Comment 2 for bug 1752026

Hi,
Thanks Andrew - I agree in general.
The following is based on the assumption that the linked discussion (kernel change) is not upstream yet.
Any clarification on that will help thou.

OTOH I want to start the discussion on the options we have early on.

I have seen the pseries-2.12 changes in the qemu 2.11.1 stable release (didn't like them).
Especially for things like those that you mentioned "... I guess by that time pseries-2.12 to be changed to cap-htm=on by default" is the reason I can't pick a 2.12 type until 2.12 is final and released.

We never can allow a case where pseries-2.12 != pseries-2.12 (for migrations and such).

So at the moment the default pseries-bionic is based on 2.11 being the usual default of qemu 2.11 and the one that is meant to be (and stay) stable.

So on the proposed change "3. Immediate fix can be Canonical defaults their machine type(pseries-bioic) to pseries-2.12" I'm reluctant to do so, as:
  - only pseries would be 2.12
  - there is a high chance we end up with 2.12 != 2.12 down the road

Suggestion #1:
If you (=IBM as the authoritative entity for Power) decide that you want htm to be off in the 2.11 machine type in the Ubuntu 18.04 (=Bionic) release we can do that (as Bionic is not released yet we can still change it).

But that would stay for the entire time of the Bionic release.
So pseries-bionic (the default) => pseries-2.11 (+htm off) will be the default until year 2023

Once (if) the host kernel at some point supports htm properly you can surely change the 2.12 type upstream, we would pick that up and later releases will default to a htm on case then.
Also people could run Bionic (which sets htm=off by default then) and run if needed with a htm=on override.

But even all that would mean that e.g. a new qemu from the Ubuntu cloud archive in a year, would fail the same on a 18.04 base kernel.

The real fix is to get that host support upstream (kernel) and get it in the Ubuntu kernel prior to the release of 18.04 - is that a realistic timeline, when do you expect this gets upstream?

I hope those clarifications helped to see why I think just choosing the 2.12 type is no option.

Thereby Counter-proposing:
1. in qemu we can make default pseries-bionic => pseries-2.11 (+htm off) if you want.
   That makes things safe to use for now, but OTOH htm an opt-in feature on Ubuntu 18.04
   That would stay that way for the support time of 18.04
OR
2. You get the kernel fix upstream asap and Ubuntu integrates before release of 18.04
   Then qemu/libvirt as is would work on P9 DD 2.2+
   (until that happens you can test with an override to set htm=off)

But any decision between #1/#2 depends very much on:
- the expected timeline of your kernel changes
- your preferenc in regard to the htm feature
So it is up to you to clarify on that as Andrew pointed out.