Re: [PATCH v5 18/24] KVM: arm64: Enforce PMU event filter at vcpu_load()

Oliver Upton
2025-12-17T00:57:21Z

Re-reading this patch...

On Tue, Dec 09, 2025 at 08:51:15PM +0000, Colton Lewis wrote:
> The KVM API for event filtering says that counters do not count when
> blocked by the event filter. To enforce that, the event filter must be
> rechecked on every load since it might have changed since the last
> time the guest wrote a value.

Just directly state that this is guarding against userspace programming
an unsupported event ID.

> +static void kvm_pmu_apply_event_filter(struct kvm_vcpu *vcpu)
> +{
> + struct arm_pmu *pmu = vcpu->kvm->arch.arm_pmu;
> + u64 evtyper_set = ARMV8_PMU_EXCLUDE_EL0 |
> + ARMV8_PMU_EXCLUDE_EL1;
> + u64 evtyper_clr = ARMV8_PMU_INCLUDE_EL2;
> + u8 i;
> + u64 val;
> + u64 evsel;
> +
> + if (!pmu)
> + return;
> +
> + for (i = 0; i < pmu->hpmn_max; i++) {

Iterate the bitmask of counters and you'll handle the cycle counter 'for
free'.

<snip>

> + val = __vcpu_sys_reg(vcpu, PMEVTYPER0_EL0 + i);
> + evsel = val & kvm_pmu_event_mask(vcpu->kvm);
> +
> + if (vcpu->kvm->arch.pmu_filter &&
> + !test_bit(evsel, vcpu->kvm->arch.pmu_filter))
> + val |= evtyper_set;
> +
> + val &= ~evtyper_clr;
> + write_pmevtypern(i, val);

</snip>

This all needs to be shared with writethrough_pmevtyper() instead of
open-coding the same thing.

Thanks,
Oliver