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

[Frederic Weisbecker <fweisbec@gmail.com>]

On Thu, Oct 08, 2009 at 10:12:22PM +0200, Ingo Molnar wrote:
> 
> * Frederic Weisbecker <fweisbec@gmail.com> 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)	||
> >  	    f_header.attr_size	!= sizeof(f_attr))
> >  		die("incompatible file format");
> >  
> > +	if (f_header.size != sizeof(f_header)) {
> > +		/* Support the previous format */
> > +		if (f_header.size == offsetof(typeof(f_header), trace_info))
> > +			f_header.trace_info.size = 0;
> > +		else
> > +			die("incompatible file format");
> > +	}
> >  	nr_attrs = f_header.attrs.size / sizeof(f_attr);
> >  	lseek(fd, f_header.attrs.offset, SEEK_SET);
> 
> Perfect - thanks Frederic!
> 
> This should handle most of the (non-trace) perf.data's out there. 


Yeah, hopefully. I've only tested in a simple old perf.data but
that should do the trick for all of the non-trace files.


> trace.info is not something we really want to be backwards compatible 
> about, if we can avoid it.
> 
> 	Ingo


Yeah, that said, it's still possible and wouldn't require too much changes.
May be let's wait for complaints and, if any, I can provide this backward
support too.

Thanks.