Re: [PATCH 16/26] perf tools: Synthesize modules with mmap3

[Arnaldo Carvalho de Melo]

Em Wed, Sep 16, 2020 at 04:17:00PM +0200, peterz@xxxxxxxxxxxxx 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.

- Arnaldo