linux-kernel - Re: [RFC 0/4] arm64: add livepatch support

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Date:	Wed, 27 May 2015 15:09:20 +0900
From:	AKASHI Takahiro <takahiro.akashi@...aro.org>
To:	Jiri Kosina <jkosina@...e.cz>
CC:	rostedt@...dmis.org, mingo@...nel.org, jpoimboe@...hat.com,
	sjenning@...hat.com, vojtech@...e.cz, catalin.marinas@....com,
	will.deacon@....com, broonie@...nel.org,
	masami.hiramatsu.pt@...achi.com, live-patching@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linaro-kernel@...ts.linaro.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 0/4] arm64: add livepatch support

Sorry for not replying soon.

On 04/24/2015 06:27 PM, Jiri Kosina wrote:
> On Fri, 24 Apr 2015, AKASHI Takahiro wrote:
>
>> This patchset enables livepatch support on arm64.
>>
>> Livepatch was merged in v4.0, and allows replacying a function dynamically
>> based on ftrace framework, but it also requires -mfentry option of gcc.
>> Currently arm64 gcc doesn't support it, but by adding a helper function to
>> ftrace, we will be able to support livepatch on arch's which don't support
>> this option.
>>
>> I submit this patchset as RFC since I'm not quite sure that I'm doing
>> in the right way, or we should definitely support -fentry instead.
>
> I don't have arm64 cross-compiler handy, could you please copy/paste how
> does function prologue, generated by gcc -pg on arm64 look like?

As other people said, my current patch has some drawbacks and was far from perfect.
I talked to a toolchain guy in Linaro, and he suggested that, instead of x86
specific -mfentry option, we should add a new generic option, -fprolog-pad=N.

It works as if "-pg -mfentry -mrecord-mcount -mnop-mcount" were specified on a
command line, but actually puts N nop instructions at the very beginning of
a function.
(Please note that -mrecord-mcount and -mnop-mcount options have been added
in x86 gcc to make it easier to implement ftrace and others.)

While it is not clear that this new option will be implemented soon (in gcc-6),
once it is supported on aarch64, I will update and re-post my livepatch patch
as well as DYNAMIC_TRACE_WITH_REGS ftrace.

Thanks,
-Takahiro AKASHI

> Thanks,
>
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