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    SubjectRe: [PATCH 16/26] perf tools: Synthesize modules with mmap3
    Em Wed, Sep 16, 2020 at 05:21:23PM +0200, Jiri Olsa escreveu:
    > 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.

    > not sure there's too many misc bits left if there would be more
    > build id kinds

    So, Ian mentioned a few types of build ids, if there are not that many
    misc bits left for PERF_RECORD_MMAP2, then yeah, we can then use one
    byte and use it as a PERF_RECORD_MMAP2 specific misc bits, me may want
    to have something else in the future, like we're now reusing those ino,
    etc, to instead store a build-id.

    But we're closing in to a final solution, and that is good. :-)

    - Arnaldo

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