linux-kernel - Re: [RFC Patch 1/2][Bugfix][x86][hw-breakpoint] Clear reserved bits of DR6 in do

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Date:	Fri, 1 Jan 2010 00:19:49 +0530
From:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Roland McGrath <roland@...hat.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Vegard Nossum <vegardno@....uio.no>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [RFC Patch 1/2][Bugfix][x86][hw-breakpoint] Clear reserved
	bits of DR6 in do_debug()

On Thu, Dec 31, 2009 at 12:45:00AM +0100, Frederic Weisbecker wrote:
> On Sat, Dec 26, 2009 at 11:57:25PM +0530, K.Prasad wrote:
> > Clear the reserved bits from the stored copy of debug status register (DR6).
> > This will help easy bitwise operations.
> > 
> > Signed-off-by: K.Prasad <prasad@...ux.vnet.ibm.com>
> > ---
> >  arch/x86/include/asm/debugreg.h |    3 +++
> >  arch/x86/kernel/traps.c         |    3 +++
> >  2 files changed, 6 insertions(+)
> > 
> > Index: linux-2.6-tip/arch/x86/include/asm/debugreg.h
> > ===================================================================
> > --- linux-2.6-tip.orig/arch/x86/include/asm/debugreg.h
> > +++ linux-2.6-tip/arch/x86/include/asm/debugreg.h
> > @@ -14,6 +14,9 @@
> >     which debugging register was responsible for the trap.  The other bits
> >     are either reserved or not of interest to us. */
> >  
> > +/* Define reserved bits in DR6 which are always set to 1 */
> > +#define DR6_RESERVED	(0xFFFF0FF0)
> > +
> 
> 
> The 12th bit seems to be also reserved.
> Shouldn't it be 0xffff1ff0 ?
> 

The 12th bit is reserved to be 0 always.

> What kind of bitwise operations do you think it could help?
> 
> All of the operations I can find on dr6 are simple masks
> test/set/clear.
> 

As you found out later, this bitmask helps us in
hw_breakpoint_handler().

Thanks,
K.Prasad

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