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qxl with wayland and 4k resolution

The default memory limit for the QXL video card is very conservative. If you’re running FullHD or higher resolution it is adviced to increase the memory by editing the xml.

Behaviour

  • Frozen screen

QXL Configuration Guide

Windows Guest

  • vgamem = (width * height * 4) / 1024
  • ram = 4 * vgamem
  • vram: Unimportant (e.g., 8 MB)

Modern Linux Guest (KMS driver)

  • vgamem = (width * height * 4 * heads) / 1024
    • Heads = number of screens
    • Wayland: Double the result
    • Round up to nearest power of 2 if using Virt-Manager
  • ram = 4 * vgamem
  • vram >= vgamem * 2

Key Notes

  • Wayland Warning: Avoid in VMs—poor performance (stuttering, low FPS). Use X11 for guests.
  • Max Resolution:
    • X11: 2560x1600 (optimal performance at ≤1080p)
    • Higher resolutions may impact performance.

Example Configs

4K (3840x2160) - Wayland Guest

<video>
  <model type="qxl" ram="524288" vram="262144" vgamem="131072" heads="1" primary="yes"/>
  <address type="pci" domain="0x0000" bus="0x00" slot="0x01" function="0x0"/>
</video>

4k (3840x2160) - X11 Guest

<video>
  <model type="qxl" ram="131072" vram="65536" vgamem="32768" heads="1" primary="yes"/>
  <address type="pci" domain="0x0000" bus="0x00" slot="0x01" function="0x0"/>
</video>

Additional Advice

  • Avoid excessive memory allocation—wastes RAM without performance gains.
  • Screen arrangement impacts memory needs (e.g., compact vs. diagonal layouts).