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

Colton Lewis
2025-12-17T23:05:28Z

Oliver Upton <oupton@kernel.org> writes:

> 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.

Sure

>> +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'.

Will do.

> <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.

Will do.

> Thanks,
> Oliver