Select cpu model from a list of cpu models¶
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.
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.
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 . If the
cpu_mode is set to custom, libvirt driver will select the first CPU model
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
cpu_models is configured like this:
[libvirt] cpu_mode = custom cpu_models = Conroe,Penryn,Nehalem,Westmere,SandyBridge,IvyBridge,Haswell,Broadwell,Skylake-Client,Skylake-Server
Haswell, the first cpu model supporting both avx and avx2 will be
chosen by libvirt.
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
cpu_model are set,
cpu_model will be
Data model impact¶
REST API impact¶
Other end user impact¶
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.
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.
Virt driver changes.
Add/modify unit tests.
Will add unit tests.
Update release note for introducing [libvirt]cpu_models.