linux-kernel - Re: [PATCH v2 1/2] arm64: implement FTRACE_WITH

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Message-ID: <20160708202455.GA4580@lst.de>
Date:	Fri, 8 Jul 2016 22:24:55 +0200
From:	Torsten Duwe <duwe@....de>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Petr Mladek <pmladek@...e.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>, takahiro.akashi@...aro.org,
	Jungseok Lee <jungseoklee85@...il.com>,
	Arnd Bergmann <arnd@...db.de>,
	Li Bin <huawei.libin@...wei.com>,
	Ingo Molnar <mingo@...hat.com>,
	Christopher Li <sparse@...isli.org>,
	Jiri Kosina <jikos@...nel.org>, andrew.wafaa@....com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	live-patching@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-sparse@...r.kernel.org
Subject: Re: [PATCH v2 1/2] arm64: implement FTRACE_WITH_REGS

On Fri, Jul 08, 2016 at 11:57:10AM -0400, Steven Rostedt wrote:
> On Fri, 8 Jul 2016 10:48:24 -0500
> Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > 
> > Here, with -fprolog-pad, it's already a nop, so no change is needed.
> > 

Yes, exactly.

> That's what I was thinking. But as I stated in another email (probably
> in the air when you wrote this), the call to ftrace_modify_code() may be
> completely circumvented by ftrace_make_nop() if the addr is MCOUNT_ADDR.

Only on the _first_ invocation. Later on, tracing can be switched on and off,
and then the instructions need to be changed just like with fentry (or
profile-kernel ;-)

	Torsten