Live migration and versioned machine types
-----------------------------------------

Today, TripleO explicitly specifes[1] versioned machine types -- to
ensure migration compatibility during upgrade windows.  E.g. to quote
TripleO's config for machine types:

    ...
    NovaHWMachineType:
      description: >
        To specify a default machine type per host architecture.
    default: 'x86_64=pc-i440fx-rhel7.6.0,aarch64=virt-rhel7.6.0,ppc64=pseries-rhel7.6.0,ppc64le=pseries-rhel7.6.0'
    type: string
    ...

Why?
----

Let's take a real-world example to make the point.  (I'm using the Red
Hat-based OpenStack, RHOS, as an example for ease of explanation.  I'm
sticking with "RHEL" here, otherwise the terminology gets muddied — the
versioned machine types reported by CentOS's QEMU binary have 'rhel' in
their names, because CentOS ships RHEL's QEMU binary as-is, minus the
branding.  You could replace "RHEL" with "CentOS", and the following
example would be true.) 

Back to the example:

  - Assume that a RHOS deployment is running on a RHEL 7.6 host, the
    Nova instances will be given a RHEL 7.6 (pc-i440fx-rhel7.6.0)
    machine type.

  - And if the same deployment gets additional compute nodes added in
    the future, which use a newer RHEL (e.g. RHEL-7.7), the guests
    launched on those new compute nodes will get a newer machine type
    (i.e. 'pc-i440fx-rhel7.7.0').

Here's is the problem: 

    It will now be _impossible_ to migrate a guest from a RHEL 7.7-based
    compute node to a RHEL 7.6-based — because RHEL 7.6 won't know about
    the RHEL 7.7's machine type.

To deal with this trouble, you need to be able to force set an explicit
machine type across all compute nodes in a deployment (as shown above).
The machine type should match the latest machine type present at the
time each RHOS version is released.  I.e. since RHOS 14 is released
initially against RHEL-7.6 base OS, Nova is configured to always use the
'rhel-7.6.0' machine type regardless of whether the compute node has
upgrade to RHEL 7.7 or 7.8.

This is done in ``nova.conf`` by setting the
``[libvirt]hw_machine_type``. There needs to be one machine type listed
per architecture, to cover all supported. architectures on KVM.