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    SubjectRe: [RFC Patch 2/2][Bugfix][x86][hw-breakpoint] Fix return-code to notifier chain in hw_breakpoint_handler
    On Fri, Jan 01, 2010 at 12:32:17AM +0530, K.Prasad wrote:
    > On Thu, Dec 31, 2009 at 01:38:09AM +0100, Frederic Weisbecker wrote:
    > > On Sat, Dec 26, 2009 at 11:58:33PM +0530, K.Prasad wrote:
    > > > The hw-breakpoint handler will return NOTIFY_DONE for user-space breakpoints
    > > > to generate SIGTRAP signal (and not for kernel-space addresses).
    > > >
    > > > Signed-off-by: K.Prasad <prasad@linux.vnet.ibm.com>
    > > > ---
    > > > arch/x86/kernel/hw_breakpoint.c | 9 +++++++--
    > > > 1 file changed, 7 insertions(+), 2 deletions(-)
    > > >
    > > > Index: linux-2.6-tip/arch/x86/kernel/hw_breakpoint.c
    > > > ===================================================================
    > > > --- linux-2.6-tip.orig/arch/x86/kernel/hw_breakpoint.c
    > > > +++ linux-2.6-tip/arch/x86/kernel/hw_breakpoint.c
    > > > @@ -502,8 +502,6 @@ static int __kprobes hw_breakpoint_handl
    > > > rcu_read_lock();
    > > >
    > > > bp = per_cpu(bp_per_reg[i], cpu);
    > > > - if (bp)
    > > > - rc = NOTIFY_DONE;
    > > > /*
    > > > * Reset the 'i'th TRAP bit in dr6 to denote completion of
    > > > * exception handling
    > > > @@ -517,6 +515,13 @@ static int __kprobes hw_breakpoint_handl
    > > > rcu_read_unlock();
    > > > break;
    > > > }
    > > > + /*
    > > > + * Further processing in do_debug() is needed for a) user-space
    > > > + * breakpoints (to generate signals) and b) when the system has
    > > > + * taken exception due to multiple causes
    > > > + */
    > > > + if (bp->attr.bp_addr < TASK_SIZE)
    > > > + rc = NOTIFY_DONE;
    > > >
    > > > perf_bp_event(bp, args->regs);
    > > >
    > > >
    > >
    > >
    > > Oh and now that I see this patch, the previous one indeed makes sense
    > > with this check:
    > >
    > > if (dr6 & (~DR_TRAP_BITS))
    > > rc = NOTIFY_DONE;
    > >
    > > That said, it means thread.debugreg6 won't get the reserved bits anymore.
    > > I see some use of them from kvm (it restores the reserved bits on guest<->host
    > > switch). Not sure if this inconsistency could affect kvm...
    > >
    >
    > Can you point me to the relevant code?


    I see various uses of DR6_VOLATILE and DR6_FIXED_1 in arch/x86/kvm/,
    DR6_FIXED_1 being the fixed unused bits in dr6. Not sure how
    this patch would affect what's set there.

    I'll wait for Jan's answer.

    Thanks.



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