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Investigating problems with dynamic resizing

  • The basic mechanisms of dynamic resizing are described on another wiki page<1>. The short summary is that the host sends "size hints" to the virtual machine (which consist of an interrupt on the virtual graphics card or the "guest" device and a way for the guest to ask for the details) and special "driver" modules from the Guest Additions ask the guest operating system to resize its virtual displays to match the hints.
  • Every time the host sends a hint there will be a matching entry in the machine log file on the host ("VMMDev: SetVideoModeHint: Got a video mode hint (...)" VMMDev is the "guest" device. A matching hint may be sent to the graphics device).
  • If for whatever reason the guest operating system chooses to modify the requested size the host windows will try to follow that. If the host window manager refuses the usual result is that the guest display will not fit the host window perfectly. The guest drivers may also (Windows guests only) modify resize requests from the guest operating system to fit the size policy chosen by the user.

The default user interface

The default user interface sends hints for every virtual screen each time one of the machine windows on the host is resized - as long as dynamic resizing is enabled (please check this).

Connecting to guests with VRDP

(To do.)

Guests running X.Org or Wayland

  • Guests running X.Org 1.17 or earlier need a "driver" module written for and loaded by the X server to control the virtual graphics card. On all but the oldest supported versions of X.Org this module will be loaded automatically if it is put in the right directory on the guest (the Guest Additions do this when they are installed). Old versions of X.Org (mostly Oracle/RedHat EL 5 and family) and XFree86 need the xorg.conf/XF86Config files to be changed to load the driver (the Additions usually get this right too).
  • If X.Org 1.17 or older is not found on a Linux guest system, and the kernel version is 3.11 or newer, a kernel driver for the graphics device will be built and installed. This should work with X.Org 1.18 and newer as well as with Wayland. This driver is Linux-only, but is written to be easily portable to other free/open-source guest operating systems which use the "drm" system for graphics. If you are interested and able, feel free to say so on the development mailing list <1>.
  • Looking at the X.Org log file on the guest (usually /var/log/Xorg.0.log) often helps understand what is and is not working.
  • If you are not sure if the kernel driver was set up correctly, see the information page about investigating that<2>.
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