From mboxrd@z Thu Jan  1 00:00:00 1970
From: Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH v2 1/2] arm64: implement FTRACE_WITH_REGS
Date: Fri, 8 Jul 2016 17:08:08 -0400
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To: Torsten Duwe <duwe@lst.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>, Petr Mladek <pmladek@suse.com>, Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will.deacon@arm.com>, takahiro.akashi@linaro.org, Jungseok Lee <jungseoklee85@gmail.com>, Arnd Bergmann <arnd@arndb.de>, Li Bin <huawei.libin@huawei.com>, Ingo Molnar <mingo@redhat.com>, Christopher Li <sparse@chrisli.org>, Jiri Kosina <jikos@kernel.org>, andrew.wafaa@arm.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, live-patching@vger.kernel.org, linux-arch@vger.kernel.org, linux-sparse@vger.kernel.org

On Fri, 8 Jul 2016 22:24:55 +0200
Torsten Duwe <duwe@lst.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@redhat.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