No live migration on aarch64

This isn't a 'bug' since it was never supported before anyway.

For us it is a regression as on Newton with libvirt 2.2.10 live migration from nova level on aarch64 worked fine.

On queens we have libvirt 3.8/3.10 and it does not on nova level but does on libvirt level.

OK removed the 'wishlist' tag if this used to work. FWIW nothing obvious jumps out at me for recent changes in the libvirt driver for the 'compare_cpu' code.

Okay, according to the BZ comment [0], it's expected that libvirt doesn't know how to detect the CPU model on AArch64. And in the past, it was wrongly being reported by libvirt, which was a bug in libvirt.

So, it seems since they fixed the bug, it breaks in nova now (because nova expects CPU model to be in the XML). When it was erroneously there, things worked. Now that it's gone, things don't work.

To fix it, I think we need to add something like this at [1]:

  # Libvirt doesn't know how to detect the host CPU model on AArch64.
  if cpu.arch == fields.Architecture.AARCH64:
      LOG.debug('Libvirt doesn't know how to detect the host CPU model on AArch64. Skipping CPU comparison')
      return

We need to double-check with someone like kashyap to confirm whether there's a more correct way to handle it other than bypassing it altogether.

[0] https://bugzilla.redhat.com/show_bug.cgi?id=1430987#c2
[1] https://github.com/openstack/nova/blob/8b6b65f/nova/virt/libvirt/driver.py#L6761

Marking it as High because it's a regression in Queens.

Fix proposed to branch: master
Review: https://review.openstack.org/589769

Lowering this to Medium as it's been regressed since Queens.

It is true that libvirt does not know how to detect host CPU model on AArch64, but even if it _wants_ to know, it cannot, because even the `/proc/cpuinfo` on AArch64 doesn't show anything interesting. There are lots of vendors making different AArch64 CPUs, and they are not easily comparable. They all differ in various ways. (This is also confirmed by Jiri Denemark of libvirt.)

So it doesn't make sense to do CPU compatibility check on AArch64. And the AArch64 folks themselves recommend that the way to run KVM guests on AArch64 is via 'host-passthrough'.

So with that in mind, I'd suggest the error to be something like:

  "Host CPU compatibility check does not make
   sense on AArch64; skip CPU comparison"

In theory those fields from /proc/cpuinfo could be used to at least know are we trying to migrate to same CPU:

X-Gene 1:

CPU implementer : 0x50
CPU architecture: 8
CPU variant : 0x0
CPU part : 0x000
CPU revision : 1

ThunderX:

CPU implementer : 0x43
CPU architecture: 8
CPU variant : 0x1
CPU part : 0x0a1
CPU revision : 0

ThunderX2:

CPU implementer : 0x43
CPU architecture: 8
CPU variant : 0x1
CPU part : 0x0af
CPU revision : 1

HiSilicon D05:

CPU implementer : 0x41
CPU architecture: 8
CPU variant : 0x0
CPU part : 0xd08
CPU revision : 2

Reviewed: https://review.opendev.org/589769
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=4bb54ae86981d186e41dcc325d171ab951beb7b6
Submitter: Zuul
Branch: master

commit 4bb54ae86981d186e41dcc325d171ab951beb7b6
Author: Kevin Zhao <email address hidden>
Date: Wed Aug 8 17:04:52 2018 +0800

    Skip cpu comparison on AArch64

    Host CPU compatibility check does not make sense on AArch64,
    this patch skips CPU comparison.

    Closes-bug: #1756118

    Change-Id: I0ef4b954b7f4ae65b6c0f96580c5f9472a2b873c
    Signed-off-by: Kevin Zhao <email address hidden>

Fix proposed to branch: stable/train
Review: https://review.opendev.org/699115

Fix proposed to branch: stable/stein
Review: https://review.opendev.org/699116

Fix proposed to branch: stable/rocky
Review: https://review.opendev.org/699117

Change abandoned by Lee Yarwood (<email address hidden>) on branch: stable/rocky
Review: https://review.opendev.org/699117

Change abandoned by Lee Yarwood (<email address hidden>) on branch: stable/stein
Review: https://review.opendev.org/699116

Change abandoned by Lee Yarwood (<email address hidden>) on branch: stable/train
Review: https://review.opendev.org/699115