Comment 29 for bug 1828495

The series was merged in 3a1acf5d47295d22ffdae0982a2fd808b802a7da as a prep to qemu 4.1.
But the changes are rather invasive and after a review I think for the SRU we will not add them.

For example the changes around:
  "model runnability guarantees won't apply to unversioned CPU models anymore"
seems non backportable to me.
I agree it is the right path going forward to keep the proliferation of CPU models under control and provide something like a "moving head" weakening the runability guarantees that can be regained by fetching the exact versions via alias-of.

But none of the old systems management stacks will be able to do so.
Yes there is [2] but I'm still scared in an SRU context to add that when it is a valid (more effort but working) option to add model+flags for the time being.

And in addition the code for making the models versionable will be another great set of backports with potential backport flaws added.

Hence at least for now my decision would be:
- make the features available at least to the last LTS (Bionic) which will reach
  LTS-1 for many (to admit not all) users via Ubuntu cloud archive
- do not backport the versioned CPU model changes (too much risk / incompatibilities)
  - pro: no changes to the higher virt stack needed
  - pro: less regression risk for the SRUs
  - pro: the new security features will be available
  - con: security features must be enabled individually via feature flags

@Rafael that should match what you had prepared in your MPs already.
I'll create a PPA with your any my changes for general regression tests.

@Quaxlan - for the versioned CPUs an all that belongs to that context I'd ask you to file a new bug. I expect that will be in qemu 4.1 (just entered rc0) and a later (yet to be defined) libvirt version. Once all of that has properly landed we can consider if we will pull it back into Eoan (as long as it isn't released and under the SRU policy) OR if we wait for these new features to properly arrive in the next Ubuntu release being 20.04. Once you happen to know which libvirt version will have the appropriate changes to properly tolerate and exploit what was added to qemu via [1] (version and commits if you have both) let us know there.

[1]: https://<email address hidden>/msg627282.html
[2]: https://<email address hidden>/msg628326.html