This article assumes you already have a fully functional libvirt VM with PCI Passthrough working on a dedicated monitor. If you do not please ensure this is configured before you proceed.
If you are using QEMU directly, this does not apply to you.
Altering the libvirt Configuration
Add the following to the libvirt machine configuration inside the 'devices' section by running "virsh edit VM" where VM is the name of your virtual machine.
<shmem name='looking-glass'> <model type='ivshmem-plain'/> <size unit='M'>32</size> </shmem>
You will need to adjust the memory size to a value that is suitable for your desired maximum resolution using the following formula:
width x height x 4 x 2 = total bytes total bytes / 1024 / 1024 = total megabytes + 2
For example, for a resolution of 1920x1080 (1080p)
1920 x 1080 x 4 x 2 = 16,588,800 bytes 16,588,800 / 1024 / 1024 = 15.82 MB + 2 = 17.82
You must round this value up to the nearest power of two, which with the above example would be 32MB
It is suggested that you create the shared memory file before starting the VM with the appropriate permissions for your system, this only needs to be done once at boot time, for example (this is a sample script only, do not use this without altering it for your requirements):
touch /dev/shm/looking-glass chown user:kvm /dev/shm/looking-glass chmod 660 /dev/shm/looking-glass