GPU passthrough with libvirt qemu kvm

GPU passthrough is a technology that allows you to Article description::directly present an internal PCI GPU to a virtual machine. The device acts as if it were directly driven by the VM, and the VM detects the PCI device as if it were physically connected. GPU passthrough is also often known as IOMMU, although this is a bit of a misnomer, since the IOMMU is the hardware technology that provides this feature but also provides other features such as some protection from DMA attacks or ability to address 64-bit memory spaces with 32-bit addresses.

As you can imagine, the most common application for GPU passthrough at least gaming, since GPU passthrough allows a VM direct access to your graphics card with the end result of being able to play games with nearly the same performance as if you were running your game directly on your computer.

QEMU (Quick EMUlator) is a generic, open source hardware emulator and virtualization suite.

BIOS and UEFI firmware
In order to utilize KVM either VT-x or AMD-V must be supported by the processor. VT-x or AMD-V are Intel and AMD's respective technologies for permitting multiple operating systems to concurrently execute operations on the processors.

To inspect hardware for visualization support issue the following command:

For a period manufacturers were shipping with virtualization turned off by default in the system BIOS

Hardware

 * A CPU that supports Intel VT-d or AMD-Vi. Check List of compatible Intel CPUs (Intel VT-x and Intel VT-d)
 * A motherboard that supports the aforementioned technologies. To find this out, check in your motherboard's BIOS configuration for an option to enable IOMMU or something similar. Chances are that your motherboard will support it if it's from 2013 or newer, but make sure to check since this is a niche technology and some manufacturers may save costs by axing it from their motherboards or delivering a defective implementation (such as Gigabyte's 2015-2016 series) simply because NORPs never use it.
 * At least two GPUs: one for your physical OS, another for your VM. (You can in theory run your computer headless through SSH or a serial console, but it might not work and you risk locking yourself away from your computer if you do so).
 * Optional but recommended: Additional monitor, keyboard and mouse

EFI configuration
Go into BIOS (EFI) settings and turn on VT-d and IOMMU support.

IOMMU
IOMMU – or input–output memory management unit – is a memory management unit (MMU) that connects a direct-memory-access–capable (DMA-capable) I/O bus to the main memory. The IOMMU maps a device-visible virtual address ( I/O virtual address or IOVA) to a physical memory address. In other words, it translates the IOVA into a real physical address.

In an ideal world, every device has its own IOVA address space and no two devices share the same IOVA. But in practice this is often not the case. Moreover, the PCI-Express (PCIe) specifications allow PCIe devices to communicate with each other directly, called peer-to-peer transactions, thereby escaping the IOMMU.

That is where PCI Access Control Services (ACS) are called to the rescue. ACS is able to tell whether or not these peer-to-peer transactions are possible between any two or more devices, and can disable them. ACS features are implemented within the CPU and the chipset.

Unfortunately the implementation of ACS varies greatly between different CPU or chip-set models.

IOMMU kernel configuration
To enable IOMMU support in kernel:

Re-build the kernel

Turn on IOMMU in GRUB. Edit file and add parameters to the kernel

Check, that IOMMU turned on and works

IOMMU groups
Passing through PCI or VGA devices requires you to pass through all devices within an IOMMU group. The exception to this rule are PCI root devices that reside in the same IOMMU group with the device(s) we want to pass through. These root devices cannot be passed through as they often perform important tasks for the host. A number of (Intel) CPUs, usually consumer-grade CPUs with integrated graphics (IGD), share a root device in the same IOMMU group as the first PCIe 16x slot.

Nvidia in IOMMU Group 13 and AMD Video Card in IOMMU group 15 and 16. Everything looks fine. But if you have buggy IOMMU support and all devices within one IOMMU group, hardware can't guarantee good device isolation. Unfortunately, it is not possible to fix that. The only workaround - use ACS override patch witch ignore IOMMU hardware check. See ACS override patch.

ACS override patch
Next re-emerge the kernel

VFIO
Kernel drivers

Search for your VGA card ids. Run

Add VGA PCI ids to VFIO

Windows
Create windows 10 as usual via libvirt manager. Edit virtual image, click Add Hardware, select AMD Ati Vega 64 and AMD Ati device. Click Apply.

Now you can start Windows 10 guest OS.

Amd card have 2 devices on PCIe bus -> one video output and another is HDMI output. Windows drivers works only if KVM will bypass to windows both AMD devices.

Fixed Vega 56/64 reset bug
AMD Vega 56/64 are unable to initialize after Guest host shutdown/reboot, because drivers set card in "garbage" state. As workaround of this bug, VFIO will load AMD card ROM at guest startup. To do that:

1) Install clear Windows 10 somewhere (not in libvirt. We need REAL windows10)

2) Install all latest windows 10 updates

3) Install AMD vga drivers

4) Reboot

5) Go again to real windows 10

6) Install GPU-z

7) In GPU-z in main tab, near BIOS version will be small button "Save ROM". Click it and save ROM on somewhere. We will need this ROM for our gentoo and libvirt. For example, for my Vega64 I save ROM as Vega64.rom

8) Boot into gentoo back

9) mkdir /etc/firmware

10) Copy to /etc/firmware your ROM file (in my case it is Vega64.rom)

11) Go to /etc/libvirt/qemu

12) Edit xml file with description of Win10 guest

13) Find section with your AMD Card Video Card device (not AMD HDMI. You can always re-check by lspci):

In my case:

14) Add path to vga ROM.



So, it should be:

Sound
create dir /home/qemu copy /home/USER/.config/pulse to /home/qemu chown qemu:qemu -R /home/qemu

change home dir for qemu: usermod -d /home/qemu qemu

Set home dir:

External resources

 * https://heiko-sieger.info/iommu-groups-what-you-need-to-consider/#What_is_IOMMU_and_why_do_I_need_it
 * https://forum.level1techs.com/t/linux-host-windows-guest-gpu-passthrough-reinitialization-fix/121097?source_topic_id=121737 - AMD GPU on windows guest
 * https://wiki.installgentoo.com/index.php/PCI_passthrough - PCI passthrough on gentoo