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

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Message-ID: <49C95334.5040106@am.sony.com>
Date:	Tue, 24 Mar 2009 14:40:04 -0700
From:	Tim Bird <tim.bird@...sony.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	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>,
	Abhishek Sagar <sagar.abhishek@...il.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?

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.
> 
> 
> _ On return hook it's pretty the same, (saving the scratch
> registers, especially the return value which should be on r0
> and r1 if I'm not wrong).
> But you'll have to get the original return address from the
> return handler and then put it on pc.
> 
> Well it's a very naive listing, there are sometimes some problems.
> For example on x86-64, I had to save even some non-scratch registers
> before calling the return hook, I still don't know why.

Thanks - this is very helpful.  I'll send patches around when
I get something going. Hopefully in the next week or two.
 -- Tim

=============================
Tim Bird
Architecture Group Chair, CE Linux Forum
Senior Staff Engineer, Sony Corporation of America
=============================

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