linux-kernel - Re: [PATCH 6/9][RFC] kprobes: Allow probe on ftrace reserved text (but move it)

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Message-ID: <4FAA26DD.4050403@hitachi.com>
Date:	Wed, 09 May 2012 17:12:13 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	"Frank Ch. Eigler" <fche@...hat.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>, yrl.pp-manager.tt@...achi.com
Subject: Re: [PATCH 6/9][RFC] kprobes: Allow probe on ftrace reserved text
 (but move it)

(2012/05/09 14:53), Masami Hiramatsu wrote:
> (2012/05/08 22:04), Steven Rostedt wrote:
>> Note, the big difference with using ftrace optimization and normal
>> kprobe jump optimization is that the ftrace one can be used on a preempt
>> kernel. But this code is still under development. I want to get a
>> solution for the current code (this patch set) now. It would be nice if
>> it was ready for 3.5.
> 
> I doubt that we can really do this. If this is possible, I can make
> jump optimization work with preemptive kernel.

Now I'm clear that it can be done only with ftrace-nop.
What I concerned with is out-of-line execution buffer releasing
timing. but with the ftrace-nop, we don't need to do that.

Theoretically, that will be done in following steps;

1. call mcount (w/ push call-site address+5)
2. mcount calls ftrace handlers
3. mcount makes pt_regs on the stack
 - on x86-32, regs->cs should be the call-site address+5, since
   regs->sp must be the top of original stack.
 - on x86-64, it is enough that regs->sp points the top of original
   stack.
4. mcount store call-site address+5 to %ax
5. mcount calls new-optprobe handler (which returns call-site address+5)
6. mcount restores registers
7. mcount returns to the call-site address+5.

Between 6 and 7, original opt-probe has to do out-of-line execution,
but this ftrace-based one doesn't need to do that. In that case,
we don't need to allocate nor to release out-of-line execution buffer
for this probe.

Thank you!

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com
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