linux-kernel - Re: [PATCH v9 2/5] perf: arm_spe: Add support for filtering on data source

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Message-ID: <20251110154827.GA278048@noisy.programming.kicks-ass.net>
Date: Mon, 10 Nov 2025 16:48:27 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: James Clark <james.clark@...aro.org>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
	Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Joey Gouly <joey.gouly@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>, 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>, Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>, Leo Yan <leo.yan@....com>,
	Anshuman Khandual <anshuman.khandual@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-perf-users@...r.kernel.org, linux-doc@...r.kernel.org,
	kvmarm@...ts.linux.dev
Subject: Re: [PATCH v9 2/5] perf: arm_spe: Add support for filtering on data
 source

On Wed, Oct 29, 2025 at 03:46:02PM +0000, James Clark wrote:
> SPE_FEAT_FDS adds the ability to filter on the data source of packets.
> Like the other existing filters, enable filtering with PMSFCR_EL1.FDS
> when any of the filter bits are set.
> 
> Each bit maps to data sources 0-63 described by bits[0:5] in the data
> source packet (although the full range of data source is 16 bits so
> higher value data sources can't be filtered on). The filter is an OR of
> all the bits, so for example clearing bits 0 and 3 only includes packets
> from data sources 0 OR 3.
> 
> Invert the filter given by userspace so that the default value of 0 is
> equivalent to including all values (no filtering). This allows us to
> skip adding a new format bit to enable filtering and still support
> excluding all data sources which would have been a filter value of 0 if
> not for the inversion.

So from that I'm reading the config4 field will only have like 16 bits,
but here:

> +#define ATTR_CFG_FLD_inv_data_src_filter_CFG	config4	/* inverse of PMSDSFR_EL1 */
> +#define ATTR_CFG_FLD_inv_data_src_filter_LO	0
> +#define ATTR_CFG_FLD_inv_data_src_filter_HI	63

you claim all 64 bits.

Also, afaict:

  #define ATTR_CFG_FLD_min_latency_CFG            config2 /* PMSLATFR_EL1.MINLAT */
  #define ATTR_CFG_FLD_min_latency_LO             0
  #define ATTR_CFG_FLD_min_latency_HI             11

Still has more than 16 bits left.


So why exactly are we needing config4? Can we please get a more solid
argument?