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

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Date:	Tue, 08 Sep 2009 08:49:19 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Peter Zijlstra <peterz@...radead.org>
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

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?

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