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

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:

quoted

On 2015/07/16 19:56, Ananth N Mavinakayanahalli wrote:

quoted

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.

quoted

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