linux-kernel - Re: [PATCH v2 1/2] perf: arm_spe: Correct setting the PERF_HES

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Message-ID: <20251124184815.GC724103@e132581.arm.com>
Date: Mon, 24 Nov 2025 18:48:15 +0000
From: Leo Yan <leo.yan@....com>
To: Will Deacon <will@...nel.org>
Cc: Mark Rutland <mark.rutland@....com>,
	Alexandru Elisei <alexandru.elisei@....com>,
	James Clark <james.clark@...aro.org>,
	linux-arm-kernel@...ts.infradead.org,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] perf: arm_spe: Correct setting the
 PERF_HES_STOPPED flag

On Mon, Nov 24, 2025 at 04:14:23PM +0000, Will Deacon wrote:
> 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@....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?

Yes, the 'limit' would never be zero unless an error occurs.

> 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?

Correct.  However, this patch is not for the flow you mentioned.

If an error is returned from arm_spe_pmu_next_off(), because hw.state
is not set to PERF_HES_STOPPED, the caller arm_spe_pmu_start() cannot
detect error properly:

    arm_spe_pmu_start()
    {
        ...

        hwc->state = 0;
        arm_spe_perf_aux_output_begin(handle, event); // Miss to set hwc->state
                                                      // for error case.
        if (hwc->state)    // Fail to detect error.
                return;

        reg = arm_spe_event_to_pmsfcr(event);
        write_sysreg_s(reg, SYS_PMSFCR_EL1);

        ...
     }

This patch consolidates arm_spe_perf_aux_output_begin() to always set
the PERF_HES_STOPPED flag for errors, ensuring consistent error
handling.

A side effect is that arm_spe_pmu_del() will skip disabling buffer unit
when "hwc->state==PERF_HES_STOPPED".  This is why we need the patch 02
to explicitly disable the buffer unit in interrupt handler.

Thanks,
Leo