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

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Date:   Tue, 6 Sep 2022 11:15:47 -0700
From:   Namhyung Kim <namhyung@...nel.org>
To:     Rob Herring <robh@...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 8:25 AM Rob Herring <robh@...nel.org> wrote:
>
> 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?

I thought we could do more flexible handling and detailed error reporting
in the C code.  But it could be done in the lex/yacc as well..

I think the general idea is that we want to run a more recent version of
perf tools than the kernel.  So if it detects the tool is older, it can show
a warning message like:

"config3 is not in the perf_event_attr.. skipping.
 Maybe you're running on a newer kernel. Please upgrade the perf tool."


> Unless there is some way to retrieve
> the PERF_ATTR_SIZE_VER* from the kernel at runtime?

Even if it can retrieve the info at runtime, perf tool might not know
how to use the new config term.

Thanks,
Namhyung