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    SubjectRe: [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@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 ?

    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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