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

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Oliver Upton <oliver.upton@xxxxxxxxx> 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.




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