[Solved] RTX 3090 GPU passthrough just displays a black screen with qemu

Full solution here to get passthrough working with ReBAR enabled on certain platforms with a 3090 and possibly other high-memory cards. TL;DR: Add this config to your domain XML (or the corresponding QEMU command-line arguments elsewhere):

  <qemu:arg value='-fw_cfg'/>
  <qemu:arg value='opt/ovmf/X-PciMmio64Mb,string=65536'/>

And you have to modify the top-level <domain/> tag to include the proper namespace as well:

<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>

It works for me on a newer “X570S” board. YMMV.