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

[Colton Lewis <coltonlewis@google.com>]

Hi James,

On Wed, Sep 30, 2026 at 04:26:23PM +0100, James Clark wrote:
> 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.

Inside a VMM, userspace can query the default PMCR_EL0.N via
KVM_GET_ONE_REG(SYS_PMCR_EL0) after KVM_ARM_VCPU_INIT (or
KVM_ARM_VCPU_PMU_V3_SET_PMU) and before calling
KVM_ARM_VCPU_PMU_V3_SET_NR_COUNTERS, since KVM initializes
nr_pmu_counters to the PMU's max_counters (which is how
vpmu_counter_access discovers the limit).

For users and management tools outside the VMM (e.g. choosing N to pass
on the QEMU command line), I agree it's awkward that
/sys/bus/event_source/devices/<pmu>/caps/ exposes slots and bus_width
but not the number of counters. I will add a patch to expose a caps
attribute for the number of counters in drivers/perf/arm_pmuv3.c.

Thanks,
Colton