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    SubjectRe: [RFC Patch 1/2][Bugfix][x86][hw-breakpoint] Clear reserved bits of DR6 in do_debug()
    On Fri, Jan 01, 2010 at 12:19:49AM +0530, K.Prasad wrote:
    > 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@linux.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.


    Ah, ok.


    > > 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().


    Yeah, ok. Just waiting for Jan's answer to be sure it has
    not side effects :)

    Thanks.



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