linux-kernel - Re: [PATCH 0/2] kprobes/x86: Allow "boost" for 10- and 11-byte instructions

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Message-ID: <556CD258.8090909@hitachi.com>
Date:	Tue, 02 Jun 2015 06:44:56 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Eugene Shatokhin <eugene.shatokhin@...alab.ru>,
	Ingo Molnar <mingo@...nel.org>
CC:	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] kprobes/x86: Allow "boost" for 10- and 11-byte instructions

On 2015/06/02 1:32, Eugene Shatokhin wrote:
> Kprobes' "boost" feature allows to avoid single-stepping in some cases, along with its overhead.
> It is useful for the Kprobes that cannot be optimized for some reason.
> 
> Currently, "boost" cannot be applied to the instructions of 10 and 11 bytes in size, including 
> some rather commonly used kinds of MOV.
> 
> The first of the two patches in this series fixes the code that checks if the jump needed for
> the boost fits in the insn slot (the conditional is too strict). This allows to apply "boost"
> to 10-byte instructions.
> 
> As a side effect of commit 91e5ed49fca0 ("x86/asm/decoder: Fix and enforce max instruction
> size in the insn decoder"), the size of the instruction slot became 1 byte smaller, 15 bytes
> VS 16 bytes before that change. The second patch makes the size of each insn slot 16 bytes
> again (while keeping MAX_INSN_SIZE as 15). This allows to apply "boost" to 11-byte
> instructions as well.
> 
> I have checked that "boost" does happen for at least "movq $0x1,0x100(%rbx)"
> (48 c7 83 00 01 00 00 01 00 00 00) in the kernel 4.1-rc6 after these changes.

Ah, I didn't expected that such long instruction existed without redundant prefixes.
I have some comment on that, but basically agree to support this.

Thank you!

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