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

[Leo Yan <leo.yan@arm.com>]

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();

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

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