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Date: Tue, 24 Mar 2009 23:48:58 +0100
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Abhishek Sagar <sagar.abhishek@gmail.com>
Cc: Tim Bird <tim.bird@am.sony.com>,
       linux-arm-kernel <linux-arm-kernel@lists.arm.linux.org.uk>,
       linux kernel <linux-kernel@vger.kernel.org>,
       Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@redhat.com>,
       Russell King <rmk@arm.linux.org.uk>,
       Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= 
	<u.kleine-koenig@pengutronix.de>
Subject: Re: Anyone working on ftrace function graph support on ARM?
Message-ID: <20090324224857.GE5975@nowhere>
References: <49C936CA.8070800@am.sony.com> <20090324213618.GC5975@nowhere> <49C95EAF.7030901@gmail.com>
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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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