There’s a feature in kvm which allows you to multiplex your serial and monitor port over a single connection. This is another one of the many features inherited from the qemu tree and was added around kvm-57 which is probably not well known. I think this is one feature that may not be used much but I think it’s still good to know thy options.

The Syntax

The syntax to enable the multiplexing of your monitor port is as follows:

mon:dev_string

Your monitor is accessed using a key sequence of Ctrl-A then c. This key sequence will toggle between your monitor and serial port. For example, if you wanted to multiplex your monitor onto a telnet server listening on port 4444, you would start your kvm guest with the following option.

qemu-system-x86_64  -hda  windows.img  -serial  mon:telnet::4445,server,nowait 

For more on accessing your monitor via telnet, see this post here on accessing your monitor across the network.

An example

For a concrete example, start your kvm guest using a syntax similar to the one described in the previous section, for example.

qemu-system-x86_64  -hda  windows.img  -m 400 -serial  mon:telnet::4445,server,nowait

Now type the following in a terminal window on your kvm host

telnet localhost 4445

When you do this you will get an output similar to the following.

Note that you will not get any response from your keyboard in this session. This is because when you initially connect, you’ll be connecting to the serial port via the telnet server. Now type Ctl-a then c and hit enter from within the same session. You’ll now see your kvm monitor come up as shown below.

Using this key sequence you can toggle between the serial and monitor sessions using your telnet server. To break out of the telnet server, issue the Ctrl-] and you’ll get the telnet prompt as shown below from which you can then quit.

The multiplexing your monitor and serial port is another one of those features that is probably not very popular but you never know when you might have to pull it out of your bag of tricks. Can you think of an application of this feature?

Share your thoughts and drop a comment if you can think of how you might use this feature.