opensm reports - Invalid requested MGID. What does it mean?

opensm reports - Invalid requested MGID. What does it mean?

I've been working on a virtualized SR-IOV infiniband interface under Xen.
If I run the OpenSM daemon under Dom0 the passed through ports appear in
DomU and work.
However, not if OpenSM is running on a different host.
I'm actually wanting to run OpenSM under Dom0, it's a good place to put
it. But, when I do I get the following error in my log after a while.
Oct 01 01:00:57 645819 [78D97700] 0x01 -> mcmr_rcv_create_new_mgrp: ERR
1B22: Invalid requested MGID
Oct 01 01:00:57 646091 [7EDA3700] 0x01 -> validate_requested_mgid: ERR
1B01: Wrong MGID Prefix 0x8000 must be 0xFF
Oct 01 01:00:57 646141 [7EDA3700] 0x01 -> mcmr_rcv_create_new_mgrp: ERR
1B22: Invalid requested MGID
Oct 01 01:00:57 646438 [7458E700] 0x01 -> validate_requested_mgid: ERR
1B01: Wrong MGID Prefix 0x8000 must be 0xFF
This error keeps repeating so quickly that the log file ends up filling
the drive up within about 30 minutes or so.
What's could be causing it? how do I fix it?