After my last post about KVM, somebody emailed me, asking how I use KVM’s serial console in combination with screen. It’s a rather simple, but really usefull solution if you think tools like virtmanager are too much for you and you don’t like to use VNC. So this post will try to explain it to you.
For starters, you have to add the -nographic option to you KVM parameters. Next, make sure your virtual machine outputs it’s console to serial device 0. For example, start the Linux kernel with console=ttyS0 and make sure there is a getty process running on that same device. Now, whenever you start KVM in a GNU screen (with that -nographic parameter) you will see the dmesg scroll by and will end up with a login prompt. You can just keep this KVM running while detaching the GNU screen session (ctrl+a d) and you can re-attach by starting screen -x.
I use this on my server to start my two virtual machines. I made a screenrc like this:
# display a nice status bar on the bottom of the screen
hardstatus alwayslastline "%-Lw%50>%n%f* %t%{-}%+Lw%< %=[%c]"
vbell off
deflogin off
# use virtio for disks also!
screen -t production kvm -m 2048m -nographic -drive file=production.raw,if=virtio,boot=on -net nic,model=virtio -net tap,ifname=tap0,script=no,downscript=no -smp 2
screen -t development kvm -m 1024m -nographic -drive file=development.raw,if=virtio,boot=on -net nic,model=virtio -net tap,ifname=tap1,script=no,downscript=no

Whenever the server boots it runs the following command in my rc.local:

# start KVM
cd /srv/
su frank - -c 'screen -c screenrc -dm'

So the server starts the GNU screen and the GNU screen start my virtual machines. If I can’t login via ssh in one of my virtual machines, I just connect to the host and attach to the screen to debug the problem.