linux-kernel - Re: [PATCH 03/24] perf: Add build id data in mmap2 event

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Date:   Tue, 10 Nov 2020 08:54:26 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jiri Olsa <jolsa@...nel.org>, lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Michael Petlan <mpetlan@...hat.com>,
        Song Liu <songliubraving@...com>,
        Ian Rogers <irogers@...gle.com>,
        Stephane Eranian <eranian@...gle.com>,
        Alexey Budankov <alexey.budankov@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Adrian Hunter <adrian.hunter@...el.com>
Subject: Re: [PATCH 03/24] perf: Add build id data in mmap2 event

Em Tue, Nov 10, 2020 at 09:07:16AM +0100, Peter Zijlstra escreveu:
> On Mon, Nov 09, 2020 at 10:53:54PM +0100, Jiri Olsa wrote:
> > Adding support to carry build id data in mmap2 event.
> > 
> > The build id data replaces maj/min/ino/ino_generation
> > fields, whichc are also used to identify map's binary,
> > so it's ok to replace them with build id data:
> > 
> >   union {
> >           struct {
> >                   u32       maj;
> >                   u32       min;
> >                   u64       ino;
> >                   u64       ino_generation;
> >           };
> >           struct {
> >                   u8        build_id[20];
> >                   u8        build_id_size;
> 
> What's the purpose of a size field for a fixed size array? Also, I'd
> flip the order of these fields, first have the size and then the array.

There can be different types of build-ids, with different sizes,
flipping the order of the fields is indeed sensible, as we could then
support even larger build_ids if the need arises :)

- Arnaldo
 
> >                   u8        __reserved_1;
> >                   u16       __reserved_2;
> >           };
> >   };
> > 
> > Replaced maj/min/ino/ino_generation fields give us size
> > of 24 bytes. We use 20 bytes for build id data, 1 byte
> > for size and rest is unused.

-- 

- Arnaldo