The Guest Additions provide services for controlling the guest
system's monitor topology. Monitor topology means the resolution
of each virtual monitor and its state (disabled/enabled). The
resolution of a virtual monitor can be modified from the host
side either by resizing the window that hosts the virtual
monitor, by using the View menu
or the VBoxManage controlvm
vmname setscreenlayout
command. On guest operating systems with X11/Wayland desktops
this is put into effect by either of the following two services:
VBoxClient --vmsvga
VBoxDRMClient
The following are some details about guest screen resolution control functionality:
On X11/Wayland desktops the resizing service is started
during desktop session initialization, that is desktop
login. On X11 desktops VBoxClient --vmsvga
handles screen topology through the RandR extension. On
Wayland clients VBoxDRMClient is used. The
decision is made automatically at each desktop session
start.
On 32-bit guest operating systems VBoxDRMClient is always used, in order to work around bugs.
Since the monitor topology control services are initialized
during the desktop session start, it is impossible to
control the monitor resolution of display managers such as
GDM or LightDM. This default behavior can be changed by
setting the guest property
/VirtualBox/GuestAdd/DRMResize of the
virtual machine to any value. See
Section 4.7, “Guest Properties” for details of how to
update guest properties. When this guest property is set
then VBoxDRMClient is started during the
guest OS boot and stays active all the time, for both the
display manager login screen and the desktop session.
VBoxDRMClient is not able to handle
arbitrary guest monitor topologies. Specifically, disabling a
guest monitor that is not the last one invalidates the monitor
topology due to limitations in the
vmwgfx.ko Linux kernel module. For example,
when the guest is configured to have four monitors it is not
recommended to disable the second or third monitor.