Comment 2 for bug 1829868

Thanks Vladyslav for the bug report and the great analysis done already.

/self rant break
Oh how I hate machine types, it is sad that we still need to add them at all for some special cases and encapsulating changes. But among all of them being sort of "ok" the wily type clearly is the top on my personal hate list.
/end rant

---

The purpose to keep these old types is only to allow people to "migrate off" the older releases as long as they are still supported. Never the less sooner or later people are strongly encouraged [1]. Given your great analysis you know that already, but others might come by this bug.
It is important to be considered when deciding on if/what to change.

No one should ever start a "new" guest of a wily type on Bionic or later.
So I'm not too concerned about the delta we introduce to people having done that. The time this took to be found confirms that even migrations from Xenial with a Wily type are rare. We can not really differ between wily types coming from e.g. Xenial (or Trusty-Mitaka) hosts and wily type migrations coming from other Bionic systems (only a problem after a fix to this bug here and if they are on a different patch level).

So as mentioned the one real case for the wily type to still exist is to migrate off of older systems, and that use-case is broken. So I'm considering the thoughts above as a known, but less important than "the main use case" regression.

---

I'd want to avoid touching pc_compat_2_3 itself (would affect other types), but just switching from wily from pc_i440fx_2_4_machine_options to pc_i440fx_2_3_machine_options seems wrong as well as this would make it loos other 2.4 attributes it had (Have I mentioned to hate the wily type most for its broken definition).

Maybe we should define a HW_COMPAT_WILY being the fuse of HW_COMPAT_2_4 / HW_COMPAT_2_3 that actually matches the past.

Let me prep a change with that and we can give it a try

[1]: https://wiki.ubuntu.com/QemuKVMMigration