[PATCH] kprobes: Mark OPTPROBES n/a for powerpc

Masami Hiramatsu masami.hiramatsu.pt at hitachi.com
Tue Jul 21 20:26:37 AEST 2015

On 2015/07/21 13:34, Ananth N Mavinakayanahalli wrote:
> On Tue, Jul 21, 2015 at 12:53:07PM +1000, Michael Ellerman wrote:
>> On Sun, 2015-07-19 at 11:21 +0900, Masami Hiramatsu wrote:
>>> On 2015/07/16 19:56, Ananth N Mavinakayanahalli wrote:
>>>> Kprobes uses a breakpoint instruction to trap into execution flow
>>>> and the probed instruction is single-stepped from an alternate location.
>>>> On some architectures like x86, under certain conditions, the OPTPROBES
>>>> feature enables replacing the probed instruction with a jump instead,
>>>> resulting in a significant perfomance boost (one single-step exception
>>>> is bypassed for each kprobe).
>>> The OPTPROBE is not only for bypassing the single-step exception, but also
>>> the breakpoint exception.
>>> Please see commit 0dc016dbd820260b (ARM: kprobes: enable OPTPROBES for ARM 32) too,
>>> which shows how it is done on RISC processor.
>>>> Powerpc has an in-kernel instruction emulator. Kprobes on powerpc uses
>>>> this emulator already and bypasses the single-step exception, with a
>>>> lot less complexity.
>>> So, this might miss the point. Since it is impossible to do on some RISC
>>> processor, I agree with this change, but it should be committed with
>>> correct comments.
>> I don't think it's impossible on powerpc.
>> So we should leave it as a TODO for now.
> OK. I put it on my TODO list.

I see, thanks Ananth! :)

Linux Technology Research Center, System Productivity Research Dept.
Center for Technology Innovation - Systems Engineering
Hitachi, Ltd., Research & Development Group
E-mail: masami.hiramatsu.pt at hitachi.com

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