Re: [PATCH v2 1/2] arm64: implement FTRACE_WITH_REGS

From: Torsten Duwe
Date: Fri Jul 08 2016 - 16:25:17 EST

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@xxxxxxxxxx> 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 ;-)