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

[Alexandru Elisei]

Hi James,

On Tue, Jul 01, 2025 at 04:31:58PM +0100, James Clark wrote:
> DEN0154 states that writes to PMBPTR_EL1 or PMBSR_EL1 must be done while
> the buffer is disabled (PMBLIMITR_EL1.E == 0). Re-arrange the interrupt
> handler to always disable the buffer for non-spurious interrupts before
> doing either.
> 
> Most of arm_spe_pmu_disable_and_drain_local() is now always done, so for
> faults the only thing left to do is clear PMSCR_EL1.
> 
> Elaborate the comment in arm_spe_pmu_disable_and_drain_local() to
> explain the ramifications of not doing it in the right order.
> 
> 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 | 33 +++++++++++++++++++++------------
>  1 file changed, 21 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c
> index 6235ca7ecd48..5829947c8871 100644
> --- a/drivers/perf/arm_spe_pmu.c
> +++ b/drivers/perf/arm_spe_pmu.c
> @@ -559,7 +559,12 @@ static void arm_spe_perf_aux_output_end(struct perf_output_handle *handle)
>  
>  static void arm_spe_pmu_disable_and_drain_local(void)
>  {
> -	/* Disable profiling at EL0 and EL1 */
> +	/*
> +	 * To prevent the CONSTRAINED UNPREDICTABLE behavior of either writing
> +	 * to memory after the buffer is disabled, or SPE reporting an access
> +	 * not allowed event, we must disable sampling before draining the
> +	 * buffer.
> +	 */
>  	write_sysreg_s(0, SYS_PMSCR_EL1);
>  	isb();
>  
> @@ -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);

I've been trying to figure out if we need an ISB here to order clearing the
service bit before the PMBLIMITR_EL1 write in arm_spe_perf_aux_output_begin().

arm_spe_perf_aux_output_begin() is called only when the buffer is full, and this
rules out the event having the discard attribute (buffer management events are
not generated in discard mode).

If a new buffer cannot be allocated (perf_aux_output_begin() returns NULL), then
PMBLIMITR_EL1.E remains 0, so no need to order the two writes.

The only other path remaining in arm_spe_perf_aux_output_begin() is
reprogramming the buffer, in which case there's an ISB before the write to
PMBLIMITR_EL1.

In conclusion, I think it's correct not to have an ISB here.

> +
>  	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);

Before:

arm_spe_pmu_buf_get_fault_act:
  <drain buffer>
  ISB
arm_spe_pmu_disable_and_drain_local:
  PMSCR_EL1 = 0
  ISB		# disables profiling
  <drain buffer>
  PMBLIMITR_EL1=0
PMBSR_EL1=0
ERET		# synchronizes the two writes above

Now:

arm_spe_pmu_buf_get_fault_act:
  <drain buffer>
  ISB
PMBLIMITR_EL1=0
ISB 		# disables profiling
PMBSR_EL1=0
PMSCR_EL1=0
ERET		# synchronizes the two writes above

This looks correct to me.

Thanks,
Alex

>  		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();
> -		}
>  		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
>