Re: [PATCH 1/3] perf: arm_spe: Add barrier before enabling profiling buffer

[James Clark]

On 04/07/2025 3:04 pm, Leo Yan wrote:
> On Tue, Jul 01, 2025 at 04:31:57PM +0100, James Clark wrote:
> > DEN0154 states that PMBPTR_EL1 must not be modified while the profiling
> > buffer is enabled. Ensure that enabling the buffer comes after setting
> > PMBPTR_EL1 by inserting an isb().
> >
> > This only applies to guests for now, but in future versions of the
> > architecture the PE will be allowed to behave in the same way.
> >
> > Fixes: d5d9696b0380 ("drivers/perf: Add support for ARMv8.2 Statistical Profiling Extension")
> > Signed-off-by: James Clark <james.clark@xxxxxxxxxx>
> > ---
> >   drivers/perf/arm_spe_pmu.c | 1 +
> >   1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c
> > index 3efed8839a4e..6235ca7ecd48 100644
> > --- a/drivers/perf/arm_spe_pmu.c
> > +++ b/drivers/perf/arm_spe_pmu.c
> > @@ -537,6 +537,7 @@ static void arm_spe_perf_aux_output_begin(struct perf_output_handle *handle,
> >   	limit += (u64)buf->base;
> >   	base = (u64)buf->base + PERF_IDX2OFF(handle->head, buf);
> >   	write_sysreg_s(base, SYS_PMBPTR_EL1);
> > +	isb();
>
> I know that you and Alexandru have discussed whether the isb() should
> be placed here or after the out_write_limit label. I should have engaged
> in the discussion earlier. Sorry for raising the question now.
>
> My understanding is that isb() is not only for synchronizing the write
> to PMBPTR_EL1. It also serves as a context synchronization event
> between any other SPE register writes and the write to
> SYS_PMBLIMITR_EL1.
>
> Let me give an example (perhaps a rare one): if we use perf snapshot
> mode or the AUX pause/resume mode, it's possible that the flow does
> not trigger an interrupt via overflow. Instead, the sequence might
> look like this:
>
>    arm_spe_pmu_stop()
>      `> arm_spe_pmu_start()
>           `> arm_spe_perf_aux_output_begin()
>
> In this case, to ensure that all SPE system registers are properly
> written to the hardware, the safest approach is to always execute isb()
> just before writing to SYS_PMBLIMITR_EL1. (In other words, after the
> label out_write_limit).
>
> Thanks,
> Leo

I think the point is that any write that enables the buffer must come 
last, but not necessarily all writes. And not all paths in 
arm_spe_perf_aux_output_begin() enable it so the isb() was only added on 
the path that does.

I couldn't see an issue with your example, are you saying 
arm_spe_pmu_stop() could call arm_spe_pmu_start()? It doesn't call it 
directly. Or do you mean the aux pause/resume thing can cause a 
arm_spe_pmu_start() from any point in time? If that was true then it 
doesn't matter where the isb() is because you can never be sure it will 
be before the write.

James

> >   out_write_limit:
> >   	write_sysreg_s(limit, SYS_PMBLIMITR_EL1);
> >
> > --
> > 2.34.1