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Date: Fri, 16 Oct 2009 09:33:41 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Ingo Molnar <mingo@elte.hu>
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: <20091016073337.GA5069@nowhere>
References: <1255641017-13958-1-git-send-email-fweisbec@gmail.com> <20091016071046.GD20388@elte.hu>
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On Fri, Oct 16, 2009 at 09:10:46AM +0200, Ingo Molnar wrote:
> 
> * 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


Yeah, I've thought about that too but feared about the limitation
of bitweight(u64), although it's probably enough, we never know.
That said I can take a bunch of four u64 to draw this bitmap and 256
features is enough.

Indeed that's way much better as a bitmap. Will do that instead.