[libvirt] [PATCH 00/22] qemu: Store default CPU in domain XML

[Andrea Bolognani]

On Thu, 2019-10-03 at 16:00 +0200, Jiri Denemark wrote:
> When starting a domain without a CPU model specified in the domain XML,
> QEMU will choose a default one. Which is fine unless the domain gets
> migrated to another host because libvirt doesn't perform any CPU ABI
> checks and the virtual CPU provided by QEMU on the destination host can
> differ from the one on the source host.
> 
> With QEMU 4.2.0 we can probe for the default CPU model used by QEMU for
> a particular machine type and store it in the domain XML. This way the
> chosen CPU model is more visible to users and libvirt will make sure
> the guest will see the exact same CPU after migration.
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=1598151
> https://bugzilla.redhat.com/show_bug.cgi?id=1598162

Unfortunately this will break all ppc64 guests that have been defined
without specifying a CPU model, because it will result in something
like

  <cpu mode='custom' match='exact' check='partial'>
    <model fallback='allow'>power8</model>
  </cpu>

being added to the domain XML, which in turn at runtime will cause

  error: internal error: Unknown CPU model power8

For a while, QEMU has been reporting (and accepting) CPU models for
ppc64 guests as lowercase strings, but historically they were all
uppercase and our CPU map, which the user-provided CPU model is
validated against, still contains those variants.

For the record, our documentation[1] recommends using something like

  <cpu mode='host-model'>
    <model>power8</model>
  </cpu>

to obtain a POWER8 CPU that can be safely migrated back and forth
between POWER8 and POWER9 machines, but we can't assume everyone is
following that recommendation.


[1] https://libvirt.org/formatdomain.html#elementsCPU
-- 
Andrea Bolognani / Red Hat / Virtualization