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