Re: [PATCH] perf tools: Provide backward compatibility with previous perf.data version

Ingo Molnar
2009-10-12T09:11:24Z

* Frederic Weisbecker <fweisbec@gmail.com> wrote:

> On Thu, Oct 08, 2009 at 10:07:29PM +0200, Frederic Weisbecker wrote:
> > We have merged the trace.info file into perf.data by adding one section
> > in the perf headers. This makes it incompatible with previous version:
> > the new perf tools can't read the older perf.data.
> >
> > To support the previous format, we check the headers size. If they
> > have the same size than in the previous format, then ignore the trace
> > info section that doesn't exist.
> >
> > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> > Cc: Mike Galbraith <efault@gmx.de>
> > Cc: Paul Mackerras <paulus@samba.org>
> > ---
> > tools/perf/util/header.c | 8 +++++++-
> > 1 files changed, 7 insertions(+), 1 deletions(-)
> >
> > diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
> > index 212fade..9aae360 100644
> > --- a/tools/perf/util/header.c
> > +++ b/tools/perf/util/header.c
> > @@ -287,10 +287,16 @@ struct perf_header *perf_header__read(int fd)
> > do_read(fd, &f_header, sizeof(f_header));
> >
> > if (f_header.magic != PERF_MAGIC ||
> > - f_header.size != sizeof(f_header) ||
>
>
>
> Few notes about this.
> I can send a separate fix for .32 that would just consist in the above line,
> or more likely I can replace it by:
>
> f_header.size < sizeof(f_header) ||
>
> and then we'll get a minimal forward compatibility from the older
> tools (can be Cc'ed to stable for .31).

Yep, would be nice.

> Another thing. We may feel the need to add yet another sections in the
> future.
>
> So just a suggestion: we could turn this trace_info section into a
> more generic one in which we could add as much subsections as we want
> in the future while always ensuring backward compatibility. That could
> be managed through a versioning of this generic section.

Definitely! (Since it's not upstream yet we can do it without supporting
the interim version.)

Ingo