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Date:   Wed, 16 Sep 2020 17:20:13 +0200
From:   peterz@infradead.org
To:     Arnaldo Carvalho de Melo <acme@kernel.org>
Cc:     Jiri Olsa <jolsa@redhat.com>, Ian Rogers <irogers@google.com>,
        Jiri Olsa <jolsa@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>,
        "Frank Ch. Eigler" <fche@redhat.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 16/26] perf tools: Synthesize modules with mmap3
Message-ID: <20200916152013.GD1362448@hirez.programming.kicks-ass.net>
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On Wed, Sep 16, 2020 at 12:10:21PM -0300, Arnaldo Carvalho de Melo wrote:
> Em Wed, Sep 16, 2020 at 04:17:00PM +0200, peterz@infradead.org escreveu:
> > On Wed, Sep 16, 2020 at 11:07:44AM -0300, Arnaldo Carvalho de Melo wrote:
> > > Em Wed, Sep 16, 2020 at 10:20:18AM +0200, Jiri Olsa escreveu:
> > 
> > > > > IIRC BUILD_ID_SIZE is 20 bytes which is the correct size for SHA-1. A
> > > > > build ID may be 128-bits (16 bytes) if md5 or uuid hashes are used.
> > > > > Should this test just be "> 0" ?
> > > > 
> > > > ah right, will check on that
> > > 
> > > And how do you deal with this in the kernel? I.e. to inform userspace,
> > > via the PERF_RECORD_MMAP3 (or MMAP2 with that misc bit trick) the size
> > > of the build-id?
> > 
> > The union size is 24 bytes, so there's plenty space to store a length
> > field with the buildid.
> 
> So, I think we should instead use a bit in the misc field, stating the
> kind of build-id, so that we don't waste a byte for that, I think.

There's no wastage:

	u32 min, maj;
	u64 ino;
	u64 ino_generation;

is 24 bytes, buildit is 20 bytes, that leaves us 4 bytes to encode the
buildid type without growing anything.