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

[Steven Rostedt]

On Fri, 8 Jul 2016 22:24:55 +0200
Torsten Duwe <duwe@xxxxxx> 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@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 ;-)
> 

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