Yes, this was it!
Before making may more changes, I looked in /proc/iomem and found this:
1802100-20020ff : PCI Bus :80
200-2000fff
200-200120f : PCI Bus :81
200-2000fff : :81:00.0
200-2257fff : efifb HERE,
I think here is the strongest hint; the host dmesg is floodeed with messages
"BAR 1: can't reserve"
2021-12-30T10:01:59.456992+ gauss.lan.incorrekt.net kernel: vfio-pci
:81:00.0: vfio_ecap_init: hiding ecap 0x1e@0x258
2021-12-30T10:01:59.457413+ gauss.lan.incorrekt.net kernel:
Some more information:
1. driver seem to be loading fine in guest
bronekk@euclid:~$ sudo dmesg | grep -E "nvidia|0d:00"
[0.810066] pci :0d:00.0: [10de:1eb1] type 00 class 0x03
[0.814518] pci :0d:00.0: reg 0x10: [mem 0xc000-0xc0ff]
[0.818518] pci :0d:00.0: reg
