[libvirt] [PATCH] qemu: ensure "pc" machine is always used as default if available

[Daniel P. Berrangé]

On Mon, Aug 06, 2018 at 07:04:32AM +0200, Christian Ehrhardt wrote:
> On Sun, Aug 5, 2018 at 10:53 AM Daniel P. Berrangé <berrange at redhat.com>
> wrote:
> 
> > It is increasingly likely that some distro is going to change the
> > default "x86" machine type in QEMU from "pc" to "q35". This will
> > certainly break existing applications which write their XML on the
> > assumption that its using a "pc" machine by default. For example they'll
> > lack a IDE CDROM and get PCI-X instad of PCI which changes the topology
> > radically.
> >
> 
> Hi,
> Distributions carry custom manchine types for quite a while now to
> encapsulate differences of backports and similar [1][2].
> That said, in all those "pc" isn't the default for a long time and it was
> actually quite comfortable to be able to switch the default from qemu where
> changes take place and not having to touch libvirt in that regard.
> 
> I agree that pc->q35 is a "bigger" switch in terms of default behavior than
> default enabling a single new feature out of the i440fx scope, therefore I
> understand the preference of libvirt to preserve the old default.
> But with the change proposed here the "default" machine type of qemu looses
> a lot of its (benficial) implications that that so far.
> 
> Ideally we'd not switch just back to "pc" here, but to something qemu can
> mark like a certain i400fx-abcd type.
> I know qemu can only have one default, and it is about to change - which
> from a pure qemu's POV makes sense.
> We always carried an alias "ubuntu" that changed with each release and
> pointed to the preferred default type, just like "pc" pointed to the most
> recent pc-i440 type.
> So maybe I just modify the patch proposed here for Ubuntu to pick "ubuntu"
> instead of "pc" - no sure yet?
> 
> Or we can carry a revert of the patch discussed here (but then would get
> all the pain of old working XML tools failing, doesn't sound like a good
> option),
> but I wanted to know if there were some more complex considerations have
> been done already how those cases are supposed to be handled?
> 
> [1]: https://bugzilla.redhat.com/show_bug.cgi?id=895959
> [2]: https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/1294823

There's multiple things at play. First all the canonical machine type
names which are versioned

  pc-i440fx-2.5.0
  pc-i440fx-2.6.0
  pc-i440fx-2.7.0
  ...
  pc-i440fx-2.10.0
  pc-q35-2.5.0
  pc-q35-2.6.0
  pc-q35-2.7.0
  ...
  pc-q35-2.10.0

Then there are the convenient aliases mapping to the most recent versioned
machine type

  pc  -> pc-i440fx-2.10.0
  q35 -> pc-q35-2.10.0

Finally, the versioned machine type that is resolved by the "pc" alias
is listed as the default.

Even when distros ship custom machine types this has little to no impact
on applications. It generally just means the version number part of the
machine type changes. 'pc' still resolves to the most recent versioned
machine type, and is listed as the default.

The key thing is that applicatons (virt-install, virt-manager, oVirt,
OpenStack, etc) need to know whether they're using "pc" or "q35" when
building guest XML, as that difference is used to trigger different
code paths.

In looking at the code for various mgmt apps we see alot of patterns
like

   if machine != NULL & strstr(machine, "q35")
      ...write XML suitable for q35...
   else
      ...write XML suitable for pc...

IOW, the application is assuming that if the user hasn't requested an
explicit type, they'll get "i440fx" based "pc".

If QEMU changes its default so that the "q35" alias is marked as the
default, then this will break every single mgmt application that we
have looked at. This is exactly the kind of thing that libvirt promises
will not happen to mgmt apps, so we must guarantee that if no machine
type is listed in the XML, then app will always get the i440fx based
"pc" machine, and not "q35".

WRT your point about the "ubuntu" machine type. I think using such a
machine type name is not a desirable thing todo. No application that
I've looked at, nor libvirt itself, has logic to know whether "ubuntu"
machine type is based on i440fx or q35 chipsets. I expect that, by
luck, they'll mostly end up treating it as i440fx based, since most
apps do an explicit check for "q35" in the name and assume everything
else is i440fx.

So the general guidance we give is it that distros should honour the
QEMU machine type name prefixes "pc-q35-" and "pc-i440fx-", and only
ever change the version suffix if they want to add custom distro
specific variants. ie don't invent new prefixes, nor new aliases,
as no application will know what todo with those.

On that basis I'd ecourage you to look at whether you can phase out
the "ubuntu" machine type alias.

Regards,
Daniel
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