linux-kernel - Re: [PATCH 4/6] perf_counter: Add PERF_COUNTER_IOC_SET

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Date:	Tue, 08 Sep 2009 10:37:52 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Tom Zanussi <tzanussi@...il.com>,
	Jason Baron <jbaron@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/6] perf_counter: Add PERF_COUNTER_IOC_SET_FILTER ioctl

On Tue, 2009-09-08 at 08:49 +0800, Li Zefan wrote:
> Peter Zijlstra wrote:
> > On Mon, 2009-09-07 at 18:48 +0200, Ingo Molnar wrote:
> >> * Peter Zijlstra <peterz@...radead.org> wrote:
> >>
> >>> On Mon, 2009-09-07 at 16:13 +0800, Li Zefan wrote:
> >>>> Allow to set profile filter via ioctl.
> >>> Hrm,.. not at all sure about this.. what are the ABI implications?
> >> I think the ABI should be fine if it's always a sub-set of C syntax. 
> >> That would be C expressions initially. Hm?
> > 
> > Right, so I've no clue what filter expressions look like, and the
> > changelog doesn't help us at all. It doesn't mention its a well
> > considered decision to henceforth freeze the expression syntax.
> > 
> > Of course, since filters so far only work with tracepoint things, and
> > since you can only come by tracepoint things through debugfs, and since
> > anything debugfs is basically a free-for-all ABI-less world, we might be
> > good, but then this is a very ill-defined ioctl() indeed.
> > 
> > So please, consider this well -- there might not be a second chance.
> > 
> 
> Ok, the expressions are:
> 
>   1. S = opr1 op opr2	(op: ==, !=, <, <=, >, >=.
> 			 opr1 should be a field in the format file)
>   2. E = S1 op S2	(op: ||, &&)
>   3. E = E1 op E2	(op: ||, &&)
>   4. () can be used
> 
> I don't the syntax will be changed, but we may extend it, like
> adding not ! operator. Like, for a func ptr, besides "func==0xccee4400",
> we may want to allow "func==foo". Those extentions are ok for the
> ABI, right?

Sure, but my point is that you need to be aware that you're creating an
ABI and the changelog was virtually non-existent which didn't inspire
much confidence.
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