arm64: Migration pre-check error: CPU doesn't have compatibility.

My initial hunch was something was wonky with the cpu configuration in nova; however:

        if config('cpu-mode'):
            ctxt['cpu_mode'] = config('cpu-mode')
        elif ctxt['arch'] in ('ppc64el', 'ppc64le', 'aarch64'):
            ctxt['cpu_mode'] = 'host-passthrough'

reveals that for aarch64, we actually do a direct host-passthrough so for identical machines the CPU features should be a 100% match; I'm wondering whether something is not exposed correctly in libvirt on aarch64 which nova is looking for for the pre-flight checks.

"Missing CPU model name" is bubbling up from libvirt's pre-migration checks.

Dann - does this work in releases prior to pike? Reading the nova code, not a huge amount has changed in this area, so I'm wondering whether this is a bug somewhere in the virt stack @ pike.

James - I never tried it with earlier releases. However, prior to pike, there was the additional reason that QEMU didn't yet support save/restore of these guests at all. See LP: #1710019.

Live migration test: https://github.com/openstack/tempest/blob/master/tempest/api/compute/admin/test_live_migration.py

The issue is that the XML does not contain the CPU model type, following is what is actually being passed.

https://github.com/openstack/nova/blob/master/nova/virt/libvirt/driver.py#L6337

This is the actual XML provided on an arm64 deploy (can be confirmed using "virsh cpu-compare file.xml" -

<cpu>
  <arch>aarch64</arch>
  <topology sockets="1" cores="48" threads="1"/>
</cpu>

more logs: https://pastebin.canonical.com/209211/

If you insert <model>anything</model> then cpu-compare works, e.g.

https://pastebin.canonical.com/209216/

However, https://github.com/openstack/nova/blob/f94c728bdddc9d1d3e3ae5f8f34b0111b9168bc9/releasenotes/notes/aarch64-set-proper-cpu-mode-8455bad7d69dc6fd.yaml indicates that no host-model will be passed.

While I agree for cpu-compare needing a cpu to compare.
I happen to migrate guests like http://paste.ubuntu.com/26530819/ all the time which have none as well.

So there must be way to pass it even with no cpu model is defined.

aarch64 does not have cpu_model = "host-model" funtionality which would be required for it.

There is no sane way of knowing how cpu1 compare to cpu2 yet.

Since Aarch64 architectures doesn't provide CPU models, but since we compare CPU models when migrating, that's why we have that issue.

I think tacking that bug requires more than a single bugfix and is probably a feature, but I leave that bug open for records, and we'll see how to address that in the future.

Anyway, I feel we should amend https://docs.openstack.org/nova/latest/user/feature-classification.html and https://docs.openstack.org/nova/latest/user/support-matrix.html to tell what ARM64 can support.