GPU passthrough only works if host boots in Legacy mode

[thierrybla]

Hey, I was trying to configuring everything with only EUFI and had a extremely hard time getting my GTX 1660 Super to passthrough until I set my host machine back to Legacy boot, it worked immediately (the passthrough)..

The weird part is when I was using UEFI my second video card (GT 730) was passing through without a problem.

Does anyone know what I should change to make the GTX 1660 Super work when the host boots in EUFI? Or should I just give up and only boot Legacy?

My motherboard is a x570 asus rog strix x570-e
My CPU is a Ryzen 3900x
The video card that is not working in EUFI is a Gigabyte GeForce GTX 1660 Super Gaming OC 6G

If any other info is needed please let me know.

Thanks in advance for your help!

 

[Stefan_R]

Some mainboards switch which GPU (PCIe socket) they use for booting if you switch from Legacy to UEFI mode. That is stupid and makes no sense, but it is what it is, I've seen this behaviour multiple times on different boards now. I suspect that's what's going on for you.

If you don't need host console access, you can attempt to disable the UEFI framebuffer for Linux, that sometimes helps. To do so, add video=efifb:off to your kernel commandline.

Also potentially look for BIOS updates, or a toggle which allows you to manually select your boot GPU in the BIOS setup.

 

[thierrybla]

Some mainboards switch which GPU (PCIe socket) they use for booting if you switch from Legacy to UEFI mode. That is stupid and makes no sense, but it is what it is, I've seen this behaviour multiple times on different boards now. I suspect that's what's going on for you.

If you don't need host console access, you can attempt to disable the UEFI framebuffer for Linux, that sometimes helps. To do so, add video=efifb:off to your kernel commandline.

Also potentially look for BIOS updates, or a toggle which allows you to manually select your boot GPU in the BIOS setup.

I already had that in my kernel and I am on the latest bios update, sadly the bios doesn't give me that option.. but I think you are right because my second gpu always works perfectly fine. Do you have any other possible fixes?

What I also noticed is that if I connect hdmi to the non working card the console won't be shown either if I boot I see the console briefly but then it bugs out and I see a bunch of lines like a glitched monitor but won't function. Either way the server will still boot fine and gui will show up in the web browser.

 

[Stefan_R]

What I also noticed is that if I connect hdmi to the non working card the console won't be shown either if I boot I see the console briefly but then it bugs out and I see a bunch of lines like a glitched monitor but won't function. Either way the server will still boot fine and gui will show up in the web browser.

That would seem like an indication of the BIOS initializing the card and then leaving it be, since linux is told not to use it (with the 'video=...' line). That's pretty much the theory I mentioned in my first post.

TBH, I'd suggest just using legacy boot if it works for you. Do you need UEFI for anything specific? In general, once the OS is booted it really doesn't matter which boot method was used.

 

[thierrybla]

That would seem like an indication of the BIOS initializing the card and then leaving it be, since linux is told not to use it (with the 'video=...' line). That's pretty much the theory I mentioned in my first post.

TBH, I'd suggest just using legacy boot if it works for you. Do you need UEFI for anything specific? In general, once the OS is booted it really doesn't matter which boot method was used.

The only thing EUFI offers to my knowledge is better failover for the bootdisks I made two disks for the OS in raid1 with zfs but in legacy it only selects one but in EUFI I can select two.

But I agree, I will use legacy just was a but curious on why this happens and if there would be a solution.. Just a last question.. Would it possibly work if I put both gpus in the bottom slots of my motherboard? And other pci devices in the first slot? Thanks for your help so far! Have a nice day!

 

[Stefan_R]

Just a last question.. Would it possibly work if I put both gpus in the bottom slots of my motherboard? And other pci devices in the first slot?

If I had to guess, probably not, as the board would still look for at least one GPU to boot on... but then again, only one way to truly find out