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

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Date:	Tue, 24 Mar 2009 23:54:58 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
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?

On Tue, Mar 24, 2009 at 11:14:39PM +0100, Ingo Molnar wrote:
> 
> * 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


Aah, ok, understood!
Thanks.

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