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

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Date:   Thu, 1 Sep 2022 23:52:56 -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

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*.

Thanks,
Namhyung


>
> Rob
>
> [1] https://lore.kernel.org/all/20220825-arm-spe-v8-7-v1-0-c75b8d92e692@kernel.org/
>
>  tools/perf/util/pmu.y | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/tools/perf/util/pmu.y b/tools/perf/util/pmu.y
> index bfd7e8509869..3096864ec9b9 100644
> --- a/tools/perf/util/pmu.y
> +++ b/tools/perf/util/pmu.y
> @@ -60,6 +60,9 @@ PP_CONFIG2 ':' bits
>                                       PERF_PMU_FORMAT_VALUE_CONFIG2,
>                                       $3));
>  }
> +|
> +error ':' bits
> +{}
>
>  bits:
>  bits ',' bit_term
> --
> 2.34.1
>