linux-kernel - Re: [RFC][PATCH] perf tools: Merge trace.info content into perf.data

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Date:	Wed, 7 Oct 2009 10:20:22 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Mike Galbraith <efault@....de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] perf tools: Merge trace.info content into
	perf.data

On Wed, Oct 07, 2009 at 08:47:30AM +0200, Peter Zijlstra wrote:
> On Tue, 2009-10-06 at 23:36 +0200, Frederic Weisbecker wrote:
> > Hi,
> > 
> > Here is an attempt to remove the trace.info file.
> > It works well for me, the reason for it to be an RFC
> > is that I have doubts about the backward compatibility.
> > 
> > A file created by perf after his patch is unsupported
> > by previous version because the size of the headers have
> > increased.
> > 
> > That said, it's two new fields that have been added in
> > the end of the headers, and those could be ignored by
> > previous versions if they just handled the dynamic header
> > size and then ignore the unknow part. The offsets guarantee
> > the compatibility.
> > But previous versions handle the header size using its
> > static size, not dynamic, then it's not backward compatible.
> > 
> > Anyway, I'm not sure exactly how to handle that.
> 
> If we're still able to read 'regular' old perf.data files after this.
> That is, allow to read short headers and assume the tail is 0 we'll be
> good. (this will break trace files that rely on the now removed
> trace.info, but that's unavoidable I guess).
> 
> So backwards compatible, but no fwd compat for the old perf.


Hmm, but forward is possible. It will just ignore the trace.info
part.

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