Re: [RFC PATCH 3/3] perf/riscv: Use Sspesa for precise sample attribution

[Sandipan Das]

On 17-08-2026 21:32, Mayuresh Chitale wrote:
> When the Sspesa (Precise Event Sample Attribution) extension is
> available and a counter overflows, the hardware records a sample PC in
> shpmspc register and the counter ID and additional implementation-defined
> data in shpmsdata register.
> 
> In the SBI PMU overflow handler, use shpmspc to report sample PC via
> PERF_SAMPLE_IP for the counter that caused the overflow. Shpmsdata is
> exported as raw data to userspace when events request PERF_SAMPLE_RAW
> sample type and it assists in deriving the sample PC from shpmspc if
> precise attribution is not supported for the event.
> 
> If sspesa is not available or for counters that don't match
> shpmsdata.CNTRID, fallback to epc for PERF_SAMPLE_IP.
> 
> Signed-off-by: Mayuresh Chitale <mayuresh.chitale@xxxxxxxxxxxxxxxx>
> ---
>  drivers/perf/riscv_pmu_sbi.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> diff --git a/drivers/perf/riscv_pmu_sbi.c b/drivers/perf/riscv_pmu_sbi.c
> index dfc886dee5ad..1b0b1cc612de 100644
> --- a/drivers/perf/riscv_pmu_sbi.c
> +++ b/drivers/perf/riscv_pmu_sbi.c
> @@ -63,6 +63,8 @@ PMU_FORMAT_ATTR(firmware, "config:62-63");
>  
>  static bool sbi_v2_available;
>  static bool sbi_v3_available;
> +static bool sspesa_available;
> +
>  static DEFINE_STATIC_KEY_FALSE(sbi_pmu_snapshot_available);
>  #define sbi_pmu_snapshot_available() \
>  	static_branch_unlikely(&sbi_pmu_snapshot_available)
> @@ -1051,6 +1053,11 @@ static irqreturn_t pmu_sbi_ovf_handler(int irq, void *dev)
>  	struct cpu_hw_events *cpu_hw_evt = dev;
>  	u64 start_clock = sched_clock();
>  	struct riscv_pmu_snapshot_data *sdata = cpu_hw_evt->snapshot_addr;
> +	unsigned long sample_pc = 0;
> +	unsigned long sample_data = 0;
> +	int sample_cntrid = -1;
> +	u64 raw_sample;
> +	struct perf_raw_record raw = { 0 };
>  
>  	if (WARN_ON_ONCE(!cpu_hw_evt))
>  		return IRQ_NONE;
> @@ -1088,6 +1095,16 @@ static irqreturn_t pmu_sbi_ovf_handler(int irq, void *dev)
>  		return IRQ_NONE;
>  
>  	regs = get_irq_regs();
> +	/*
> +	 * Sspesa records the PC and metadata of the overflowing counter in
> +	 * hardware. The PC is precise only for events that support precise
> +	 * attribution; otherwise it is best-effort.
> +	 */
> +	if (sspesa_available) {
> +		sample_pc = csr_read(CSR_SHPMSPC);
> +		sample_data = csr_read(CSR_SHPMSDATA);
> +		sample_cntrid = sample_data & SHPMSDATA_CNTRID;
> +	}
>  
>  	for_each_set_bit(lidx, cpu_hw_evt->used_hw_ctrs, RISCV_MAX_COUNTERS) {
>  		struct perf_event *event = cpu_hw_evt->events[lidx];
> @@ -1123,6 +1140,15 @@ static irqreturn_t pmu_sbi_ovf_handler(int irq, void *dev)
>  		riscv_pmu_event_update(event);
>  		hw_evt->state |= PERF_HES_UPTODATE;
>  		perf_sample_data_init(&data, 0, hw_evt->last_period);
> +		if (sspesa_available && hidx == sample_cntrid) {
> +			data.ip = sample_pc;

Does this need to be tied to event->attr.precise_ip?

> +			data.sample_flags |= PERF_SAMPLE_IP;
> +
> +			raw_sample = sample_data;
> +			raw.frag.size = sizeof(raw_sample);
> +			raw.frag.data = &raw_sample;
> +			perf_sample_save_raw_data(&data, event, &raw);
> +		}
>  		if (riscv_pmu_event_set_period(event)) {
>  			/*
>  			 * Unlike other ISAs, RISC-V don't have to disable interrupts
> @@ -1194,6 +1220,9 @@ static int pmu_sbi_setup_irqs(struct riscv_pmu *pmu, struct platform_device *pde
>  	struct cpu_hw_events __percpu *hw_events = pmu->hw_events;
>  	struct irq_domain *domain = NULL;
>  
> +	if (riscv_isa_extension_available(NULL, SSPESA))
> +		sspesa_available = true;
> +
>  	if (riscv_isa_extension_available(NULL, SSCOFPMF)) {
>  		riscv_pmu_irq_num = RV_IRQ_PMU;
>  		riscv_pmu_use_irq = true;