linux-kernel - Re: [PATCH] perf: Ignore format attributes with an unknown perf_event

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Message-ID: <CAL_JsqLmszK=NzD_2nYnZG1dD=tU1fKGUtYL-dUG2a497LkEcg@mail.gmail.com>
Date:   Fri, 2 Sep 2022 10:25:36 -0500
From:   Rob Herring <robh@...nel.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     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>,
        Jiri Olsa <jolsa@...nel.org>,
        James Clark <james.clark@....com>,
        linux-perf-users <linux-perf-users@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] perf: Ignore format attributes with an unknown
 perf_event_attr field

On Fri, Sep 2, 2022 at 1:53 AM Namhyung Kim <namhyung@...nel.org> wrote:
>
> Hello,
>
> On Thu, Sep 1, 2022 at 11:55 AM Rob Herring <robh@...nel.org> wrote:
> >
> > If the kernel exposes a new perf_event_attr field in a format attr, perf
> > will return an error stating the specified PMU can't be found. For
> > example, a format attr with 'config3:0-63' causes an error if config3 is
> > unknown to perf. This causes a compatibility issue between a newer
> > kernel and an older perf tool.
> >
> > The addition here makes any attr string up to the ':' ignored, but
> > still checks the 'bits' portion.
> >
> > Signed-off-by: Rob Herring <robh@...nel.org>
> > ---
> > This is the YACC mud I threw and seems to stick. Maybe there's a better
> > way to handle this. It doesn't seem like there's a way to do wildcards
> > (i.e. config.*) in YACC.
> >
> > This is needed for this series[1]. Unfortunately the best we do to avoid
> > the issue is applying this to stable. I think there's some time before
> > v8.7 h/w is deployed, too.
>
> Maybe you could change the format_term rule to take an identifier instead
> of PP_CONFIG* directly and pass it to perf_pmu__new_format().  Then
> it could check the string and create an appropriate PERF_PMU_FORMAT_VALUE_*
> or ignore it according to the PERF_ATTR_SIZE_VER*.

That only moves parsing of configN from YACC to strcmp in C. In doing
so, we'd be left with just the 'error' token case which seems a bit
odd (if there's another way to do it, I don't know. yacc is not my
thing). Is that really better? Unless there is some way to retrieve
the PERF_ATTR_SIZE_VER* from the kernel at runtime?

Rob