Re: [PATCH v9 00/22] ARM64 PMU Partitioning

[James Clark]

On 24/09/2026 18:29, Colton Lewis wrote:
> This series creates a new PMU scheme on ARM, a partitioned PMU that
> allows reserving a subset of counters for more direct guest access,
> significantly reducing overhead. More details, including performance
> benchmarks, can be read in the v1 cover letter linked below.
>
> There is no longer a kernel command line parameter
> (`arm_pmuv3.reserved_host_counters`); PMU partitioning is now completely
> controlled via the KVM API using `KVM_ARM_VCPU_PMU_V3_ENABLE_PARTITION`
> and `KVM_ARM_VCPU_PMU_V3_SET_NR_COUNTERS` vCPU device attributes. When

As far as I know there isn't a straightforward way to get the number of 
hw counters from userspace. You have to either parse dmesg or attempt to 
open incrementally more counters in a group until it fails.

This makes doing things like "use all counters for guest" or "reserve 
two for the host" difficult. Maybe it's time to add a file to the PMU 
that userspace can use query this.