linux-kernel - Re: [PATCH] ftrace: mcount record based dynamic tracing for ARM

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Date:	Fri, 21 Nov 2008 15:38:27 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Jim Radford <radford@...vanix.net>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sam Ravnborg <sam@...nborg.org>,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ftrace: mcount record based dynamic tracing for ARM

On Fri, Nov 21, 2008 at 09:27:17AM -0500, Steven Rostedt wrote:
> On Thu, 20 Nov 2008, Jim Radford wrote:
> > -	select HAVE_FUNCTION_TRACER if (!XIP_KERNEL)
> > +	select HAVE_FTRACE_MCOUNT_RECORD
> > +	select HAVE_DYNAMIC_FTRACE if (!XIP_KERNEL)
> 
> Russell mentioned something about the code not being compatible with 
> Thumb2, is the above if statement enough?

I think I said (or should've said) "upcoming Thumb 2" - it's not yet in
the kernel but there's a patch series floating around for it.  We've
started on merging some of the pre-requisits, and it will mean that
the instruction length is no longer constant.  (It may be a 16bit or
32bit instruction.)

I suspect that ftrace won't be able to handle that, so it may have to
depend on !THUMB2_KERNEL for the time being.

> > diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
> > index 06269ea..06216af 100644
> > --- a/arch/arm/kernel/entry-common.S
> > +++ b/arch/arm/kernel/entry-common.S
> > @@ -104,14 +104,7 @@ ENDPROC(ret_from_fork)
> >  #ifdef CONFIG_FUNCTION_TRACER
> >  #ifdef CONFIG_DYNAMIC_FTRACE
> >  ENTRY(mcount)
> > -	stmdb sp!, {r0-r3, lr}
> > -	mov r0, lr
> > -	sub r0, r0, #MCOUNT_INSN_SIZE
> > -
> > -	.globl mcount_call
> > -mcount_call:
> > -	bl ftrace_stub
> > -	ldmia sp!, {r0-r3, pc}
> > +	mov pc, lr
> 
> This looks OK. The new mcount stub for dynamic ftrace is a simple return.
> It has been a while since I've programmed ARM (7 years ago), but I'm 
> assuming that loading the program counter with the link register is 
> a simple return.

Yes, that's all that's required.
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