Hi Mike,
Thanks for the note. Not feasible to move to AMD. The 3090 is factory supported for passthrough. In fact, there's even a hack to make it virtualizable under KVM--unfortunately this hack does not appear to be portable to Xen.
I need some pointers on what to look for or look at. I'm happy to do some legwork, but I'm not sure where to start. I'm hoping the community will help me. Otherwise Qubes is DOA on this system, and I'm going to have to give up on Qubes and look at a different solution (I think someone has cobbled up a Qubes-like config using KVM--maybe I'll give that a try).
Thanks,
--Ben