Re: [Qemu-devel] [PATCH v4] Add machine parameter qemu-kvm-migration for live migrate compatibility with qemu-kvm

[Paolo Bonzini]

> > > If you were just objecting to the fact that pc-1.0 was made to
> > > be an alias of either one or the other at compile time, simply
> > > drop the second patch of the v2 patchset.
> > 
> > I was objecting to making pc-1.0 special.  There's nothing special in
> > pc-1.0, other machine types also had differences between qemu-kvm and
> > qemu.  And I do not think that upstream has any reason to make pc-1.0
> > special.
> 
> OK, so in v5, pc-1.0 is unchanged, and not made special. A new machine
> type is added which allows import from something (unfortunately) called
> pc-1.0 in something in qemu's past, as well as some distributions.

The very fact that a clone of pc-1.0 is added, but not pc-0.15 or
pc-1.2, makes pc-1.0 special.

My proposal has always been that _all_ PC machines should have a property.
This could be done, for example, by starting with Eduardo's patches that
make a class hierarchy of PC machine types.

pc-1.0 is special in Ubuntu world, because it was in an LTS release.  This
is why this patch should be added to Ubuntu, not to upstream.  pc-1.0 is
not necessarily special in other distros.  And some of them (Fedora for
example) are _always_ treating their machine types as the qemu-kvm variants,
and have been doing so for
a couple years.

I very strongly object to including a patch upstream that is tailored
after a particular downstream, and just because that particular downstream
has failed in doing the integration testing that it was supposed to do.

> > So, if Ubuntu is okay with breaking pc-1.0 migration from 14.04-old to
> > 14.04-new, the right thing to do is simply that Ubuntu makes its pc-1.0
> > machine type the qemu-kvm one.  No new machine types, no aliases, no
> > anything.
> 
> That would not allow Ubuntu (or Suse - similarly affected I think)
> to import pc-1.0 VMs from things actually running pc-1.0, and would
> mean that newly created pc-1.0 VMs would be 'wrong', perpetuating
> the problem.

The problem _is_ perpetual.  "pc-1.0" makes no sense without a context
(the distro).  This is why it is not a problem for Fedora to always use
the qemu-kvm variants.

"pc-1.0" will always be the qemu-kvm variant in Ubuntu context (apart
from the 6 months passed since the release of 14.04), because most
"pc-1.0" machines will have been created with the qemu-kvm package in
Ubuntu 12.04.

It can be confusing---to avoid confusion, RHEL drops the upstream machine
types apart from the "pc" generic type and adopts a completely different
nomenclature.

In fact as time passes the benefit of 12.04->14.04 migration becomes
smaller and smaller and, by now, it should have gone almost completely.
It's much simpler to ignore the problem at this point.  For the next
Ubuntu LTS, Canonical should include migration compatibility in their
test plans.  And start well in advance, for it may only take one
person to fix the bugs, but it takes months of testing to find them.

Paolo