This is how I turn on IOMMU in Proxmox before trying PCIe passthrough.

Enable IOMMU in GRUB

vi /etc/default/grub
Find the line with "GRUB_CMDLINE_LINUX_DEFAULT"
Add `intel_iommu=on` or `amd_iommu=on`
Save
update-grub

PCIe ACS Override

There are motherboards that bundle peripherals together (does not support ACS).

A workaround is to override PCIe ACS by adding pcie_acs_override=downstream,multifunction in GRUB_CMDLINE_LINUX_DEFAULT.

This flags the kernel that a device can be “isolated”. It might have security and stability issues.

References:

• [PATCH] pci: Enable overrides for missing ACS capabilities https://lkml.org/lkml/2013/5/30/513
• IOMMU Groups, inside and out http://vfio.blogspot.com/2014/08/iommu-groups-inside-and-out.html

IOMMU groups on real hardware

Whether passthrough works depends on how the board groups its devices. These are two machines I checked.

Asus Chromebox CN62

It cannot passthrough the GPU

Lenovo ThinkCentre M920s

iGPU and SATA controller is in its own group

Related: Proxmox iGPU Passthrough only works on Windows guests and Attempted to make XFX RX 460 work on a macOS KVM.

For the past couple of days (when I’m procrastinating), I’ve been attempting to make an XFX RX 460 work as a GPU passthrough on a macOS KVM.

Ready-made macOS VM

Flashing a different BIOS

# Display all adaptors detected
./amdvbflash -ai
# Get info about the rom file
./amdvbflash -biosfileinfo vbios.rom
# Save bios
./amdvbflash -s 0 original_rom
# Force flash a rom
./amdvbflash -f -p 0 ./original_rom

Where to get BIOS rom files

https://www.techpowerup.com/vgabios/

How to know which BIOS are compatible

Run GPU-Z. Take note of the memory type and size.

Setting Proxmox to use a rom file

Where are the rom files located

/usr/share/kvm/romfile.rom

How to use the romfile

vi /etc/pve/qemu

Add romfile=romfilename.rom to hostpci0.

Giving up

I think I’ve tried almost all BIOS compatible with my graphics card. I was not able to make it work.

Too much time has been spent. I could have bought a fully compatible one if I worked for the same amount of time I spent on this.

It was a good try, but I need to cut my losses (of time).

Related: Enable IOMMU on Proxmox, Plus Real IOMMU Groups.