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Controlling virtual machines with VNC and Spice
Well Seasoned
For graphical output to work on KVM-based virtual machines, several pieces of a puzzle need to fit. For one thing, KVM needs to provide a graphics adapter to the underlying QEMU system on the virtual machine. By default, a (legacy) Cirrus graphics card is emulated. If you need a resolution above 1024x768 pixels, other virtual graphics cards are available; however, on Linux guests, this setup often means a manual X configuration.
Additionally, you must consider which protocol you should use to transfer the graphical data from the virtual machine to the client. VNC is the standard choice for a remote desktop protocol; it is widespread beyond the world of Linux and causes the least problems in production use. One alternative to VNC, however, is the new Spice protocol, which promises superior speed and a number of additional features.
The Virtual Graphics Adapter
For the virtual machine to see a graphics system, QEMU emulates a graphics card. In combination with KVM, you have a choice of four models:
cirrus: This graphics card is correctly detected by just about any guest system and offers acceptable speed with no configuration overhead. However, the maximum resolution is 1024x768 pixels.vga: The benefit of this virtual graphics card is that it supports far higher resolutions (tested up to 2560x1600 pixels). The graphics card is correctly detected on Windows, but some manual configuration work is necessary on Linux. You need to set up the Vesa driver explicitly inxorg.conf(Listing 1).
Listing 1
Vesa Configuration
01 Section "Device" 02 Identifier "device0" 03 Driver "vesa" 04
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