GPU Passthrough -> lightdm.service won't start

Okay, so I followed the Create a Gaming HVM guide and I got as far as modifying the grub config with rd.qubes.hide_pci to ignore my second gpu, after I apply the changes and reboot, I get asked my password. After inputting my password everything loads like it normally does but then I get [ERROR] Failed to start lightdm.service - Light Display Manager.

Extra info:

  • Both GPUs are AMD
  • Qubes 4.2

I really hope there is a fix since I already borked my installation twice.

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If you’re looking for a way to fix the boot into Qubes OS then in GRUB menu edit the entry and remove rd.qubes.hide_pci.

The problem is that GPU passthrough won’t work if I won’t hide the second GPU.

I saw somewhere on this forum that you can’t hide the primary GPU, only the secondary GPU, so primary GPU passthrough is not possible.
Maybe you can change the GPU used as primary in BIOS. Or switch the GPUs in motherboard slots.
But I’m not sure, you can search the forum for more info.

Right so I don’t have that option in my bios because my vendor doesn’t include it only Gigabyte and others do that, I did saw that my motherboard automatically sets the lowest slot as primary gpu so I tried using my lowest slot as my output and tried disabling the top slot one which I wasn’t using for any output. The problem is that after a fresh install I still get the same Failed to start lightdm.service error whenever I reboot my system after changing the grub with rd.qubes.hide_pci.
I also don’t think switching the place of the GPUs will help since I tried disabling both gpus.

Why do you think so? Is it stated somewhere in your BIOS info?

Maybe it’s the other way around and the GPU in top slot will be primary?

Did you try both to only hide GPU in top slot and to only GPU in bottom slot? Or did you mean you’ve tried to disable both of them at the same time?

I have first tried having my top slot gpu as my primary and the bottom one as secondary (which I tried to hide from dom0 but failed) then I tried the other way around to have my bottom slot gpu as my only display out and the top one to be hidden but still getting the same error.
I’m very curious to get the perspective of other people that managed to get GPU passthrough working and if they encountered similar behavior to mine.

It probably try to use the GPU you just hide from dom0.
Assuming you used the correct value for rd.qubes.hide_pci:

  • Only plug your primary GPU to your primary screen.
  • Once you successfully booted into Qubes, plug the secondary GPU to the secondary screen

Also did you checked my comment about this subject

Note: if after this step when you reboot the computer you get stuck in the QubesOS startup that means you are trying to use the GPU you just hide. Check your BIOS options. Also check the cables, BIOS have some GPU priority based on the type of cable. For example, DisplayPort can be favoured over HDMI.

I am in this exact same situation, but in my case:

  • I am also on Qubes 4.2 (rc4)
  • My CPU has integrated graphics
  • I’m trying to hide an Nvidia GPU

I had a previous setup working on 4.1 with the same extra boot parameters. So my hypothesis is that rd.qubes.hide_pci is not working as expected in Qubes 4.2.

What makes it particularly challenging to debug is that when I switch to another tty to get access to a console, it moves back to tty1 after half a second or so. So I can never login and see debugging information.

Are you suggesting GPU hot-plugging?


  • Both GPU correctly plugged in their respective PCI slots
  • Only connect 1 cable, from the primary GPU to the primary screen

That way you physically ensure that if you have something on your screen then it come from the primary GPU. Should be easier to debug that way

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Yes, these are the exact steps I followed from the beginning and as I said above I get to write in my LUKS password after other stuff loads I get stuck with the lightdm.service error .
To be sure I wasn’t doing something wrong I also switched between the output of my 2 GPUs. Once I tried with top slot gpu as primary (plugged in the monitor) and the second one hidden and then the other way around but still got the same error!

Would it be a viable option to downgrade to 4.1 to try it out and see if it works?

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weird, out of curiosity, if in the grub, on the xen line, you add the parameter x2apic_phys=true , do you see any difference ?

OK. Then this is my situation already with the integrated graphics.

Still the same for me, unfortunately.

Turns out lightdm trying to start was the source of this issue. In tty2 I ran the command sudo systemctl stop lightdm after fighting the system and it stopped.

Right so I tried running x2apic_phys under the command line option of xen but still getting the same outcome.
I have tried downgrading to 4.12 but when I boot into the .iso I get “A start job is running for udev Wait for Complete Device Initialization 1 min / 3 min 4s” then nothing commences further than that.
Is there any type of issue related to lightdm.service open on github?

I inspected the error log for lightdm on my system and what caused it to fail was Xorg. So the issue is with Xorg, I think.

Somehow it’s still trying to use the secondary graphics card even through technically it should be hidden

I have managed to get pass the rd.qubes.hide_pci part of the guide the only problem is that I had to downgrade to 4.12 .
I suspect that something got broken in 4.2 .

I was able to pass through my GPU without even hiding it using boot parameters by just configuring the correct GPU to boot Xorg on. Since I’m using my iGPU for Qubes, the config file is this:

vim /etc/X11/xorg.conf.d/20-intel.conf

Section "Device"
  Identifier    "Intel Graphics"
  Driver        "Intel"
  Option "DRI" "3"

I think this might fix your situation. If it doesn’t work, you can simply just delete the 20-intel.conf file you created and go back to normal.

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