Re: [PATCH v9 20/22] KVM: arm64: Add vCPU device attr to partition the PMU

[James Clark <james.clark@linaro.org>]

On 24/09/2026 18:29, Colton Lewis wrote:
> Add the KVM_ARM_VCPU_PMU_V3_ENABLE_PARTITION vCPU device attribute to
> enable a Partitioned PMU for a VM where PMUv3 and VHE are supported.
> 
> When partitioning is enabled (tracked via
> KVM_ARCH_FLAG_PARTITION_PMU_ENABLED), userspace must explicitly
> configure the number of guest event counters via
> KVM_ARM_VCPU_PMU_V3_SET_NR_COUNTERS with a value strictly less than the
> maximum number of general-purpose counters implemented by the PMU (or
> 0 only when FEAT_HPMN0 is supported), leaving at least one
> general-purpose counter reserved for host profiling prior to calling
> KVM_ARM_VCPU_PMU_V3_INIT.
> 
> Signed-off-by: Colton Lewis <coltonlewis@google.com>
> ---
>   Documentation/virt/kvm/devices/vcpu.rst | 42 ++++++++++++++-
>   arch/arm64/include/asm/kvm_host.h       |  1 +
>   arch/arm64/include/uapi/asm/kvm.h       |  2 +
>   arch/arm64/kvm/pmu.c                    | 71 ++++++++++++++++++++++++-
>   arch/arm64/kvm/sys_regs.c               |  5 +-
>   5 files changed, 118 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/devices/vcpu.rst b/Documentation/virt/kvm/devices/vcpu.rst
> index deb5c51bc00c8..fb5921ed9dea2 100644
> --- a/Documentation/virt/kvm/devices/vcpu.rst
> +++ b/Documentation/virt/kvm/devices/vcpu.rst
> @@ -57,6 +57,9 @@ Returns:
>                     hardware PMU, or interrupt number not set (non-GICv5
>                     guests, only)
>   	 -EBUSY   PMUv3 already initialized
> +	 -EINVAL  Partitioning enabled without explicitly configuring
> +		  fewer than max_counters via
> +		  KVM_ARM_VCPU_PMU_V3_SET_NR_COUNTERS
>   	 =======  ======================================================
>   
>   Request the initialization of the PMUv3.  If using the PMUv3 with an in-kernel
> @@ -162,7 +165,8 @@ the cpu field to the processor id.
>   	 -EFAULT  Error accessing the value pointed to by addr
>   	 -ENODEV  PMUv3 not supported or GIC not initialized
>   	 -EINVAL  No PMUv3 explicitly selected, or value of N out of
> -	 	  range
> +	 	  range (or N >= max_counters when partitioning is
> +		  enabled)

"No PMUv3 explicitly selected, or value of N > max_counters, or N > 
max_counters - 1 when partitioning is enabled".

That might be clearer, otherwise it seems like max_counters being 
relevant to "out of range" only applies to when partitioning is enabled, 
not that it's max_counters all the time but one is -1.