Arch Linux

andreigrig

Member

Registered: 2016-02-04

Posts: 20

Set main GPU before Xorg

Hi all,

I've made a very satisfactory setup with Arch as the main OS and with a Windows VM with GPU passthrough for gaming. The GPU passthrough works very well and so far I have encountered no issues. However, in order to make this possible I have placed my best video card into a PCI-E x8 slot (x4 electrical) witch sometimes limits the capacity of the GPU.

What I would like now is to put this GPU into the x16 slot and pass it through from there. So far, I cannot seem to make the OS use the other card as the main GPU. The problem comes from the fact that I cannot select the main GPU in BIOS, the MB setting automatically the x16 card as the main GPU.

Now, when the computer boots, it uses the card that I want to pass through. So far, I managed to make Xorg use the cheaper card as the main GPU, but for some reason KVM still sees the most capable card as the main one.

Does anyone have any idea how to force the main GPU from UDEV and at the same time not to break the vfio driver assignment (when I try to remove the card from UDEV, the card does not show in vfio anymore). Maybe it works from GRUB or are there any kernel parameters that are can help me set just that ? - as I think this needs to be set before Xorg loads. Another idea is to override the passing through of the main GPU but I only found out it is possible with XEN and didn't find any information about KVM.

I do not want to change the MB since I have no guarantee that another one will give me the option to select the primary GPU, and it is also brand new.

Would anyone be so kind and share any ideas?

Thanks!
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Later edit:

I found that /sys/bus/pci/devices/0000:01:00.0/boot_vga is set to 1. Maybe someone knows how to change the boot flag right after vgaarb does it? Or how to specify a device for vgaarb and not let it get the default GPU passed by BIOS settings?

Last edited by andreigrig (2016-02-04 21:06:57)