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

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Message-Id: <1254898050.26976.146.camel@twins>
Date:	Wed, 07 Oct 2009 08:47:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
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 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.
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