linux-kernel - Re: [PATCH v5 16/24] KVM: arm64: Account for partitioning in PMCR

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Message-ID: <aTiWsAvK9krnEw7V@kernel.org>
Date: Tue, 9 Dec 2025 13:37:52 -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 16/24] KVM: arm64: Account for partitioning in
 PMCR_EL0 access

On Tue, Dec 09, 2025 at 08:51:13PM +0000, Colton Lewis wrote:
> Make sure reads and writes to PMCR_EL0 conform to additional
> constraints imposed when the PMU is partitioned.
> 
> Signed-off-by: Colton Lewis <coltonlewis@...gle.com>
> ---
>  arch/arm64/kvm/pmu.c      | 2 +-
>  arch/arm64/kvm/sys_regs.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kvm/pmu.c b/arch/arm64/kvm/pmu.c
> index 1fd012f8ff4a9..48b39f096fa12 100644
> --- a/arch/arm64/kvm/pmu.c
> +++ b/arch/arm64/kvm/pmu.c
> @@ -877,7 +877,7 @@ u64 kvm_pmu_accessible_counter_mask(struct kvm_vcpu *vcpu)
>  u64 kvm_vcpu_read_pmcr(struct kvm_vcpu *vcpu)
>  {
>  	u64 pmcr = __vcpu_sys_reg(vcpu, PMCR_EL0);
> -	u64 n = vcpu->kvm->arch.nr_pmu_counters;
> +	u64 n = kvm_pmu_guest_num_counters(vcpu);

Why can't the value of vcpu->kvm->arch.nr_pmu_counters be trusted?

> @@ -1360,7 +1360,7 @@ static int set_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
>  	 */
>  	if (!kvm_vm_has_ran_once(kvm) &&
>  	    !vcpu_has_nv(vcpu)	      &&
> -	    new_n <= kvm_arm_pmu_get_max_counters(kvm))
> +	    new_n <= kvm_pmu_hpmn(vcpu))
>  		kvm->arch.nr_pmu_counters = new_n;

This is the legacy UAPI for setting the number of PMU counters by
writing to PMCR_EL0.N.

The 'partitioned' implementation should take a dependency on the
SET_NR_COUNTERS attribute and reject attempts to change the value of
PMCR_EL0.N. Just like nested.

Thanks,
Oliver