Re: [PATCH 2/3] perf: arm_spe: Factor HW discard mode out of arm_spe_perf_aux_output_begin()

[Will Deacon]

On Wed, Aug 05, 2026 at 04:01:52PM +0100, James Clark wrote:
> Discard mode doesn't call perf_aux_output_begin() or emit aux records,
> and enabling it never fails. We'll add a second discard mode in the
> next commit so it will be cleaner if there is a separate begin function
> just for discard mode.
> 
> No functional changes intended.

[...]

> +static void arm_spe_pmu_begin_discard(struct perf_output_handle *handle,
> +				      struct perf_event *event)
> +{
> +	u64 limit;
> +
> +	limit = FIELD_PREP(PMBLIMITR_EL1_FM, PMBLIMITR_EL1_FM_DISCARD);
> +	limit |= PMBLIMITR_EL1_E;
> +
> +	write_sysreg_s(limit, SYS_PMBLIMITR_EL1);
> +}
> +
>  static int arm_spe_perf_aux_output_begin(struct perf_output_handle *handle,
>  					 struct perf_event *event)
>  {
>  	u64 base, limit;
>  	struct arm_spe_pmu_buf *buf;

Is it worth checking that we have a discard event in begin_discard() and
similarly that we don't have a discard event in output_begin()? It looks
like quite an error-prone interface for the caller, otherwise.

> @@ -775,6 +785,12 @@ static irqreturn_t arm_spe_pmu_irq_handler(int irq, void *dev)
>  		arm_spe_pmu_disable_and_drain_local();
>  		break;
>  	case SPE_PMU_BUF_FAULT_ACT_OK:
> +		if (arm_spe_discard_mode(event)) {
> +			arm_spe_pmu_begin_discard(handle, event);
> +			isb();
> +			break;
> +		}

This looks like new behaviour?

Will