Re: [PATCH v5 22/24] KVM: arm64: Add KVM_CAP to partition the PMU

On Tue, Dec 09, 2025 at 08:51:19PM +0000, Colton Lewis wrote:

+7.245 KVM_CAP_ARM_PARTITION_PMU +-------------------------------------

Why can't this be a vCPU attribute similar to the other vPMU controls? Making the UAPI consistent will make it easier for userspace to reason about it.

Better yet, we could make the UAPI such that userspace selects a PMU implementation and the partitioned-ness of the PMU at the same time.

@@ -132,6 +134,16 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm, } mutex_unlock(&kvm->lock); break;

• case KVM_CAP_ARM_PARTITION_PMU:

• if (kvm->created_vcpus) {
  

• 	r = -EBUSY;
  

• } else if (!kvm_pmu_partition_ready()) {
  

• 	r = -EPERM;
  

• } else {
  

• 	r = 0;
  

• 	kvm_pmu_partition_enable(kvm, cap->args[0]);
  

• }
  

• break;
  

  default: break; }

@@ -388,6 +400,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext) case KVM_CAP_ARM_PMU_V3: r = kvm_supports_guest_pmuv3(); break;

• case KVM_CAP_ARM_PARTITION_PMU:

• r = kvm_pmu_partition_ready();
  

"ready" is very confusing in this context, as KVM will never be ready to support the feature on a system w/o the prerequisites.

Thanks, Oliver