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

Raghavendra Rao Ananta
2023-09-18T17:11:35Z

On Fri, Sep 15, 2023 at 2:54 PM Oliver Upton <oliver.upton@linux.dev> wrote:
>
> On Fri, Sep 15, 2023 at 08:53:16PM +0000, Oliver Upton wrote:
> > Hi Raghu,
> >
> > On Thu, Aug 17, 2023 at 12:30:25AM +0000, Raghavendra Rao Ananta wrote:
> > > From: Reiji Watanabe <reijiw@google.com>
> > >
> > > KVM does not yet support userspace modifying PMCR_EL0.N (With
> > > the previous patch, KVM ignores what is written by upserspace).
> >
> > typo: userspace
> >
> > > 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?
> >
> > > 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.
>
> I hacked up my reply and dropped some context; this doesn't read right.
> Shaoqin made the point about preventing changes to PMCR_EL0.N after the
> VM has started and I firmly agree. The behavior should be:
>
> - Writes to PMCR always succeed
>
> - PMCR_EL0.N values greater than what's supported by hardware are
> ignored
>
> - Changes to N after the VM has started are ignored.
>
Reiji and I were wondering if we should proceed with this as this
would change userspace expectation. BTW, when you said "ignored", does
that mean we silently return to userspace with a success or with EBUSY
(changing the expectations)?

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