linux-kernel - Re: [RFC] microblaze: Support FRAME

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Message-Id: <1268261476.3076.100.camel@iscandar.digidescorp.com>
Date:	Wed, 10 Mar 2010 16:51:16 -0600
From:	"Steven J. Magnani" <steve@...idescorp.com>
To:	Paul Mundt <lethal@...ux-sh.org>
Cc:	microblaze-uclinux@...e.uq.edu.au, linux-kernel@...r.kernel.org
Subject: Re: [RFC] microblaze: Support FRAME_POINTER for better backtrace

I've had some time to bake a Microblaze implementation of this and have
come up with a question...

On Mon, 2010-03-01 at 15:26 -0600, Steven J. Magnani wrote:
> On Mon, 2010-03-01 at 10:43 +0900, Paul Mundt wrote: 
> > This doesn't look that bad compared to what some of the other
> > architectures have to deal with. If the frame pointer is always setup
> > using these addik/swi pairs then you can trivially scan an arbitrary
> > number of instructions attempting to match before giving up. We do
> > similar things for sh64 where we also have to figure out how stack frames
> > were created in order to roll them back.
> > 
> > In any event, take a look at arch/sh/kernel/cpu/sh5/unwind.c, you should
> > be able to use a similar scheme without the need for undue complexity.
> > 
> 
> Thanks for the tip. This looks manageable. I had thought to search for
> instructions that create frames but didn't think working backwards from
> return addresses was a good idea. Using kallsyms to get the "top" of
> each function is a nice way around that.

The kallsyms approach to unwinding depends on as many functions as
possible being in the table, for two reasons:

* The unwind can't get past an address that kallsyms can't identify
* A mis-lookup, such as reporting a nearby global function when the
  actual address lies in a static function, can send the unwind off the
  rails

To get static (local) functions into kallsyms, I've found that I need to
specify -fno-unit-at-a-time to Microblaze gcc 4.1.2. I suspect that
particular option has other side-effects, but despite much searching I
couldn't find any mention of a gcc option that would do what I need. My
x86_64 gcc (4.4.1) seems to emit local symbols by default even with
-funit-at-a-time. 

Any suggestions?

------------------------------------------------------------------------
Steven J. Magnani               "I claim this network for MARS!
www.digidescorp.com              Earthling, return my space modulator!"

#include <standard.disclaimer>


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