Well.. I'm a little astonished but after modifying vm-bhyve to write the bare minimum vlan stanzas to the cloud-init network-config it generates - the basic functionality seems to be working. I do have a guest side network-online timeout to resolve.

I did have to have iovctl turn off all the mac filtering/anti-spoofing/etc for ixl0.

I am accumulating tweaks to vm-bhyve for adding functionality. I need more. So far, I need to be able to set an explicit mac for a VM - to match the mac assigned to the VF. And be able to specify pre-built cloud-init config files.

The cidata seed.iso is definitely better as a virtio-blk rather than ahci-cd because ahci-cd adds about one extra second of boot time for linux guest kernels. I forsee some patches heading upstream soon.

Anyway, to answer my first question: yes, AMD pci passthrough does seem to work - when enabled.