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

[Will Deacon <will@kernel.org>]

On Mon, Sep 08, 2025 at 02:54:32PM +0100, James Clark wrote:
> 
> 
> On 08/09/2025 2:41 pm, Will Deacon 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@linaro.org>
> > > ---
> > >   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();
> > 
> > 
> > Hmm.
> > 
> > arm_spe_perf_aux_output_begin() is only called in two places:
> > 
> > 1. From arm_spe_pmu_start()
> > 2. From arm_spe_pmu_irq_handler()
> > 
> > For (1), we know that profiling is disabled by PMSCR_EL1.ExSPE.
> > For (2), we know that profiling is disabled by PMBSR_EL1.S.
> > 
> > In both cases, we already have an isb() before enabling profiling again
> > so I don't understand what this additional isb() is achieving.
> > 
> 
> It's to prevent PMBPTR_EL1 from being written to after the PMBLIMITR_EL1
> write than enables the buffer again. So you're right it's already disabled
> up to this point, which is why we didn't need to add another isb(). This
> change is only for the re-enabling bit.
> 
> If the instructions were reordered you could get this ordering at the end of
> arm_spe_perf_aux_output_begin():
> 
>   write_sysreg_s(limit, SYS_PMBLIMITR_EL1); // Enables buffer
> 
>   write_sysreg_s(base, SYS_PMBPTR_EL1);  // Invalid write to PMBPTR
> 
> Instead of the new version with the barrier where PMBPTR must come before:
> 
>   write_sysreg_s(base, SYS_PMBPTR_EL1);
>   isb()
>   write_sysreg_s(limit, SYS_PMBLIMITR_EL1);

... but my point is that profiling is still disabled after writing to
PMBLIMITR_EL1.

Will