In other words, will it work to reserve the first PCIe slot with vfio-pci, and then commit the first slot to a VM for passthrough?
A little background: I have a B450 mobo with a primary x16 slot as well as a physical secondary x16 slot. The problem is that second slot is electrically PCIe 2.0 x4. So I’d like to use the secondary “x16” slot for the host GPU.
I think gigabyte are the only vendors that offer switching which GPU you can boot from in the BIOS. So if you have a gigabyte board, check in there.
On high end (threadripper) and server boards (epyc) you can put the GPU in whatever slot you like.
I originally misread your post. As far as regular consumer boards, I’ve heard of occasional board specific issues. One complication is if the card expects to use lots of pcie supplied power, but the slot doesn’t/can’t provide it. Not sure if a x16 physical x4 electrical slot is specified to provide the possibly expected 75watts of a full fat x16 slot.
Basically try it and see.
I’m running an ASUS Prime X470-Pro with a 3080Ti in the top slot for vfio/looking-glass and an RX 6700 XT in the second slot for host. This works for me, but I always unplug the dummy plug from the 3080Ti whenever I boot the system – if I don’t do that, I think sometimes it makes the 3080 Ti primary? I think you also need Legacy Boot disabled for this to work too.
Unfortunately this is an MSI board not Gigabyte, and I do not recall any options in the EFI menus to make the second GPU primary. @darcagn 's report is somewhat encouraging, though obviously it might not work in my environment. Interesting that it has to be unplugged from anything even remotely resembling a monitor. That may be what went wrong with my efforts years ago.
I purposely bought Gigabyte because it specifically has this feature to add flexibility. People have asked for this for EVGA boards before but the BIOS engineers haven’t seen a reasonable use case for this on their motherboards.
If you want this feature in the upcoming EVGA X570S motherboard, better ask and request this feature now while the Beta BIOSes are being made.
I have an asrock x570 itx and would love to be able to set my thunderbolt egpu as the primary so that I could use the primary gpu for vfio, but i guess we are not there yet…