linux-kernel - Re: [RFC] libperf: Add support for user space counter access

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Message-ID: <20200811105027.GD699846@krava>
Date:   Tue, 11 Aug 2020 12:50:27 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Rob Herring <robh@...nel.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>
Subject: Re: [RFC] libperf: Add support for user space counter access

On Mon, Aug 10, 2020 at 12:11:23PM -0600, Rob Herring wrote:
> On Sat, Aug 8, 2020 at 4:22 AM Jiri Olsa <jolsa@...hat.com> wrote:
> >
> > On Fri, Aug 07, 2020 at 05:05:17PM -0600, Rob Herring wrote:
> > > x86 and arm64 can both support direct access of event counters in
> > > userspace. The access sequence is less than trivial and currently exists
> > > in perf test code (tools/perf/arch/x86/tests/rdpmc.c) with copies in
> > > projects such as PAPI and libpfm4.
> > >
> > > Patches to add arm64 userspace support are pending[1].
> > >
> > > For this RFC, looking for a yes, seems like a good idea, or no, go away we
> > > don't want this in libperf.
> >
> > hi,
> > looks great!
> >
> > I wanted to add this for very long time.. so yes, we want this ;-)
> 
> Thanks for the quick feedback. Would this be better implemented as a
> fast path for perf_evsel__read()? If so, how to get the mmap data

if it works for all events, which I'm not sure of

> which is associated with a evlist rather than a evsel?

not sure what you mean, you can mmap evsel, not evlist

jirka