linux-kernel - Re: [PATCH RFC v1 2/3] perf: Add perf_event

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Date:   Wed, 31 Aug 2022 17:07:25 -0300
From:   Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
To:     Rob Herring <robh@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-perf-users <linux-perf-users@...r.kernel.org>
Subject: Re: [PATCH RFC v1 2/3] perf: Add perf_event_attr::config3



On August 31, 2022 4:06:40 PM GMT-03:00, Rob Herring <robh@...nel.org> wrote:
>On Thu, Aug 25, 2022 at 1:08 PM Rob Herring <robh@...nel.org> wrote:
>>
>> Arm SPEv1.2 adds another 64-bits of event filtering control. As the
>> existing perf_event_attr::configN fields are all used up for SPE PMU, an
>> additional field is needed. Add a new 'config3' field.
>
>In testing this, just exposing 'config3' in the format attributes
>causes the SPE PMU to be disabled. So we can't add this without
>breaking existing perf. Shouldn't perf just skip any format fields it
>doesn't know about?

Something it doesn't know about, even more at the main perf_event_attr level, may preclude it to grok new records.

"May" is the key word, as perf_event_hdr has a size field, maybe sometimes it's possible to do as you say.

If you're interested propose patches to make perf ignore things when possible.

- Arnaldo