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

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Date:	Thu, 31 Dec 2009 00:45:00 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"K.Prasad" <prasad@...ux.vnet.ibm.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 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 ?

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.



>  #define DR_TRAP0	(0x1)		/* db0 */
>  #define DR_TRAP1	(0x2)		/* db1 */
>  #define DR_TRAP2	(0x4)		/* db2 */
> Index: linux-2.6-tip/arch/x86/kernel/traps.c
> ===================================================================
> --- linux-2.6-tip.orig/arch/x86/kernel/traps.c
> +++ linux-2.6-tip/arch/x86/kernel/traps.c
> @@ -534,6 +534,9 @@ dotraplinkage void __kprobes do_debug(st
>  
>  	get_debugreg(dr6, 6);
>  
> +	/* Filter out all the reserved bits which are preset to 1 */
> +	dr6 &= ~DR6_RESERVED;
> +
>  	/* Catch kmemcheck conditions first of all! */
>  	if ((dr6 & DR_STEP) && kmemcheck_trap(regs))
>  		return;
> 

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