linux-kernel - Re: [RFC/PATCH 1/1] tools headers arm64: Sync arm64's cputype.h with the kernel sources

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Date: Tue, 4 Jun 2024 19:55:42 +0100
From: Mark Rutland <mark.rutland@....com>
To: Leo Yan <leo.yan@....com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>,
	Besar Wicaksono <bwicaksono@...dia.com>,
	Will Deacon <will@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Ian Rogers <irogers@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
	Kan Liang <kan.liang@...ux.intel.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-perf-users@...r.kernel.org, James Clark <james.clark@....com>
Subject: Re: [RFC/PATCH 1/1] tools headers arm64: Sync arm64's cputype.h with
 the kernel sources

On Tue, Jun 04, 2024 at 06:14:22PM +0100, Leo Yan wrote:
> On 6/4/24 10:11, Mark Rutland wrote:
> > Hi Arnaldo,
> > 
> > On Mon, Jun 03, 2024 at 03:33:07PM -0300, Arnaldo Carvalho de Melo wrote:
> > > To get the changes in:
> > > 
> > >    0ce85db6c2141b7f ("arm64: cputype: Add Neoverse-V3 definitions")
> > >    02a0a04676fa7796 ("arm64: cputype: Add Cortex-X4 definitions")
> > >    f4d9d9dcc70b96b5 ("arm64: Add Neoverse-V2 part")
> > 
> > As a heads-up, there are likely to be a couple more updates here
> > shortly:
> > 
> >    https://lore.kernel.org/linux-arm-kernel/20240603111812.1514101-1-mark.rutland@arm.com/
> > 
> > > That makes this perf source code to be rebuilt:
> > > 
> > >    CC      /tmp/build/perf-tools/util/arm-spe.o
> > > 
> > > The changes in the above patch add MIDR_NEOVERSE_V[23] and
> > > MIDR_NEOVERSE_V1 is used in arm-spe.c, so probably we need to add those
> > > and perhaps MIDR_CORTEX_X4 to that array? Or maybe we need to leave this
> > > for later when this is all tested on those machines?
> > 
> > Hmm... looking at where that was added this is somewhat misnamed, this
> > is really saying that these cores use the same IMPLEMENTATION DEFINED
> > encoding of the source field. That's not really a property of Neoverse
> > specifically, and I'm not sure what Arm's policy is here going forwards.
> > 
> > We should probably rename that to something like
> > common_data_source_encoding, with a big comment about exactly what it
> > implies.
> 
> Agreed.
> 
> I went through Neoverse-V2/V3/V4, Cortex-X4, Cortex-X4, Cortex-A720, and
> Cortex-X925, all of them have the same definition for data source packet
> format. It makes sense to change name to Neoverse specific to a more general
> name.

Cool.

> > I would not touch this for now -- someone would have to go audit the
> > TRMs to check that those other cores have the same encoding, and I think
> > it'd be better to do that as a follow-up.
> 
> So far, it's fine to not change the file util/arm-spe.c.
> 
> Now more and more Arm CPUs support the data source in SPE and share the same
> data source format. It's not scalable for us to adding every CPU variant
> into the file util/arm-spe.c.
> 
> I would like to expose the PMSIDR_EL1.LDS bit (Data source indicator for
> sampled load instructions) via the 'cap' folder, and then we can save this
> info into the perf meta data during record phase.

I'd be happy to expose fields from PMSIDR_EL1.

> In the perf report, we can parse the meta data and if the
> PMSIDR_EL1.LDS bit is set, the tool will parse the data source packet
> based on the common format.

I don't believe that's right.

PMSIDR_EL1.LDS indicates that the loaded data source field is
implemented, but even when it is implemented, the format is
IMPLEMENTATION DEFINED.

Today, Arm Ltd implementations happen to share a format, but that isn't
implied by PMSIDR_EL1.LDS, and there's no guarantee that future CPUs
will all use the same format.

For the moment we'll have to keep adding to this list.

Mark.