Re: [PATCH v2 1/2] perf: arm_spe: Correct setting the PERF_HES_STOPPED flag

[Will Deacon <will@kernel.org>]

On Mon, Nov 10, 2025 at 04:28:31PM +0000, Leo Yan wrote:
> In arm_spe_perf_aux_output_begin(), if the calculation of limit fails
> and arm_spe_pmu_next_off() returns zero, the driver misses to set the
> PERF_HES_STOPPED flag for the event.  As a result, hwc->state does not
> reflect the latest state, which can mislead subsequent operations.
> 
> Validate the limit when exiting the function: if the limit is 0,
> that tracing is disabled, set the PERF_HES_STOPPED flag accordingly.
> 
> Fixes: d5d9696b0380 ("drivers/perf: Add support for ARMv8.2 Statistical Profiling Extension")
> Signed-off-by: Leo Yan <leo.yan@arm.com>
> ---
>  drivers/perf/arm_spe_pmu.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c
> index fa50645feddadbea5dc1e404f80f62cf5aa96fd4..fc8f908c2c3a270f2d1ae574c2badb1fbcf51484 100644
> --- a/drivers/perf/arm_spe_pmu.c
> +++ b/drivers/perf/arm_spe_pmu.c
> @@ -597,7 +597,6 @@ static void arm_spe_perf_aux_output_begin(struct perf_output_handle *handle,
>  	/* Start a new aux session */
>  	buf = perf_aux_output_begin(handle, event);
>  	if (!buf) {
> -		event->hw.state |= PERF_HES_STOPPED;
>  		/*
>  		 * We still need to clear the limit pointer, since the
>  		 * profiler might only be disabled by virtue of a fault.
> @@ -608,15 +607,19 @@ static void arm_spe_perf_aux_output_begin(struct perf_output_handle *handle,
>  
>  	limit = buf->snapshot ? arm_spe_pmu_next_snapshot_off(handle)
>  			      : arm_spe_pmu_next_off(handle);
> -	if (limit)
> -		limit |= PMBLIMITR_EL1_E;
> +	if (!limit)
> +		goto out_write_limit;

Is 'limit == 0' always indicative of an error, even in snapshot mode?

If __arm_spe_pmu_next_off() fails, it will call perf_aux_output_end()
with the TRUNCATED flag set, which should then disable the event
via arm_spe_pmu_del() and update the state there.

Is that not happening?

Will