linux-kernel - Re: [PATCH v5 18/24] KVM: arm64: Enforce PMU event filter at vcpu

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Message-ID: <aTicD5hlKhZ3rkIn@kernel.org>
Date: Tue, 9 Dec 2025 14:00:47 -0800
From: Oliver Upton <oupton@...nel.org>
To: Colton Lewis <coltonlewis@...gle.com>
Cc: kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
	Jonathan Corbet <corbet@....net>,
	Russell King <linux@...linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Mingwei Zhang <mizhang@...gle.com>, Joey Gouly <joey.gouly@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	Mark Rutland <mark.rutland@....com>, Shuah Khan <shuah@...nel.org>,
	Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
	linux-perf-users@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v5 18/24] KVM: arm64: Enforce PMU event filter at
 vcpu_load()

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 <coltonlewis@...gle.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