I have 5900x 64G, resbar enabled in bios, uefi boot and 3x 7900 xfx, this worked for me, but I had to adjust rebar to 4MB/32G or I get black screen, I have no idea why more can not work
I am on acs 6.3 kernel as I need to have 3 GPU in 3 VM so their own group each
maybe I need to adjust opt/ovmf/X-PciMmio64Mb,string=65536 for 24G GPU or I need to set more ram per VM (atm 14G)