KVM QEMU - Guide

KVM QEMU: GPU Passthrough and Virtualization Support

This guide covers essential steps for setting up GPU passthrough in a KVM/QEMU virtual machine with an NVIDIA GPU. It also details enabling virtualization support and configuring Looking Glass for enhanced performance.

1. GPU Passthrough Setup#

When setting up GPU passthrough in QEMU, two key elements need attention for NVIDIA GPUs: the hidden KVM flag and the vendor ID for Hyper-V.

Without properly configuring these elements, the VM will not recognize or fully utilize the GPU’s power, and installing NVIDIA drivers may fail due to virtualization restrictions.

1.1. Hidden KVM Flag#

Enabling the hidden KVM flag prevents NVIDIA drivers from detecting the virtual machine, allowing the GPU passthrough to work correctly.

<features>
  ...
  <kvm>
    <hidden state="on"/>
  </kvm>
  ...
</features>

1.2. Vendor ID for Hyper-V#

Set a custom vendor_id in the XML file of your VM. This helps in fooling the drivers into thinking the VM is running on actual hardware.

<features>
  ...
  <hyperv>
    <vendor_id state="on" value="whatever"/>
  </hyperv>
  ...
</features>

2. Enabling Virtualization Support#

To run certain features like Windows Subsystem for Linux (WSL) within your virtual machine, ensure that Intel VT-x or AMD-V virtualization support is enabled. This can be done by adding the following line to your XML configuration:

<feature policy='require' name='vmx'/>

3. Optimizing Looking Glass for High Resolutions#

Looking Glass is a tool that allows you to share the GPU between the host and guest, providing a smooth KVM experience without needing a second monitor.

3.1. Disabling Memory Ballooning#

In some cases, memory ballooning may prevent Looking Glass from working with higher resolutions (such as 3440x1440). You can disable this feature in your VM’s XML configuration:

<devices>
  ...
  <memballoon model="none"/>
  <shmem name="looking-glass">
    <model type="ivshmem-plain"/>
    <size unit="M">128</size>
    <address type="pci" domain="0x0000" bus="0x10" slot="0x01" function="0x0"/>
  </shmem>
</devices>

By disabling memballoon, you should be able to get higher refresh rates and more resolutions to work seamlessly.

Tips & Troubleshooting#

• NVIDIA Code 43: If you encounter this error when using an NVIDIA GPU, ensure the hidden KVM flag is properly configured.
• Looking Glass Performance: Adjust the shared memory size in the shmem section to fine-tune performance. Increasing this may improve the VM’s ability to handle high frame rates and resolutions.

This guide should help you configure your virtual machines with KVM/QEMU to achieve GPU passthrough with optimal performance. Let me know if you need further adjustments or additional sections.