Hi Dave,
 One thing re: x86_64 -- unlike x86, please create your own
xen-specific
 backtrace function to plug into the machdep->back_trace function pointer.
 I'm pretty sure you won't be able to make the current version work
 for xen! 
Yes, I'm investigating now. "bt" is more difficult than other command.
Thanks.
-- 
Itsuro ODA <oda(a)valinux.co.jp>