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Date:   Tue, 10 Nov 2020 09:07:16 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Jiri Olsa <jolsa@kernel.org>
Cc:     Arnaldo Carvalho de Melo <acme@kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Michael Petlan <mpetlan@redhat.com>,
        Song Liu <songliubraving@fb.com>,
        Ian Rogers <irogers@google.com>,
        Stephane Eranian <eranian@google.com>,
        Alexey Budankov <alexey.budankov@linux.intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Adrian Hunter <adrian.hunter@intel.com>
Subject: Re: [PATCH 03/24] perf: Add build id data in mmap2 event
Message-ID: <20201110080716.GU2594@hirez.programming.kicks-ass.net>
References: <20201109215415.400153-1-jolsa@kernel.org>
 <20201109215415.400153-4-jolsa@kernel.org>
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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.

>                   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.