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

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Message-ID: <20150528085807.GA31001@arm.com>
Date:	Thu, 28 May 2015 09:58:07 +0100
From:	Will Deacon <will.deacon@....com>
To:	Li Bin <huawei.libin@...wei.com>
Cc:	Jiri Kosina <jkosina@...e.cz>,
	AKASHI Takahiro <takahiro.akashi@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"sjenning@...hat.com" <sjenning@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"rostedt@...dmis.org" <rostedt@...dmis.org>,
	"vojtech@...e.cz" <vojtech@...e.cz>,
	"broonie@...nel.org" <broonie@...nel.org>,
	"jpoimboe@...hat.com" <jpoimboe@...hat.com>,
	"masami.hiramatsu.pt@...achi.com" <masami.hiramatsu.pt@...achi.com>,
	"live-patching@...r.kernel.org" <live-patching@...r.kernel.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC 0/4] arm64: add livepatch support

On Thu, May 28, 2015 at 07:08:42AM +0100, Li Bin wrote:
> On 2015/4/24 17:27, 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?
> > 
> 
> The function prologue on arm64 with gcc -pg look like as following:
> func:
>         stp     x29, x30, [sp, -48]!
>         add     x29, sp, 0
>         mov     x1, x30
>         str     w0, [x29,28]
>         mov     x0, x1
>         bl      _mcount

Just for the avoidance of confusion, this looks like a function with
a live parameter in x0, which explains the str to the stack and the
juggling of x30 into x0. I don't think there's necessarily a golden
template for the prologue code.

Will
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