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

Raghavendra Rao Ananta
2023-09-18T17:25:09Z

On Fri, Sep 15, 2023 at 12:22 PM Oliver Upton <oliver.upton@linux.dev> wrote:
>
> Hi Raghu,
>
> On Thu, Aug 17, 2023 at 12:30:18AM +0000, Raghavendra Rao Ananta wrote:
> > From: Reiji Watanabe <reijiw@google.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@google.com>
> > Signed-off-by: Raghavendra Rao Ananta <rananta@google.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