linux-kernel - Re: [PATCH v5 01/12] KVM: arm64: PMU: Introduce a helper to set the guest's PMU

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Date:   Mon, 18 Sep 2023 10:24:53 -0700
From:   Raghavendra Rao Ananta <rananta@...gle.com>
To:     Oliver Upton <oliver.upton@...ux.dev>
Cc:     Marc Zyngier <maz@...nel.org>,
        Alexandru Elisei <alexandru.elisei@....com>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Shaoqin Huang <shahuang@...hat.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        Reiji Watanabe <reijiw@...gle.com>,
        Colton Lewis <coltonlewis@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v5 01/12] KVM: arm64: PMU: Introduce a helper to set the
 guest's PMU

On Fri, Sep 15, 2023 at 12:22 PM Oliver Upton <oliver.upton@...ux.dev> wrote:
>
> Hi Raghu,
>
> On Thu, Aug 17, 2023 at 12:30:18AM +0000, Raghavendra Rao Ananta wrote:
> > From: Reiji Watanabe <reijiw@...gle.com>
> >
> > Introduce a new helper function to set the guest's PMU
> > (kvm->arch.arm_pmu), and use it when the guest's PMU needs
> > to be set. This helper will make it easier for the following
> > patches to modify the relevant code.
> >
> > No functional change intended.
> >
> > Signed-off-by: Reiji Watanabe <reijiw@...gle.com>
> > Signed-off-by: Raghavendra Rao Ananta <rananta@...gle.com>
> > ---
> >  arch/arm64/kvm/pmu-emul.c | 52 +++++++++++++++++++++++++++------------
> >  1 file changed, 36 insertions(+), 16 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> > index 5606509724787..0ffd1efa90c07 100644
> > --- a/arch/arm64/kvm/pmu-emul.c
> > +++ b/arch/arm64/kvm/pmu-emul.c
> > @@ -865,6 +865,32 @@ static bool pmu_irq_is_valid(struct kvm *kvm, int irq)
> >       return true;
> >  }
> >
> > +static int kvm_arm_set_vm_pmu(struct kvm *kvm, struct arm_pmu *arm_pmu)
> > +{
> > +     lockdep_assert_held(&kvm->arch.config_lock);
> > +
> > +     if (!arm_pmu) {
> > +             /*
> > +              * No PMU set, get the default one.
> > +              *
> > +              * The observant among you will notice that the supported_cpus
> > +              * mask does not get updated for the default PMU even though it
> > +              * is quite possible the selected instance supports only a
> > +              * subset of cores in the system. This is intentional, and
> > +              * upholds the preexisting behavior on heterogeneous systems
> > +              * where vCPUs can be scheduled on any core but the guest
> > +              * counters could stop working.
> > +              */
> > +             arm_pmu = kvm_pmu_probe_armpmu();
> > +             if (!arm_pmu)
> > +                     return -ENODEV;
> > +     }
> > +
> > +     kvm->arch.arm_pmu = arm_pmu;
> > +
> > +     return 0;
> > +}
> > +
>
> I'm not too big of a fan of adding the 'default' path to this helper.
> I'd prefer it if kvm_arm_set_vm_pmu() does all the necessary
> initialization for a valid pmu instance. You then avoid introducing
> unexpected error handling where it didn't exist before.
>
>   static void kvm_arm_set_pmu(struct kvm *kvm, struct arm_pmu *arm_pmu)
>   {
>         lockdep_assert_held(&kvm->arch.config_lock);
>
>         kvm->arch.arm_pmu = arm_pmu;
>   }
>
>   /*
>    * Blurb about default PMUs I'm too lazy to copy/paste
>    */
>   static int kvm_arm_set_default_pmu(struct kvm *kvm)
>   {
>         struct arm_pmu *arm_pmu = kvm_pmu_probe_armpmu();
>
>         if (!arm_pmu)
>                 return -ENODEV;
>
>         kvm_arm_set_pmu(kvm, arm_pmu);
>         return 0;
>   }
>
Sounds good. We can adapt to your suggestion.

Thank you.
Raghavendra
> --
> Thanks,
> Oliver