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Date: Fri, 16 Oct 2009 09:10:46 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>, Peter Zijlstra <peterz@infradead.org>,
       Arnaldo Carvalho de Melo <acme@redhat.com>,
       Mike Galbraith <efault@gmx.de>, Paul Mackerras <paulus@samba.org>,
       Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH] perf tools: Add a new generic section in perf.data
Message-ID: <20091016071046.GD20388@elte.hu>
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* Frederic Weisbecker <fweisbec@gmail.com> wrote:

> +/*
> + * Version 0: contains trace_info section only
> + */
> +struct perf_file_additionals {
> +	u64				version;
> +	struct perf_file_section	trace_info;
> +};

i dont disagree with the change - but it would be even nicer to simply 
define a features bitmask, instead of a flat version - and add the 
trace_info section as a feature.

That way it's all a lot more manageable: we dont know about versions per 
se, we know about features. Individual features could be developed (and 
backported) in a distributed way - without having to worry about a flat 
version model.

So i'd suggest something like a bitmask in the perf.data file header:

	DECLARE_BITMAP(features, 256);

Plus every perf version knows about the features it supports:

	DECLARE_BITMAP(features_supported, 256);

The compatibility rule is: perf only touches attributes that belong to 
features it knows about.

	Ingo