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Date: Tue, 21 Jul 2015 10:04:16 +0530
From: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
 linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH] kprobes: Mark OPTPROBES n/a for powerpc
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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.

Ananth