linux-kernel - Re: Anyone working on ftrace function graph support on ARM?

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Date:	Tue, 24 Mar 2009 23:48:58 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Abhishek Sagar <sagar.abhishek@...il.com>
Cc:	Tim Bird <tim.bird@...sony.com>,
	linux-arm-kernel <linux-arm-kernel@...ts.arm.linux.org.uk>,
	linux kernel <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Russell King <rmk@....linux.org.uk>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
Subject: Re: Anyone working on ftrace function graph support on ARM?

On Tue, Mar 24, 2009 at 06:29:03PM -0400, Abhishek Sagar wrote:
> Frederic Weisbecker wrote:
> > Yes, ie:
> >
> > _Before jumping to the function entry hook, you must save
> > the arguments for the traced function on the stack.
> > On x86, its eax, edx and ecx.
> > On arm, it will be r0-r3.
> > Then you have to transmit the address of the traced function
> > (it's on r14) and it's parent (must rely on fp for that).
> > Then you call the entry hook and restore the old scratch/arg
> > registers.
> >   
> Instead of just restoring the old backed-up args, lr can be fixed up
> inside the entry hook to point to the return hook. So when the traced
> function returns, it actually returns to the return hook (where we can
> restore the original return address). This means that
> -finstrument-functions is not required at all. This is analogous to how
> kretprobes work. The only difference here is that instead of planting a
> kprobe at the function entry and redirecting the function return to the
> profiling exit routine, we can use mcount. This is slightly more
> complicated to implement but can be a very efficient alternative to
> kretprobes.
> --
> Abhishek
> 

Indeed, you need to override lr, that even the only solution.
I was still thinking in an x86 way with its on stack return address.

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