Re: [Patch 00/11] Hardware Breakpoint interfaces

["K.Prasad" <prasad@linux.vnet.ibm.com>]

On Wed, Apr 01, 2009 at 12:22:08PM -0400, Alan Stern wrote:
> On Sat, 28 Mar 2009, K.Prasad wrote:
> 
> > On Wed, Mar 25, 2009 at 03:48:31PM -0400, Alan Stern wrote:
> > > > +
> > > > +/*
> > > > + * Handle debug exception notifications.
> > > > + */
> > > > +int __kprobes hw_breakpoint_handler(struct die_args *args)
> > > > +{
> > > > +	int i, rc = NOTIFY_DONE;
> > > > +	struct hw_breakpoint *bp;
> > > > +	/* The DR6 value is stored in args->err */
> > > > +	unsigned long dr7, dr6 = args->err;
> > > 
> > > Please change this.  (I should have changed it long ago, but I never
> > > got around to it.)  Instead of passing the DR6 value in args->err,
> > > pass a pointer to the dr6 variable in do_debug().  That way the
> > > handler routines can turn off bits in that variable and do_debug() can
> > > see which bits remain set at the end.
> > > 
> > > Of course, this will require a corresponding change to the
> > > post_kprobe_handler() routine.
> > >
> > 
> > As I looked at the code with an intention of changing it, I don't find a
> > place - in hw_breakpoint_handler() and in functions called by
> > kprobe_exceptions_notify() where bits in dr6 are written into.
> 
> That's because such writes wouldn't do any good in the old code!  :-)
> 
> > The thread-specific thread->debugreg6 is updated with causative bits in
> > ptrace_triggered() to help send signals to the user-space. I don't see a
> > user for the change you propose.
> 
> For each breakpoint where we decide it's a case of lazy DR switching or
> we invoke a "triggered" callback, the corresponding bit in dr6 should
> be cleared.  This is a way of indicating to do_debug() that the handler 
> has taken care of these causes of the exception.
> 
> Similarly, the kprobe routine should clear the single-step bit in dr6 
> when it handles a single-step exception.  When the notifier chain 
> completes, the only bits remaining in dr6 should be for events that 
> still need to be handled.
> 
> Alan Stern
>

This does sound like good design, but unfortunately there are pieces in
do_debug() which rely upon bits in dr6 being set even after the actual
breakpoint is handled (the get_si_code() is one such example).

Do we go about changing them to use thread->debugreg6 instead of dr6? If
yes, wouldn't that be better done outside the HW Breakpoint patches as a
part of some cleanup initiative?

Thanks,
K.Prasad