Comment 14 for bug 1959547

Hi Harald, well, this is an Ubuntu 22.04 feature (like the Launchpad ticket title indicated), hence need to be used/tested on a Ubuntu 22.04 codename 'jammy' system.
(You may upgrade your 20.04 / focal system directly to 22.04 / jammy using 'do-release-upgrade -d' - '-d' because jammy is still in development).

Once on a 22.04 / jammy system, things are relatively straight forward - let me go into the details:

First of all check what's available in the (jammy) archives, in case of this tickets, it's about the kernel (package called 'linux-generic'):
$ rmadison --arch=s390x linux-generic | grep $(lsb_release -cs)
 linux-generic | 5.15.0.25.27 | jammy | s390x
Well, this tells us that there is only one kernel available,
and that one is in jammy(-release) and there is no kernel in jammy-proposed right now.

Means the kernel is already rolled out and you don't need -proposed anymore.
So you can go with the kernel that you get by default if running jammy/22.04.

[
If there is another kernel in -proposed for testing, things look like in this mockup:
 linux-generic | 5.15.0.25.27 | jammy | s390x
 linux-generic | 5.15.0.26.30 | jammy-proposed | s390x
]
___

Anyway, just for the reason of completeness, here are the steps needed in case a kernel from
 -proposed should be installed and tested:

1) make sure your system is at the latest level:
sudo apt -y -q update && sudo apt -y -q full-upgrade

2) activate '-proposed' with:
sudo add-apt-repository -y "deb http://us.ports.ubuntu.com/ubuntu-ports/ $(lsb_release -sc)-proposed main universe"
(and update the package index with 'sudo apt -y -q update' if not already triggered automatically)

3) now check for the available versions that can be installed:
$ apt-cache policy linux-generic
linux-generic:
  Installed: 5.15.0.25.27
  Candidate: 5.15.0.26.30
  Version table:
...
And the version table shows you where the kernels are coming from (either main, proposed, updates, or security).

[Btw. ignore the right-most digit, since it's just the build/meta digit.
If there is a request to test 5.15.0-25.25, it usually points to the kernel source package,
and the binary kernel is (in this case) 5.15.0.25.27. So it's save to ignore '27'.]

4) update the kernel by either 'installing' the latest one that is available, like:
sudo apt install linux-generic
or explicitly install a specific version, like:
sudo apt install linux-generic=5.15.0.26.30

5) don't forget to reboot afterwards to activate the newly installed kernel.