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

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Message-ID: <aV_aHi9BLFzjnmx7@willie-the-truck>
Date: Thu, 8 Jan 2026 16:23:58 +0000
From: Will Deacon <will@...nel.org>
To: Leo Yan <leo.yan@....com>
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 Tue, Nov 25, 2025 at 02:20:36PM +0000, Leo Yan wrote:
> On Mon, Nov 24, 2025 at 07:02:06PM +0000, Will Deacon wrote:
> 
> [...]
> 
> > > > 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.
> > 
> > How is it not for this flow? You're talking about:
> > 
> > arm_spe_pmu_start
> > 	=> arm_spe_perf_aux_output_begin
> > 		=> arm_spe_pmu_next_off // Returns error
> > 
> > The only way arm_spe_pmu_next_off() returns an error is if
> > __arm_spe_pmu_next_off() fails, and that's the flow I'm talking about.
> 
> My bad.  Because you mentioned the TRUNCATED flag, I incorrectly assumed
> it had to be used in interrupt handler with the disable irq work.
> 
> > > 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:
> > 
> > But why isn't PERF_HES_STOPPED set by the sequence I described?
> 
> Fair point.  I can confirm after settting the TRUNCATED flag,
> arm_spe_pmu_del() will be invoked to disable the trace unit and state
> will be updated to PERF_HES_STOPPED.
> 
> > I have a feeling you're right, but I can't piece it together from the
> > information here.
> 
> Let me explain in another way:
> 
> The issue is a mismatch between the state machine and the hardware
> state.  When arm_spe_perf_aux_output_begin() detects an error and does
> not set PMBLIMITR_EL1_E, the trace unit is effectively stopped, but
> the state machine is not updated to PERF_HES_STOPPED. This causes
> callers to handle errors incorrectly [1][2].
> 
> It is arguable that the disable IRQ work will eventually disable the
> trace unit and update hw.state, but the state should be updated in the
> first place by the PMU driver to notify even core layer.

>From what I can tell, perf_aux_output_end() will call
perf_event_disable_inatomic() which should end up invoking
perf_pending_disable() via an IPI-to-self to disable the event and put
it in the PERF_HES_STOPPED state before we return to userspace.

So I still struggle to see the problem here.

Will