Nova's use of libvirt's compareCPU() API served its purpose
over the years, but its design limitations break live migration in
subtle ways. For example, the compareCPU() API compares against the
host physical CPUID. Some of the features from this CPUID aren not
exposed by KVM, and then there are some features that KVM emulates that
are not in the host CPUID. The latter can cause bogus live migration
failures.
With QEMU >=2.9 and libvirt >= 4.4.0, libvirt will do the right thing in
terms of CPU compatibility checks on the destination host during live
migration. Nova satisfies these minimum version requirements by a good
margin. So, provide a workaround to skip the CPU comparison check on
the destination host before migrating a guest, and let libvirt handle it
correctly. This workaround will be removed once Nova replaces the older
libvirt APIs with their newer and improved counterparts[1][2].
- - -
Note that Nova's libvirt driver calls compareCPU() in another method,
_check_cpu_compatibility(); I did not remove its usage yet. As it needs
more careful combing of the code, and then:
- where possible, remove the usage of compareCPU() altogether, and
rely on libvirt doing the right thing under the hood; or
- where Nova _must_ do the CPU comparison checks, switch to the better
libvirt CPU APIs -- baselineHypervisorCPU() and
compareHypervisorCPU() -- that are described here[1]. This is work
in progress[2].
[1] https://opendev.org/openstack/nova-specs/commit/70811da221035044e27
[2] https://review.opendev.org/q/topic:bp%252Fcpu-selection-with-hypervisor-consideration
Change-Id: I444991584118a969e9ea04d352821b07ec0ba88d
Closes-Bug: #1913716
Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
Signed-off-by: Balazs Gibizer <bgibizer@redhat.com>
(cherry picked from commit 267a40663c)
277f88e387