guest_pmu_is_enabled(struct kvm_pmu *pmu)
> > +{
> > +       /*
> > +        * Currently VMs do not have PMU settings in configs which defaults
> > +        * to "pmu=off".
> > +        *
> > +        * For Intel currently this means pmu->version will be 0.
> > +        * For AMD currently PMU cannot be disabled:
> 
> Isn't that what KVM_PMU_CAP_DISABLE is for?

Yeah, see my response.  KVM doesn't clear the metadata, so internally it looks
like the PMU is enabled even though it's effectively disabled from the guest's
perspective.