Re: [PATCH 2/3] perf: arm_spe: Disable buffer before writing to PMBPTR_EL1 or PMBSR_EL1

[James Clark]

On 04/07/2025 4:50 pm, Leo Yan wrote:
> On Tue, Jul 01, 2025 at 04:31:58PM +0100, James Clark wrote:
>
> [...]
>
> > @@ -661,16 +666,24 @@ static irqreturn_t arm_spe_pmu_irq_handler(int irq, void *dev)
> >   	 */
> >   	irq_work_run();
> >   
> > +	/*
> > +	 * arm_spe_pmu_buf_get_fault_act() already drained, and PMBSR_EL1.S == 1
> > +	 * means that StatisticalProfilingEnabled() == false. So now we can
> > +	 * safely disable the buffer.
> > +	 */
> > +	write_sysreg_s(0, SYS_PMBLIMITR_EL1);
> > +	isb();
> > +
> > +	/* Status can be cleared now that PMBLIMITR_EL1.E == 0 */
> > +	write_sysreg_s(0, SYS_PMBSR_EL1);
> > +
>
> An important thing is about sequence:
> As described in arm_spe_pmu_disable_and_drain_local(), should we always
> clear ELs bits in PMSCR_EL1 before clear PMBLIMITR_EL1.E bit? As a
> reference, we could see TRBE always clear ELx bits before disable trace
> buffer.
>
> And a trivial flaw:
>
> If the TRUNCATED flag has been set, the irq_work_run() above runs the
> IRQ work to invoke the arm_spe_pmu_stop() to disable trace buffer, which
> clear SYS_PMBLIMITR_EL1.E bit. This is why the current code does not
> explictly clear SYS_PMBLIMITR_EL1.E bit.
>
> With this patch, the interrupt handler will clear SYS_PMBLIMITR_EL1.E
> bit twice for a trunacated case.

I suppose that's a rarer case that we don't necessarily have to optimize 
for. I don't think it will do any harm, but is it even possible to avoid?

There are already some other duplications in the driver, for example in 
arm_spe_pmu_stop() we call arm_spe_pmu_disable_and_drain_local() which 
drains, and then arm_spe_pmu_buf_get_fault_act() which also drains again.

> >   	switch (act) {
> >   	case SPE_PMU_BUF_FAULT_ACT_FATAL:
> >   		/*
> > -		 * If a fatal exception occurred then leaving the profiling
> > -		 * buffer enabled is a recipe waiting to happen. Since
> > -		 * fatal faults don't always imply truncation, make sure
> > -		 * that the profiling buffer is disabled explicitly before
> > -		 * clearing the syndrome register.
> > +		 * To complete the full disable sequence, also disable profiling
> > +		 * at EL0 and EL1, we don't want to continue at all anymore.
> >   		 */
> > -		arm_spe_pmu_disable_and_drain_local();
> > +		write_sysreg_s(0, SYS_PMSCR_EL1);
> >   		break;
> >   	case SPE_PMU_BUF_FAULT_ACT_OK:
> >   		/*
> > @@ -679,18 +692,14 @@ static irqreturn_t arm_spe_pmu_irq_handler(int irq, void *dev)
> >   		 * PMBPTR might be misaligned, but we'll burn that bridge
> >   		 * when we get to it.
> >   		 */
> > -		if (!(handle->aux_flags & PERF_AUX_FLAG_TRUNCATED)) {
> > +		if (!(handle->aux_flags & PERF_AUX_FLAG_TRUNCATED))
> >   			arm_spe_perf_aux_output_begin(handle, event);
> > -			isb();
>
> I am a bit suspecious we can remove this isb().
>
> As a reference to the software usage PKLXF in Arm ARM (DDI 0487 L.a),
> after enable TRBE trace unit, an ISB is mandatory. Maybe check a bit
> for this?
>
> Thanks,
> Leo

Wasn't this isb() to separate the programming of the registers with the 
status register clear at the end of this function to enable profiling?

But now we enable profiling with the write to PMBLIMITR_EL1 in 
arm_spe_perf_aux_output_begin() and the last thing here is the ERET. 
That's specifically mentioned as enough synchronization in PKLXF:

  In the common case, this is an ERET instruction that returns to a
  different Exception level where tracing is allowed.

> > -		}
> >   		break;
> >   	case SPE_PMU_BUF_FAULT_ACT_SPURIOUS:
> >   		/* We've seen you before, but GCC has the memory of a sieve. */
> >   		break;
> >   	}
> >   
> > -	/* The buffer pointers are now sane, so resume profiling. */
> > -	write_sysreg_s(0, SYS_PMBSR_EL1);
> >   	return IRQ_HANDLED;
> >   }
> >   
> >
> > --
> > 2.34.1