Re: [PATCH 05/21] perf: arm_pmuv3: Check cntr_mask before using pmccntr

[Robin Murphy <robin.murphy@arm.com>]

On 12/06/2026 8:28 pm, Colton Lewis wrote:
> Check cntr_mask before using pmccntr to ensure it's available. With a
> partitioned PMU, there may be instances where pmccntr is being used by
> the guest and will be absent from cntr_mask.
> 
> Signed-off-by: Colton Lewis <coltonlewis@google.com>
> ---
>   drivers/perf/arm_pmuv3.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/perf/arm_pmuv3.c b/drivers/perf/arm_pmuv3.c
> index 1cceb1f614515..17bb1cfdc271c 100644
> --- a/drivers/perf/arm_pmuv3.c
> +++ b/drivers/perf/arm_pmuv3.c
> @@ -1028,7 +1028,8 @@ static int armv8pmu_get_event_idx(struct pmu_hw_events *cpuc,
>   
>   	/* Always prefer to place a cycle counter into the cycle counter. */
>   	if (armv8pmu_can_use_pmccntr(cpuc, event)) {

Something smells wrong here - we can use PMCCNTR but we can't?

> -		if (!test_and_set_bit(ARMV8_PMU_CYCLE_IDX, cpuc->used_mask))
> +		if (test_bit(ARMV8_PMU_CYCLE_IDX, cpu_pmu->cntr_mask) &&

...i.e. this test would seem to belong in the helper function.

> +		    !test_and_set_bit(ARMV8_PMU_CYCLE_IDX, cpuc->used_mask))
>   			return ARMV8_PMU_CYCLE_IDX;
>   		else if (armv8pmu_event_is_64bit(event) &&
>   			   armv8pmu_event_want_user_access(event) &&

Plus it also doesn't make much sense to fall through to the "we _could_ 
accommodate this event if only PMCCNTR was free" case here if we know 
PMCCNTR can never be free because it's not even available.

Thanks,
Robin.