linux-kernel - Re: [PATCH 16/17] KVM: arm64: Add ioctl to partition the PMU when supported

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Message-ID: <aD4oS1_tnMPlgDJ6@linux.dev>
Date: Mon, 2 Jun 2025 15:40:11 -0700
From: Oliver Upton <oliver.upton@...ux.dev>
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>,
	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>,
	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 16/17] KVM: arm64: Add ioctl to partition the PMU when
 supported

On Mon, Jun 02, 2025 at 07:27:01PM +0000, Colton Lewis wrote:
> +	case KVM_ARM_PARTITION_PMU: {

This should be a vCPU attribute similar to the other PMUv3 controls we
already have. Ideally a single attribute where userspace tells us it
wants paritioning and specifies the PMU ID to use. None of this can be
changed after INIT'ing the PMU.

> +		struct arm_pmu *pmu;
> +		u8 host_counters;
> +
> +		if (unlikely(!kvm_vcpu_initialized(vcpu)))
> +			return -ENOEXEC;
> +
> +		if (!kvm_pmu_partition_supported())
> +			return -EPERM;
> +
> +		if (copy_from_user(&host_counters, argp, sizeof(host_counters)))
> +			return -EFAULT;
> +
> +		pmu = vcpu->kvm->arch.arm_pmu;
> +		return kvm_pmu_partition(pmu, host_counters);

Yeah, we really can't be changing the counters available to the ARM PMU
driver at this point. What happens to host events already scheduled on
the CPU?

Either the partition of host / KVM-owned counters needs to be computed
up front (prior to scheduling events) or KVM needs a way to direct perf
to reschedule events on the PMU based on the new operating constraints.

Thanks,
Oliver