Select cpu model from a list of cpu models

https://blueprints.launchpad.net/nova/+spec/cpu-model-selection

Problem description

In the libvirt virt driver, currently we use cpu_model in nova.conf (when cpu_mode is set to custom) to specify the CPU model the instance should use on this host. This could have implications on availability of compute nodes for live migration if you ended up with an advanced CPU model when all you really cared about was an older feature flag.

Use Cases

As a user, I would like to boot an instance on a host supporting specific CPU features and for the instance to be live-migratable to as many other hosts as possible that also support the instance.

Proposed change

Replace cpu_model with cpu_models which is an ordered list of CPU models the host supports. It is expected that the list is ordered so that the more common and less advanced CPU models are listed earlier. The reported cpu feature traits will be the union of features of all the cpu models.

End users specify CPU features they required through traits 1. If the cpu_mode is set to custom, libvirt driver will select the first CPU model in the cpu_models list (combined with cpu_model_extra_flags if it is specified) that can provide the required feature traits. This would make it more likely that the instance could be live-migrated later on. If no CPU feature traits are specified then the instance will be configured with the first CPU model in the list.

For example, if the end user specifies CPU features avx and avx2 as following:

openstack flavor set 1 --property trait:HW_CPU_X86_AVX=required --property trait:HW_CPU_X86_AVX2=required

and cpu_models is configured like this:

[libvirt]
cpu_mode = custom
cpu_models = Conroe,Penryn,Nehalem,Westmere,SandyBridge,IvyBridge,Haswell,Broadwell,Skylake-Client,Skylake-Server

then Haswell, the first cpu model supporting both avx and avx2 will be chosen by libvirt.

If cpu_model_extra_flags is specified, it should be checked against each cpu model in the cpu_models list using host.compare_cpu() to make sure it is compatible with all models in the cpu_models list. Any incompatibility should prevent compute host from starting and user needs to correct the configuration.

If both cpu_models and cpu_model are set, cpu_model will be ignored.

Alternatives

None

Data model impact

None

REST API impact

None

Security impact

None

Notifications impact

None

Other end user impact

None

Performance Impact

Negligible.

Other deployer impact

Add some information in the config option help text indicating that the operator should be careful to only specify models which can be fully supported in hardware. If they specify models with CPU features that are emulated by qemu it could result in performance degredation.

Developer impact

None

Upgrade impact

The operator needs to set the config option appropriately after an upgrade. If cpu_models is not set it should default to the value of cpu_model.

Implementation

Assignee(s)

TBD

Work Items

  • Conf: define [libvirt]cpu_models. [libvirt]cpu_model is deprecated.

  • Virt driver changes.

  • Add/modify unit tests.

Dependencies

None

Testing

Will add unit tests.

Documentation Impact

Update release note for introducing [libvirt]cpu_models.

History

Revisions

Release Name

Description

Stein

Introduced