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

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Message-ID: <gsnt1ps033ch.fsf@coltonlewis-kvm.c.googlers.com>
Date: Tue, 03 Jun 2025 21:46:54 +0000
From: Colton Lewis <coltonlewis@...gle.com>
To: Oliver Upton <oliver.upton@...ux.dev>
Cc: kvm@...r.kernel.org, pbonzini@...hat.com, corbet@....net, 
	linux@...linux.org.uk, catalin.marinas@....com, will@...nel.org, 
	maz@...nel.org, joey.gouly@....com, suzuki.poulose@....com, 
	yuzenghui@...wei.com, mark.rutland@....com, 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

Oliver Upton <oliver.upton@...ux.dev> writes:

> 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.

Okay

>> +		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?

Okay. I remember talking about this before.

> 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.

Yes. I will think about it.