libvirt/QEMU Installation

This article assumes you already have a fully functional libvirt domain with PCI passthrough working. If you use virt-manager, this guide also applies to you, since virt-manager uses libvirt as its back end.

Determining memory

You will first need to calculate the memory size to be suitable for your desired maximum resolution using the following formula:

\[ \begin{align}\begin{aligned}\text{WIDTH} \times \text{HEIGHT} \times \text{BPP} \times 2 = \text{frame size in bytes}\\\text{frame size in bytes} \div 1024 \div 1024 = \text{ frame size in MiB}\\\text{frame size in MiB} + 10 = \text{ required size in MiB}\\2^{\lceil \log_2(\text {required size in MiB}) \rceil} = \text{ total MiB}\end{aligned}\end{align} \]

Where BPP is 4 for 32-bit RGB (SDR) or 8 for 64-bit (HDR *).

Hint

The final step in this calculation is simply rounding the value up to the nearest power of two.

For example, for a resolution of 1920x1080 (1080p) SDR:

\[ \begin{align}\begin{aligned}1920 \times 1080 \times 4 \times 2 = 16,588,800 \text{ bytes}\\16,588,800 \div 1024 \div 1024 = 15.82 \text{ MiB}\\15.82 \text{ MiB} + 10 \text{ MiB} = 25.82 \text{ MiB}\\2^{\lceil \log_2(25.82) \rceil} = 32 \text { MiB}\end{aligned}\end{align} \]

Failure to provide enough memory will cause Looking Glass to truncate the bottom of the screen and will trigger a message popup to inform you of the size you need to increase the value to.

Note

Increasing this value beyond what you need does not yield any performance improvements, it simply will block access to that RAM making it unusable by your system.

Common Values
Resolution	Standard Dynamic Range	High Dynamic Range (HDR) *
1920x1080 (1080p)	32	64
1920x1200 (1200p)	32	64
2560x1440 (1440p)	64	128
3840x2160 (2160p/4K)	128	256

Warning

While Looking Glass can capture and display HDR, at the time of writing neither Xorg or Wayland can make use of it and it will be converted by the GPU drivers/hardware to SDR. Additionally using HDR doubles the amount of memory, bandwidth, and CPU load and as such should generally not be used unless you have a special reason to do so.

IVSHMEM

There are two methods of configuring IVSHMEM, using shared memory directly, or using the KVMFR kernel module. While the KVMFR module is slightly more complicated to configure, it substantially improves performance as it allows Looking Glass to use your GPUs DMA engine to transfer the frame data.

Keyboard/mouse/display/audio

Looking Glass makes use of the SPICE protocol to provide keyboard and mouse input, audio input and output, and display fallback.

Note

The default configuration that libvirt uses is not optimal and must be adjusted. Failure to perform these changes will cause input issues along with failure to support 5 button mice.

If you would like to use SPICE to give you keyboard and mouse input along with clipboard sync support, make sure you have a <graphics type='spice'> device, then:

Find your <video> device, and set <model type='vga'/>
- If you can’t find it, make sure you have a <graphics> device, save and edit again.
Remove the <input type='tablet'/> device, if you have one.
Create an <input type='mouse' bus='virtio'/> device, if you don’t already have one.
Create an <input type='keyboard' bus='virtio'/> device to improve keyboard usage.

Note

Be sure to install the the vioinput driver from virtio-win in the guest

To enable audio support add a standard Intel HDA audio device to your configuration as per below:

<sound model='ich9'>
  <audio id='1'/>
</sound>
<audio id='1' type='spice'/>

If you also want clipboard synchronization please see Clipboard synchronization

Clipboard synchronization

Looking Glass can synchronize the clipboard between the host and guest using the SPICE guest agent.

1. Install the SPICE guest tools from https://www.spice-space.org/download.html#windows-binaries.

Configure your VM to enable the SPICE guest agent:

QEMU

-device virtio-serial-pci \
-chardev spicevmc,id=vdagent,name=vdagent \
-device virtserialport,chardev=vdagent,name=com.redhat.spice.0

libvirt

<channel type="spicevmc">
  <target type="virtio" name="com.redhat.spice.0"/>
  <address type="virtio-serial" controller="0" bus="0" port="1"/>
</channel>
<!-- No need to add a VirtIO Serial device, it will be added automatically -->

AppArmor

For libvirt versions before 5.10.0, if you are using AppArmor, you need to add permissions for QEMU to access the shared memory file. This can be done by adding the following to /etc/apparmor.d/local/abstractions/libvirt-qemu:

/dev/shm/looking-glass rw,

then, restart AppArmor.

sudo systemctl restart apparmor

Memballoon

The VirtIO memballoon device enables the host to dynamically reclaim memory from your VM by growing the balloon inside the guest, reserving reclaimed memory. Libvirt adds this device to guests by default.

However, this device causes major performance issues with VFIO passthrough setups, and should be disabled.

Find the <memballoon> tag and set its type to none:

<memballoon model="none"/>

Additional tuning

Looking Glass is latency sensitive and as such it may suffer microstutters if you have not properly tuned your virtual machine. The physical display output of your GPU will usually not show such issues due to the nature of the hardware but be sure that if you are experiencing issues the following tuning is required to obtain optimal performance.

Do not assign all your CPU cores to your guest VM, you must at minimum reserve two CPU cores (4 threads) for your host system to use. For example, if you have a 6 core CPU, only assign 4 cores (8 threads) to the guest.
Ensure you correctly pin your VMs vCPU threads to the correct cores for your CPU architecture.
If you are on a NUMA architecture (dual CPU, or early Threadripper) be sure that you pin the vCPU threads to the physical CPU/die attached to your GPU.
Just because your GPU is in a slot that is physically x16 in size, does not mean your GPU is running at x16, this is dependent on how your motherboard is physically wired and the physical slot may be limited to x4 or x8.
Be sure to set your CPU model type to host-passthrough so that your guest operating system is aware of the acceleration features of your CPU and can make full use of them.
AMD users be sure that you have the CPU feature flag topoext enabled or your guest operating system will not be aware of which CPU cores are hyper-thread pairs.

How to perform these changes is left as an exercise to the reader.