Re: [Patch 4/6] Modify process and processor handling code to recognise hardware debug registers

["K.Prasad" <prasad@linux.vnet.ibm.com>]

On Wed, Jun 17, 2009 at 02:14:20PM +1000, David Gibson wrote:
> On Wed, Jun 10, 2009 at 02:38:18PM +0530, K.Prasad wrote:
> > Modify process handling code to recognise hardware debug registers during copy
> > and flush operations. Introduce a new TIF_DEBUG task flag to indicate a
> > process's use of debug register. Load the debug register values into a
> > new CPU during initialisation.
> > 
> > Signed-off-by: K.Prasad <prasad@linux.vnet.ibm.com>
> > ---
> >  arch/powerpc/kernel/process.c |   15 +++++++++++++++
> >  arch/powerpc/kernel/smp.c     |    2 ++
> >  2 files changed, 17 insertions(+)
> > 
> > Index: linux-2.6-tip.hbkpt/arch/powerpc/kernel/process.c
> > ===================================================================
> > --- linux-2.6-tip.hbkpt.orig/arch/powerpc/kernel/process.c
> > +++ linux-2.6-tip.hbkpt/arch/powerpc/kernel/process.c
> > @@ -50,6 +50,7 @@
> >  #include <asm/syscalls.h>
> >  #ifdef CONFIG_PPC64
> >  #include <asm/firmware.h>
> > +#include <asm/hw_breakpoint.h>
> >  #endif
> >  #include <linux/kprobes.h>
> >  #include <linux/kdebug.h>
> > @@ -254,8 +255,10 @@ void do_dabr(struct pt_regs *regs, unsig
> >  			11, SIGSEGV) == NOTIFY_STOP)
> >  		return;
> >  
> > +#ifndef CONFIG_PPC64
> >  	if (debugger_dabr_match(regs))
> >  		return;
> > +#endif
> 
> Won't this disable the check for breakpoints set by xmon - but I don't
> see anything in this patch series to convert xmon to use the new
> breakpoint interface instead.
> 

As noted by me here:
http://lists.ozlabs.org/pipermail/linuxppc-dev/2009-May/071832.html the
Xmon integration is pending. When I tried to study and integrate Xmon, I
found that the HW Breakpoint triggering was broken as of 2.6.29 kernel
(tested on a Power5 box).

This would mean that if Xmon's hardware breakpoint infrastructure is
used in tandem with the given breakpoint interfaces, they would conflict
with each other resulting in difficult-to-predict behaviour (the last to
grab the register will use it).

I think that tidying up do_dabr() is best done along with Xmon
integration.

> -- 
> David Gibson			| I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
> 				| _way_ _around_!
> http://www.ozlabs.org/~dgibson

Thanks,
K.Prasad