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

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Date:	Tue, 24 Mar 2009 23:14:39 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...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>,
	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 <fweisbec@...il.com> wrote:

> On Tue, Mar 24, 2009 at 10:48:46PM +0100, Ingo Molnar wrote:
> > 
> > * Frederic Weisbecker <fweisbec@...il.com> wrote:
> > 
> > > 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.
> > 
> > btw., which are those registers?
> > 
> > 	Ingo
> 
> 
> I would expect to only save rax,rdi,rsi,rdx,rcx,r8,r9 which are 
> used for parameters.

> And I had some crashes until I append r10 and r11 which actually 
> are scratch if I'm not wrong, but since they are scratch and are 
> not used for arguments, I thought they didn't need to be saved.
> 
> Well, I think there were some code flow cases I was missing.

Correct, r10 and r11 are clobbered registers too - and you need to 
save them too in mcount methods.

The reason is that mcount has a special calling convention - it's 
not just about not destroying arguments - GCC can keep data in r10 
or r11 scratch registers across function calls as well - for example 
for relatively static functions that are in its local optimization 
scope.

If GCC can prove that the local scope function itself does not 
clobber r10/r11, it does not have to clobber them across the 
function call. But the mcount() callback still gets inserted.

So the rule is: mcount must not destroy _any_ register state. 
(beyond flags)

	ngo
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