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Subject: Re: [PATCH] kprobes: Mark OPTPROBES n/a for powerpc
From: Michael Ellerman <mpe@ellerman.id.au>
To: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
 linuxppc-dev@lists.ozlabs.org
Date: Tue, 21 Jul 2015 12:53:07 +1000
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References: <20150716105639.9151.75671.stgit@thinktux.in.ibm.com>
 <55AB09BE.5040506@hitachi.com>
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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.

cheers