Re: [PATCH v5 08/12] KVM: arm64: PMU: Allow userspace to limit PMCR_EL0.N for the guest

[Raghavendra Rao Ananta]

On Fri, Sep 15, 2023 at 1:53 PM Oliver Upton <oliver.upton@xxxxxxxxx> wrote:
>
> Hi Raghu,
>
> On Thu, Aug 17, 2023 at 12:30:25AM +0000, Raghavendra Rao Ananta wrote:
> > From: Reiji Watanabe <reijiw@xxxxxxxxxx>
> >
> > KVM does not yet support userspace modifying PMCR_EL0.N (With
> > the previous patch, KVM ignores what is written by upserspace).
>
> typo: userspace
>
Noted.
> > diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> > index ce7de6bbdc967..39ad56a71ad20 100644
> > --- a/arch/arm64/kvm/pmu-emul.c
> > +++ b/arch/arm64/kvm/pmu-emul.c
> > @@ -896,6 +896,7 @@ int kvm_arm_set_vm_pmu(struct kvm *kvm, struct arm_pmu *arm_pmu)
> >        * while the latter does not.
> >        */
> >       kvm->arch.pmcr_n = arm_pmu->num_events - 1;
> > +     kvm->arch.pmcr_n_limit = arm_pmu->num_events - 1;
>
> Can't we just get at this through the arm_pmu instance rather than
> copying it into kvm_arch?
>
Yeah, I suppose we can directly access it in set_pmcr().

Thank you.
Raghavendra
> >       return 0;
> >  }
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index 2075901356c5b..c01d62afa7db4 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -1086,6 +1086,51 @@ static int get_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
> >       return 0;
> >  }
> >
> > +static int set_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
> > +                 u64 val)
> > +{
> > +     struct kvm *kvm = vcpu->kvm;
> > +     u64 new_n, mutable_mask;
> > +     int ret = 0;
> > +
> > +     new_n = FIELD_GET(ARMV8_PMU_PMCR_N, val);
> > +
> > +     mutex_lock(&kvm->arch.config_lock);
> > +     if (unlikely(new_n != kvm->arch.pmcr_n)) {
> > +             /*
> > +              * The vCPU can't have more counters than the PMU
> > +              * hardware implements.
> > +              */
> > +             if (new_n <= kvm->arch.pmcr_n_limit)
> > +                     kvm->arch.pmcr_n = new_n;
> > +             else
> > +                     ret = -EINVAL;
> > +     }
>
> Hmm, I'm not so sure about returning an error here. ABI has it that
> userspace can write any value to PMCR_EL0 successfully. Can we just
> ignore writes that attempt to set PMCR_EL0.N to something higher than
> supported by hardware? Our general stance should be that system register
> fields responsible for feature identification are immutable after the VM
> has started.
>
> --
> Thanks,
> Oliver