Re: [PATCH v3 06/22] perf: arm_pmuv3: Introduce method to partition the PMU

Mark Rutland
2025-07-07T16:57:23Z

On Thu, Jun 26, 2025 at 08:04:42PM +0000, Colton Lewis wrote:
> For PMUv3, the register field MDCR_EL2.HPMN partitiones the PMU
> counters into two ranges where counters 0..HPMN-1 are accessible by
> EL1 and, if allowed, EL0 while counters HPMN..N are only accessible by
> EL2.
>
> Create module parameter reserved_host_counters to reserve a number of
> counters for the host. This number is set at boot because the perf
> subsystem assumes the number of counters will not change after the PMU
> is probed.
>
> Introduce the function armv8pmu_partition() to modify the PMU driver's
> cntr_mask of available counters to exclude the counters being reserved
> for the guest and record reserved_guest_counters as the maximum
> allowable value for HPMN.
>
> Due to the difficulty this feature would create for the driver running
> at EL1 on the host, partitioning is only allowed in VHE mode. Working
> on nVHE mode would require a hypercall for every counter access in the
> driver because the counters reserved for the host by HPMN are only
> accessible to EL2.

It would be good if we could elaborate on this last point. When exactly
do we intend to configure HPMN (e.g. is that static, dynamic at
load/put, or dynamic at finer granularity)?

I ask becuase it's not immediately clear to me how this would break nVHE
without also breaking direct userspace access on VHE, unless we flip
HPMN dynamically at load/put, and this is only broken in some transient
windows on nVHE.

>
> Signed-off-by: Colton Lewis <coltonlewis@google.com>
> ---
> arch/arm/include/asm/arm_pmuv3.h | 14 ++++++
> arch/arm64/include/asm/arm_pmuv3.h | 5 ++
> arch/arm64/include/asm/kvm_pmu.h | 6 +++
> arch/arm64/kvm/Makefile | 2 +-
> arch/arm64/kvm/pmu-part.c | 23 ++++++++++

Maybe I'll contradict Oliver and Marc here (and whatever they say
rules), but IMO it'd be nice to spell out "partition" rather than "part"
here for clarity.

Mark.