libvirt: Migrate a VM from qemu:///session to qemu:///system

In recent versions of the libvirt virtualisation libraries, you to create and manage virtual machines as a regular user, using the qemu:///session connection.

This is great, but the networking is quite limited. I found that machines defined in Gnome Boxes could not speak to each-other, and that libvirt commands for networking were unavailable.

For this reason, I’ve written this quick guide for booting up an existing same VM image under the qemu:///system instance, which is faster than re-installing the machine. Unlike most sorts of migrations, this leaves the disk image at the same location on the same host machine.

There’s many different ways to do VM’s in Linux. This setup will be useful only if you use libvirt/kvm using qcow2 images on Debian. As always, consider doing a backup before trying new things.

Configurations

First, find your virtual machine in virsh, and dump its configuration to a text file in your home directory, as a regular user.

$ virsh list --all
 Id    Name                           State
----------------------------------------------------
 -     foo-machine                    shut off
$ virsh dumpxml foo-machine > foo-machine.xml

Now remove the VM definition from your user:

$ virsh undefine foo-machine
Domain foo-machine has been undefined

Import the definitions into virsh as the root user:

$ sudo virsh define foo-machine.xml 
Domain foo-machin defined from foo-machine.xml

Attempt to start the new VM definition. Depending on where the disk image is, expect an error.

$ sudo virsh start foo-machine

Disk images

The disk image needs to be accessible to the libvirt-qemu user. There’s two basic ways to achieve this: Re-permission the directories above it, or move it.

I chose to just re-permission it, since it’s not an issue to have world-readable directories on this particular box:

$ cat foo-machine.xml | grep source
      <source file='/home/example/.local/share/gnome-boxes/images/foo-machine'/>

This one-liner outputs the commands to run to make a directory work/navigable:

$ dir=`pwd`; while [ "$dir" != "/" ]; do echo "chmod o+x,g+x \"$dir\""; dir=`dirname $dir`; done
chmod o+x,g+x "/home/example/.local/share/gnome-boxes/images"
chmod o+x,g+x "/home/example/.local/share/gnome-boxes"
chmod o+x,g+x "/home/example/.local/share"
chmod o+x,g+x "/home/example/.local"
chmod o+x,g+x "/home/example"
chmod o+x,g+x "/home"

And the user account needs to be able to write as well:

$ sudo chown libvirt-qemu /home/example/.local/share/gnome-boxes/images/foo-machine

Once you have the permissions right, the VM should start, using the same command as before:

$ sudo virsh start foo-machine

More importantly, you can now hook up virt-manager and view your machine on qemu:///system, allowing you to configure the VM with any network settings you need.