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Table Of Contents

Enable & Verify IOMMU

BIOS Settings
Enable Intel VT-d or AMD-Vi in BIOS settings.
If you can't find those virtualization options in BIOS, your hardware probably doesn't support it.

Set the kernel paramater depending on your CPU.
For GRUB user, edit grub configuration.

/etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT="... intel_iommu=on iommu=pt ..."
OR
GRUB_CMDLINE_LINUX_DEFAULT="... amd_iommu=on iommu=pt ..."

Generate grub.cfg

grub-mkconfig -o /boot/grub/grub.cfg

Reboot your system for the changes to take effect.

To verify IOMMU, run the following command, which should return result.

dmesg | grep 'IOMMU enabled'

Now, you need to make sure that your IOMMU groups are valid.
Run the following script to view the IOMMU groups and attached devices. \

#!/bin/bash
shopt -s nullglob
for g in `find /sys/kernel/iommu_groups/* -maxdepth 0 -type d | sort -V`; do
    echo "IOMMU Group ${g##*/}:"
    for d in $g/devices/*; do
        echo -e "\t$(lspci -nns ${d##*/})"
    done;
done;

During passthrough, you need to pass every device (except PCI) in the group which includes your GPU.
You can avoid having to pass everything by using ACS override patch.

Install required tools

Gentoo Linux RECOMMENDED USE FLAGS: app-emulation/virt-manager gtk
             app-emulation/qemu spice usb usbredir pulseaudio 
emerge -av qemu virt-manager libvirt ebtables dnsmasq
Arch Linux 
pacman -S qemu libvirt edk2-ovmf virt-manager dnsmasq ebtables
Fedora 
dnf install @virtualization
Ubuntu 
apt install qemu-kvm qemu-utils libvirt-daemon-system libvirt-clients bridge-utils virt-manager ovmf

Enable required services

SystemD 
systemctl enable --now libvirtd
OpenRC 
rc-update add libvirtd default
rc-service libvirtd start

Sometimes, you might need to start default network manually.

virsh net-start default
virsh net-autostart default

Setup Guest OS

NOTE: You should replace win10 with your VM's name where applicable
You should add your user to libvirt group to be able to run VM without root. And, input and kvm group for passing input devices.

usermod -aG kvm,input,libvirt username

Download virtio driver.
Launch virt-manager and create a new virtual machine. Select Customize before install on Final Step.
In Overview section, set Chipset to Q35, and Firmware to UEFI
In CPUs section, set CPU model to host-passthrough, and CPU Topology to whatever fits your system.
For SATA disk of VM, set Disk Bus to virtio.
In NIC section, set Device Model to virtio
Add Hardware > CDROM: virtio-win.iso
Now, Begin Installation. Windows can't detect the virtio disk, so you need to Load Driver and select virtio-iso/amd64/win10 when prompted.
After successful installation of Windows, install virtio drivers from virtio CDROM. You can then remove virtio iso.

Attaching PCI devices

Remove Channel Spice, Display Spice, Video QXL, Sound ich* and other unnecessary devices.
Now, click on Add Hardware, select PCI Devices and add the PCI Host devices for your GPU's VGA and HDMI Audio.

Libvirt Hooks

Libvirt hooks automate the process of running specific tasks during VM state change.
More info at: PassthroughPost

Note: Comment Unbind/rebind EFI framebuffer line from start and stop script if you're using AMD 6000 series cards, thanks to cdgriffith. Also, move the line to unload AMD kernal module below detaching devices from host. These might also apply to older AMD cards.

Create Libvirt Hook 
mkdir /etc/libvirt/hooks
touch /etc/libvirt/hooks/qemu
chmod +x /etc/libvirt/hooks/qemu
/etc/libvirt/hooks/qemu 
#!/bin/bash

GUEST_NAME="$1"
HOOK_NAME="$2"
STATE_NAME="$3"
MISC="${@:4}"

BASEDIR="$(dirname $0)"

HOOKPATH="$BASEDIR/qemu.d/$GUEST_NAME/$HOOK_NAME/$STATE_NAME"
set -e # If a script exits with an error, we should as well.

if [ -f "$HOOKPATH" ]; then
eval \""$HOOKPATH"\" "$@"
elif [ -d "$HOOKPATH" ]; then
while read file; do
  eval \""$file"\" "$@"
done <<< "$(find -L "$HOOKPATH" -maxdepth 1 -type f -executable -print;)"
fi
Create Start Script 
mkdir -p /etc/libvirt/hooks/qemu.d/win10/prepare/begin
touch /etc/libvirt/hooks/qemu.d/win10/prepare/begin/start.sh
chmod +x /etc/libvirt/hooks/qemu.d/win10/prepare/begin/start.sh
/etc/libvirt/hooks/qemu.d/win10/prepare/begin/start.sh 
#!/bin/bash
set -x

# Stop display manager
systemctl stop display-manager
# rc-service xdm stop

# Unbind EFI Framebuffer
echo efi-framebuffer.0 > /sys/bus/platform/drivers/efi-framebuffer/unbind

# Unload NVIDIA kernel modules
modprobe -r nvidia_drm nvidia_modeset nvidia_uvm nvidia

# Unload AMD kernel module
# modprobe -r amdgpu

# Detach GPU devices from host
# Use your GPU and HDMI Audio PCI host device
virsh nodedev-detach pci_0000_01_00_0
virsh nodedev-detach pci_0000_01_00_1

# Load vfio module
modprobe vfio-pci
Create Stop Script 
mkdir -p /etc/libvirt/hooks/qemu.d/win10/release/end
touch /etc/libvirt/hooks/qemu.d/win10/release/end/stop.sh
chmod +x /etc/libvirt/hooks/qemu.d/win10/release/end/stop.sh
/etc/libvirt/hooks/qemu.d/win10/release/end/stop.sh 
#!/bin/bash
set -x

# Unload vfio module
modprobe -r vfio-pci

# Attach GPU devices to host
# Use your GPU and HDMI Audio PCI host device
virsh nodedev-reattach pci_0000_01_00_0
virsh nodedev-reattach pci_0000_01_00_1

# Rebind framebuffer to host
echo "efi-framebuffer.0" > /sys/bus/platform/drivers/efi-framebuffer/bind

# Load NVIDIA kernel modules
modprobe nvidia_drm
modprobe nvidia_modeset
modprobe nvidia_uvm
modprobe nvidia

# Load AMD kernel module
# modprobe amdgpu

# Restart Display Manager
systemctl start display-manager
# rc-service xdm start

Keyboard/Mouse Passthrough

Modify libvirt configuration of your VM. Change first line to:

virsh edit win10 
<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>

Find your keyboard and mouse devices in /dev/input/by-id. You'd generally use the devices ending with event-kbd and event-mouse. And the devices in your configuration right before closing </domain> tag.
Replace MOUSE_NAME and KEYBOARD_NAME with your device id.

virsh edit win10 
...
  <qemu:commandline>
    <qemu:arg value='-object'/>
    <qemu:arg value='input-linux,id=mouse1,evdev=/dev/input/by-id/MOUSE_NAME'/>
    <qemu:arg value='-object'/>
    <qemu:arg value='input-linux,id=kbd1,evdev=/dev/input/by-id/KEYBOARD_NAME,grab_all=on,repeat=on'/>
  </qemu:commandline>
</domain>

You need to include these devices in your qemu config.

/etc/libvirt/qemu.conf 
...
user = "YOUR_USERNAME"
group = "kvm"
...
cgroup_device_acl = [
    "/dev/input/by-id/KEYBOARD_NAME",
    "/dev/input/by-id/MOUSE_NAME",
    "/dev/null", "/dev/full", "/dev/zero",
    "/dev/random", "/dev/urandom",
    "/dev/ptmx", "/dev/kvm", "/dev/kqemu",
    "/dev/rtc","/dev/hpet", "/dev/sev"
]
...

Also, switch from PS/2 devices to virtio devices. Add the devices inside <devices> block

virsh edit win10 
...
<devices>
  ...
  <input type='mouse' bus='virtio'/>
  <input type='keyboard' bus='virtio'/>
  ...
</devices>
...

Audio Passthrough

VM's audio can be routed to the host. You need Pulseaudio. It's hit or miss.
You can also use Scream instead of Pulseaudio.
Modify the libvirt configuration of your VM.

virsh edit win10 
...
  <qemu:commandline>
    ...
    <qemu:arg value="-device"/>
    <qemu:arg value="ich9-intel-hda,bus=pcie.0,addr=0x1b"/>
    <qemu:arg value="-device"/>
    <qemu:arg value="hda-micro,audiodev=hda"/>
    <qemu:arg value="-audiodev"/>
    <qemu:arg value="pa,id=hda,server=/run/user/1000/pulse/native"/>
  </qemu:commandline>
</devices>

Video card driver virtualisation detection

Video Card drivers refuse to run in Virtual Machine, so you need to spoof Hyper-V Vendor ID.

virsh edit win10 
...
<features>
  ...
  <hyperv>
    ...
    <vendor_id state='on' value='whatever'/>
    ...
  </hyperv>
  ...
</features>
...

NVIDIA guest drivers also require hiding the KVM CPU leaf:

virsh edit win10 
...
<features>
  ...
  <kvm>
    <hidden state='on'/>
  </kvm>
  ...
</features>
...

vBIOS Patching

NOTE: You only need patch the dumped ROM file. You don't need to make changes on the hardware BIOS.
While most of the GPU can be passed with stock vBIOS, some GPU requires vBIOS patching depending on your host distro.
In order to patch vBIOS, you need to first dump the GPU vBIOS from your system.
If you have Windows installed, you can use GPU-Z to dump vBIOS.
To dump vBIOS on Linux, you can use following command (replace PCI id with yours):
If it doesn't work on your distro, you can try using live cd.

echo 1 > /sys/bus/pci/devices/0000:01:00.0/rom
cat /sys/bus/pci/devices/0000:01:00.0/rom > path/to/dump/vbios.rom
echo 0 > /sys/bus/pci/devices/0000:01:00.0/rom

To patch vBIOS, you need to use Hex Editor (eg., Okteta) and trim unnecessary header.
For NVIDIA GPU, using hex editor, search string “VIDEO”, and remove everything before HEX value 55.
This is probably the same for AMD device.

To use patched vBIOS, edit VM's configuration to include patched vBIOS inside hostdev block of VGA

virsh edit win10 
...
<hostdev mode='subsystem' type='pci' managed='yes'>
  <source>
    ...
  </source>
  <rom file='/home/me/patched.rom'/>
  ...
</hostdev>
...

Troubleshooting

Where is the log?

The logs are stored in /var/log/libvirt/qemu/vm_name.log, where vm_name is the name of the VM you're trying to boot.
By reading the few lines at the bottom of the log file, you get general idea of what might be an issue.

warning: host doesn't support requested feature, what is this?

This warning usually means that the feature guest is requesting is not available on host.
After the warning message, on same line, you'll see something like MSR(490H).vmx-entry-load-perf-global-ctrl
That's the feature it's missing, but I have no idea what most of them indicate.

How to fix it?

Make sure you've enabled Intel VT-d or AMD-Vi in BIOS Settings.
If you've set BIOS as Firmware for Guest, change to UEFI.
In the cpu section in log file, you might see something like this:
-cpu Skylake,vme=on,ss=on,vmx=on,.... Although, it works in most cases, it doesn't in some.
Make sure you've set host-passthrough CPU model which'll produce -cpu host,... instead.

shutting down, reason=failed, but no actual error?

If the VM fails without an error, SSH into your host machine, and run libvirt start script from SSH Client.
If the script works without producing error, you'd get blank screen, and you can try starting VM from SSH Client, which might provide error message.
Both commands should be run as superuser.

sh /etc/libvirt/hooks/qemu.d/win10/prepare/begin/start.sh
virsh start win10

See Also

Single GPU Passthrough by joeknock90
Single GPU Passthrough by YuriAlek
ArchLinux PCI Passthrough
Gentoo GPU Passthrough