unable to compose VM from Ubuntu 22.04 pod

[Christian]

1. Installed a Ubuntu 22.04 physical server via MAAS adding it as KVM host
2. Configured a volume group named vg_os and added it as a KVM logical pool storage so it can be used to create my VM disk devices in it

<pool type='logical'>
  <name>vg_os</name>
  <uuid>xxxxx</uuid>
  <capacity unit='bytes'>3356915400704</capacity>
  <allocation unit='bytes'>68719476736</allocation>
  <available unit='bytes'>3288195923968</available>
  <source>
    <name>vg_os</name>
    <format type='lvm2'/>
  </source>
  <target>
    <path>/dev/vg_os</path>
  </target>
</pool>

3. Refreshed pod from console and can see my vg_os available in MAAS
4. When composing (via API or console):
   - i can see a logical being created in vg_os and the associate volume (virsh vol-list vg_os)
   - domain creation suceeds but later maas tries to attach volumes and fails

Pod unable to compose machine: Unable to compose machine because: Failed talking to pod: Unable to compose test22: Virsh command ['start', '--paused', 'test22'] failed: Failed to start domain test22 error: internal error: qemu unexpectedly closed the monitor: 2022-12-12T14:15:43.356030Z qemu-system-x86_64: -blockdev {"driver":"file","filename":"/dev/vg_os/7ea03407-bdec-4803-8255-d9acc3a7a99c","node-name":"libvirt-1-storage","auto-read-only":true,"discard":"unmap"}: 'file' driver requires '/dev/vg_os/7ea03407-bdec-4803-8255-d9acc3a7a99c' to be a regular file

This works fine if KVM is running Ubuntu 20.04.

I found that QEMU introduced a breaking change in QEMU 6.0 as stated
in https://gitlab.com/libvirt/libvirt/-/issues/212

"This seems to be a breaking change introduced in QEMU 6.0, as changelog says the following: The file block driver no longer permits use with block devices. Downgrading to QEMU 5.2 allows for domain to successfully boot up.
Judging from current documentation, in order to pass a block device, you have to use raw block driver instead."

Looking at /snap/maas/current/lib/python3.8/site-packages/provisioningserver/drivers/pod/virsh.py (from snap 3.2.6-12016-g.19812b4da - 3.2-stable), i can see maas enforces file driver usage which was working for earlier version of QEMU:

 def attach_local_volume(self, domain, pool, volume, device):
        """Attach `volume` in `pool` to `domain` as `device`."""
        vol_path = self.get_volume_path(pool, volume)
        serial = os.path.basename(vol_path)
        self.run(
            [
                "attach-disk",
                domain,
                vol_path,
                device,
                "--targetbus",
                "virtio",
                "--sourcetype",
                "file",
                "--config",
                "--serial",
                serial,
            ]
        )

[Adam Collard (adam-collard)]

Let's figure out how we need to adapt the QEMU commands to work with QEMU and libvirt in Jammy