ddb in domU

[Greg Troxel <gdt%lexort.com@localhost>]

I am trying to debug zfs lockups, and using a domU.  I have succeeded in
provoking a lockup, I think.  I have an 'xl console' open.

ddb(4) says +++++ will enter ddb on a domU, but that's for a domU kernel
presumably.   My domU is HVM with GENERIC.  So I can send "break".  But
I haven't been able to find what to type to send break.

It is seeming like one needs a kernel option in the domUs kernel, or to
get hw.cnmagic, or something.