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

[Oliver Upton <oliver.upton@linux.dev>]

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