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

[Oliver Upton]

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. If the event is filtered, exclude
> counting at all exception levels before writing the hardware.
> 
> Signed-off-by: Colton Lewis <coltonle...@google.com>
> ---
>  arch/arm64/kvm/pmu-direct.c | 44 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 44 insertions(+)
> 
> diff --git a/arch/arm64/kvm/pmu-direct.c b/arch/arm64/kvm/pmu-direct.c
> index 71977d24f489a..8d0d6d1a0d851 100644
> --- a/arch/arm64/kvm/pmu-direct.c
> +++ b/arch/arm64/kvm/pmu-direct.c
> @@ -221,6 +221,49 @@ u8 kvm_pmu_hpmn(struct kvm_vcpu *vcpu)
>       return nr_host_cnt_max;
>  }
>  
> +/**
> + * kvm_pmu_apply_event_filter()
> + * @vcpu: Pointer to vcpu struct
> + *
> + * To uphold the guarantee of the KVM PMU event filter, we must ensure
> + * no counter counts if the event is filtered. Accomplish this by
> + * filtering all exception levels if the event is filtered.
> + */
> +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++) {
> +             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);
> +     }
> +
> +     val = __vcpu_sys_reg(vcpu, PMCCFILTR_EL0);
> +
> +     if (vcpu->kvm->arch.pmu_filter &&
> +         !test_bit(ARMV8_PMUV3_PERFCTR_CPU_CYCLES, 
> vcpu->kvm->arch.pmu_filter))
> +             val |= evtyper_set;
> +
> +     val &= ~evtyper_clr;
> +     write_pmccfiltr(val);
> +}

This doesn't work for nested. I agree that the hardware value of
PMEVTYPERn_EL0 needs to be under KVM control, but depending on whether
or not we're in a hyp context the meaning of the EL1 filtering bit
changes. Have a look at kvm_pmu_create_perf_event().

Thanks,
Oliver