linux-kernel - Re: [PATCH v2 1/5] perf: arm_spe: Add format option for discard mode

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Message-ID: <20250107173950.GA8111@willie-the-truck>
Date: Tue, 7 Jan 2025 17:39:51 +0000
From: Will Deacon <will@...nel.org>
To: James Clark <james.clark@...aro.org>
Cc: linux-arm-kernel@...ts.infradead.org, linux-perf-users@...r.kernel.org,
	irogers@...gle.com, yeoreum.yun@....com, mark.rutland@....com,
	robh@...nel.org, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	"Liang, Kan" <kan.liang@...ux.intel.com>,
	John Garry <john.g.garry@...cle.com>,
	Mike Leach <mike.leach@...aro.org>, Leo Yan <leo.yan@...ux.dev>,
	Graham Woodward <graham.woodward@....com>,
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH v2 1/5] perf: arm_spe: Add format option for discard mode

On Tue, Dec 24, 2024 at 10:44:08AM +0000, James Clark wrote:
> FEAT_SPEv1p2 (optional from Armv8.6) adds a discard mode that allows all
> SPE data to be discarded rather than written to memory. Add a format
> bit for this mode.
> 
> If the mode isn't supported, the format bit isn't published and attempts
> to use it will result in -EOPNOTSUPP. Allocating an aux buffer is still
> allowed even though it won't be written to so that old tools continue to
> work, but updated tools can choose to skip this step.
> 
> Reviewed-by: Yeoreum Yun <yeoreum.yun@....com>
> Signed-off-by: James Clark <james.clark@...aro.org>
> ---
>  drivers/perf/arm_spe_pmu.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c
> index fd5b78732603..9aaf3f98e6f5 100644
> --- a/drivers/perf/arm_spe_pmu.c
> +++ b/drivers/perf/arm_spe_pmu.c
> @@ -193,6 +193,9 @@ static const struct attribute_group arm_spe_pmu_cap_group = {
>  #define ATTR_CFG_FLD_store_filter_CFG		config	/* PMSFCR_EL1.ST */
>  #define ATTR_CFG_FLD_store_filter_LO		34
>  #define ATTR_CFG_FLD_store_filter_HI		34
> +#define ATTR_CFG_FLD_discard_CFG		config	/* PMBLIMITR_EL1.FM = DISCARD */
> +#define ATTR_CFG_FLD_discard_LO			35
> +#define ATTR_CFG_FLD_discard_HI			35
>  
>  #define ATTR_CFG_FLD_event_filter_CFG		config1	/* PMSEVFR_EL1 */
>  #define ATTR_CFG_FLD_event_filter_LO		0
> @@ -216,6 +219,7 @@ GEN_PMU_FORMAT_ATTR(store_filter);
>  GEN_PMU_FORMAT_ATTR(event_filter);
>  GEN_PMU_FORMAT_ATTR(inv_event_filter);
>  GEN_PMU_FORMAT_ATTR(min_latency);
> +GEN_PMU_FORMAT_ATTR(discard);
>  
>  static struct attribute *arm_spe_pmu_formats_attr[] = {
>  	&format_attr_ts_enable.attr,
> @@ -228,9 +232,15 @@ static struct attribute *arm_spe_pmu_formats_attr[] = {
>  	&format_attr_event_filter.attr,
>  	&format_attr_inv_event_filter.attr,
>  	&format_attr_min_latency.attr,
> +	&format_attr_discard.attr,
>  	NULL,
>  };
>  
> +static bool discard_unsupported(struct arm_spe_pmu *spe_pmu)
> +{
> +	return spe_pmu->pmsver < ID_AA64DFR0_EL1_PMSVer_V1P2;
> +}

Why not add a new SPE_PMU_FEAT_* for this and handle it in a similar
way to other optional hardware features?

Will