QEMU Barrier Client

Generally, mouse and keyboard are grabbed through the QEMU video interface emulation.

But when we want to use a video graphic adapter via a PCI passthrough there is no way to provide the keyboard and mouse inputs to the VM except by plugging a second set of mouse and keyboard to the host or by installing a KVM software in the guest OS.

The QEMU Barrier client avoids this by implementing directly the Barrier protocol into QEMU.

Barrier is a KVM (Keyboard-Video-Mouse) software forked from Symless’s synergy 1.9 codebase.

This protocol is enabled by adding an input-barrier object to QEMU.


input-barrier,id=<object-id>,name=<guest display name>
[,server=<barrier server address>][,port=<barrier server port>]

The object can be added on the QEMU command line, for instance with:

-object input-barrier,id=barrier0,name=VM-1

where VM-1 is the name the display configured in the Barrier server on the host providing the mouse and the keyboard events.

by default <barrier server address> is localhost, <port> is 24800, <x-origin> and <y-origin> are set to 0, <width> and <height> to 1920 and 1080.

If the Barrier server is stopped QEMU needs to be reconnected manually, by removing and re-adding the input-barrier object, for instance with the help of the HMP monitor:

(qemu) object_del barrier0
(qemu) object_add input-barrier,id=barrier0,name=VM-1