> dr_control_mirror is never supposed to be updated from the target.  It's a
> one way street - the state we _want_ dr7 to have on next resume.  Where is this
> 0x400 coming from then?
> 
> Also, what's different in native GDB?  This debug registers code is
> shared between GDB and GDBserver now.

It is read back in win32-i386-low.c::i386_get_thread_context:

  if (th->tid == current_event->dwThreadId)
    {
      /* Copy dr values from the current thread.  */
      struct x86_debug_reg_state *dr = &debug_reg_state;
      dr->dr_mirror[0] = th->context.Dr0;
      dr->dr_mirror[1] = th->context.Dr1;
      dr->dr_mirror[2] = th->context.Dr2;
      dr->dr_mirror[3] = th->context.Dr3;
      dr->dr_status_mirror = th->context.Dr6;
      dr->dr_control_mirror = th->context.Dr7;
    }

debug_reg_state is a global, which is passed to x86_dr_insert_watchpoint
in win32-i386-low.c::i386_insert_point:

        return x86_dr_insert_watchpoint (&debug_reg_state,
                                         hw_type, addr, size);

i386_get_thread_context gets called after we receive the watchpoint
signal (TARGET_WAITKIND_STOPPED) in win32_wait:

        case TARGET_WAITKIND_STOPPED:
        case TARGET_WAITKIND_LOADED:
          OUTMSG2 (("Child Stopped with signal = %d \n",
                    ourstatus->value.sig));

          regcache = get_thread_regcache (current_thread, 1);
          child_fetch_inferior_registers (regcache, -1);
          return debug_event_ptid (&current_event);

It looks like just removing the statement that sets dr_control_mirror
from the thread's DR7 is sufficient to allow hardware watchpoints
to work again.

gdb/gdbserver/ChangeLog:

        * win32-i386-low.c (i386_get_thread_context): Do not set
        dr->dr_control_mirror.

Tested on x86-windows using AdaCore's testsuite. No regression while
all watchpoint-related issues get resolved.

I admit I could have spent a little more time trying to understand
the bigger picture, but I'm a bit under time pressure, so I'm hoping
this is enough. Otherwise, I'll look at a better fix.

Once we have a fix, I think we'll want it for GDB 7.8.1. I haven't
completely checked eitherr, but I think it's there as well. I'll create
a PR as soon as I have a minute.

Thanks!
-- 
Joel