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

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Date:	Fri, 8 Jul 2016 17:08:08 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Torsten Duwe <duwe@....de>
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, 8 Jul 2016 22:24:55 +0200
Torsten Duwe <duwe@....de> wrote:

> 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 ;-)
> 

Understood, but ftrace_modify_code() will only receive addr ==
MCOUNT_ADDR on boot up or when a module is loaded. In both cases, with
-fprolog-pad it will already be a nop, hence no need to call
ftrace_modify_code(), in those cases.

In all other cases, addr will point to a ftrace trampoline.

-- Steve