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

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Date:	Thu, 8 Oct 2009 22:12:22 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH] perf tools: Provide backward compatibility with
	previous perf.data version


* Frederic Weisbecker <fweisbec@...il.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@...il.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
> Cc: Mike Galbraith <efault@....de>
> Cc: Paul Mackerras <paulus@...ba.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. 
trace.info is not something we really want to be backwards compatible 
about, if we can avoid it.

	Ingo
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