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I'm running a Windows 11 VM via qemu/libvirt on my Fedora 40 (Plasma Wayland) host. The VM is configured to use a passthrough GPU, 25% of my cores (6), and 25% of my memory (16GB). For input it has a dummy PS/2 keyboard and mouse. It has no USB sharing or passthrough configured. When I'm using it, I connect via Looking Glass.

Sometimes I play a fighting game on my host OS while the VM is running. I exit Looking Glass, but even when the VM is running in the background, there is some noticeable input delay in the game (1-3 frames). It's jittery and inconsistent. I don't get frame lag or stutters. I don't suspect resource load to be the culprit—The game is old and light on resources, and my load stays low. I experience the jittery inputs when playing locally offline.

If I shut the VM off and restart the game, it's resolved and inputs are consistent again.

When a host machine is running a VM, is some amount of input jitter expected on the host? If so, why?

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This happened with me when I use any device over a USB 3.0 XHCI interface. It lags on both the host (Wayland Debian) and guest machines. Disabling XHCI on the bios or trying to use any interface that enables USB 2.0 support solved for me.

Another possibility is to take a look into the mouse DPI/frequency. There are multiple reports mentioning that a high-frequency mouse can cause the virtualization drive to became slow and lag both the host and guest OS.

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