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From: "Dale Farnsworth" <dale@farnsworth.org>
Date: Thu, 23 Feb 2006 07:23:37 -0700
To: Josh Boyer <jwboyer@jdub.homelinux.org>
Subject: Re: [PATCH] ppc32: handle Book E debug exceptions on kernel stack
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Cc: David Gibson <david@gibson.dropbear.id.au>,
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On Thu, Feb 23, 2006 at 10:54:02AM +0000, Josh Boyer wrote:
> On Wed, 2006-02-22 at 22:19 -0600, Kumar Gala wrote:
> > On Tue, 14 Feb 2006, Dale Farnsworth wrote:
> > 
> > > From: Dale Farnsworth <dale@farnsworth.org>
> > > 
> > > On PPC Book E processsors, we currently handle debug
> > > exceptions on the critical exception stack (debug stack
> > > for E200).  This causes problems with the kgdb single
> > > step handler, which calls smp_processor_id() and spin_lock(),
> > > which reference current_thread_info(), which only works when
> > > we are on the kernel stack.
> > > 
> > > We address this by switching to the kernel stack early while
> > > handling debug exceptions.  Note that the entry values of r10
> > > and r11 are still saved on the critical exception (or debug) stack.
> > > 
> > > Signed-off-by: Dale Farnsworth <dale@farnsworth.org>
> > 
> > Paul,
> > 
> > We were wondering if you or David remember why a specific critical
> > exception stack was added in the 40x port from 2.4 to 2.6?
> 
> I think Matt did that.  And if I remember correctly, it was to avoid
> corruption if you were in the middle of handling a normal interrupt and
> a critical interrupt came in.

Thanks Josh, Matt was one of the "we" who were wondering.  If indeed
you are remembering correctly, then I think we're safe to use the
normal kernel stack rather than the critical exception stack.

-Dale