libvirt: use 'host-passthrough' as default on AArch64

[Marcin Juszkiewicz]

At Linaro we have several instances of OpenStack running. Some are public, some private. Some at Newton, some at Pike and now some on Queens.

And always we have to set cpu_mode = 'host-passthrough' in [libvirt] section of nova.conf file.

As we use Kolla to build and deploy Queens I added workaround there which sets cpu_mode in proper way for AArch64. But this is workaround...

So I looked at code of Nova and created some quick hack: https://review.openstack.org/#/c/530965/

It works for us but may not be proper way of doing it.

We use Debian 'stretch' as host system with libvirt 3.10.0 backported from 'testing'.

[Marcin Juszkiewicz (hrw)]

snippet from nova-compute.log

[Marcin Juszkiewicz (hrw)]

https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1673467 has long description of issue

[Marcin Juszkiewicz (hrw)]

Reported at libvirt upstream: https://bugzilla.redhat.com/show_bug.cgi?id=1531076

[Marcin Juszkiewicz (hrw)]

https://bugzilla.redhat.com/show_bug.cgi?id=1430987 also has interesting information on this issue.

[Marcin Juszkiewicz (hrw)]

With cpu_mode = custom + cpu_model = cortex-a53 (lowest nominator in AArch64) Nova errors out.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/530965
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=8bc7b950b7c0a3c80cdd120fe4df97c14848c344
Submitter: Zuul
Branch: master

commit 8bc7b950b7c0a3c80cdd120fe4df97c14848c344
Author: Marcin Juszkiewicz <email address hidden>
Date: Wed Jan 3 17:44:27 2018 +0100

    libvirt: use 'host-passthrough' as default on AArch64

    Nova is assuming 'host-model' for KVM/QEMU setup.

    On AArch64 it results in "libvirtError: unsupported configuration: CPU
    mode 'host-model' for aarch64 kvm domain on aarch64 host is not
    supported by hypervisor" message.

    AArch64 lacks 'host-model' support because neither libvirt nor QEMU
    are able to tell what the host CPU model exactly is. And there is no
    CPU description code for ARM(64) at this point.

    So instead we fallback to 'host-passthrough' to get VM instances
    running. This will completely break live migration, *unless* all the
    Compute nodes (running libvirtd) have *identical* CPUs.

    Small summary: https://marcin.juszkiewicz.com.pl/2018/01/04/today-i-was-fighting-with-nova-no-idea-who-won/

    Closes-bug: #1741230

    Co-authored-by: Kevin Zhao <email address hidden>
    Co-authored-by: Marcin Juszkiewicz <email address hidden>

    Change-Id: Iafb5f1790d68489db73b9f0549333108c6426a00

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: stable/pike
Review: https://review.openstack.org/532504

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/nova 17.0.0.0b3 development milestone.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/532504
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=50d8a07b3fa2fd8ee605691ee4f6148dd3a18a5f
Submitter: Zuul
Branch: stable/pike

commit 50d8a07b3fa2fd8ee605691ee4f6148dd3a18a5f
Author: Marcin Juszkiewicz <email address hidden>
Date: Wed Jan 3 17:44:27 2018 +0100

    libvirt: use 'host-passthrough' as default on AArch64

    Nova is assuming 'host-model' for KVM/QEMU setup.

    On AArch64 it results in "libvirtError: unsupported configuration: CPU
    mode 'host-model' for aarch64 kvm domain on aarch64 host is not
    supported by hypervisor" message.

    AArch64 lacks 'host-model' support because neither libvirt nor QEMU
    are able to tell what the host CPU model exactly is. And there is no
    CPU description code for ARM(64) at this point.

    So instead we fallback to 'host-passthrough' to get VM instances
    running. This will completely break live migration, *unless* all the
    Compute nodes (running libvirtd) have *identical* CPUs.

    Small summary: https://marcin.juszkiewicz.com.pl/2018/01/04/today-i-was-fighting-with-nova-no-idea-who-won/

    Closes-bug: #1741230

    Co-authored-by: Kevin Zhao <email address hidden>
    Co-authored-by: Marcin Juszkiewicz <email address hidden>

    Change-Id: Iafb5f1790d68489db73b9f0549333108c6426a00
    (cherry-picked from commit 8bc7b950b7c0a3c80cdd120fe4df97c14848c344)

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/nova 16.1.0 release.