linux-kernel - Re: [PATCH -tip v5 00/10] kprobes: Kprobes jump optimization support

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Message-ID: <4B0C0469.2020903@redhat.com>
Date:	Tue, 24 Nov 2009 11:06:01 -0500
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Frederic Weisbecker <fweisbec@...il.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	lkml <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jim Keniston <jkenisto@...ibm.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Anders Kaseorg <andersk@...lice.com>,
	Tim Abbott <tabbott@...lice.com>,
	Andi Kleen <andi@...stfloor.org>,
	Jason Baron <jbaron@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	systemtap <systemtap@...rces.redhat.com>,
	DLE <dle-develop@...ts.sourceforge.net>
Subject: Re: [PATCH -tip v5 00/10] kprobes: Kprobes jump optimization support

Ingo Molnar wrote:
> 
> * Frederic Weisbecker <fweisbec@...il.com> wrote:
> 
>> On Tue, Nov 24, 2009 at 03:03:19AM +0100, Frederic Weisbecker wrote:
>>> On Mon, Nov 23, 2009 at 06:21:16PM -0500, Masami Hiramatsu wrote:
>>>>   When the optimized-kprobe is hit before optimization, its handler
>>>>  changes IP(instruction pointer) to copied code and exits. So, the
>>>>  instructions which were copied to detour buffer are executed on the detour
>>>>  buffer.
>>>
>>>
>>>
>>> Hm, why is it playing such hybrid game there?
>>> If I understand well, we have executed int 3, executed the
>>> handler and we jump back to the detour buffer?
>>>
>>
>> I got it, I think. We have instructions to patch. And the above turn 
>> this area into dead code, safe to patch.
>>
>> But still, stop_machine() seem to make it not necessary anymore.
> 
> i think 'sending an IPI to all online CPUs' might be an adequate 
> sequence to make sure patching effects have propagated. I.e. an 
> smp_call_function() with a dummy function?

Hmm, I assume that you mean waiting for all int3 handler.

We have to separate below issues:
 - int3-based multi-bytes code replacement
 - multi-instruction replacement with int3-detour code

The former is implemented on patch 9/10 and 10/10. As you can see,
these patches are RFC status, because I'd like to wait for official
reply of safeness from processor architects.
And it may be able to use a dummy IPI for 2nd IPI because it
just for waiting int3 interrupts. But again, it is just estimated that
replacing with/recovering from int3 is automatically synchronized...

However, at least stop_machine() method is officially described
at "7.1.3 Handling Self- and Cross-Modifying Code" on the intel's
software developer's manual 3A . So currently we can use it.

For the latter issue, as I explained on previous reply, we need
to wait all running interrupts including hardware interrupts.
Thus I used synchronize_sched().

Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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