Enable vSphere Nested Virtualization

[Adam Dyess]

The juju vsphere provider can launch VMs by first building with `buildConfigSpec`[0] and I wondered how difficult it would be to expose another api flag `NestedVMEnabled`[1] in vsphere before booting it.

This flag would allow vsphere instances to be started which could also host VMs by passing `/dev/kvm` through to the host machine.

[0] https://github.com/juju/juju/blob/3.3/provider/vsphere/internal/vsphereclient/client.go#L822-L826
[1] https://github.com/vmware/govmomi/blob/2fab8d5337155ac7e6162b4cddd0b87fbfdf7515/vim25/types/helpers.go#L152C3-L152C18

[Adam Dyess (addyess)]

Proposal of a new machine constraint:

   nested-virtualization=[true|false]

My expectation is that one would want to add machines in a model, some with this enabled, and others without. This would facilitate something like Charmed Kubernetes with kubevirt enabled. The worker machines in the model could be started with a constraint like `nested-virtualization=true' which would allow `/dev/kvm` from the metal to be passed through and used to start VMs inside the workers without adding `/dev/kvm` the machines that host kubernetes-control-plane, etcd, vault, kubeapi-load-balancer and other Charmed Kubernetes machines

[Adam Dyess (addyess)]

This is pretty poor - and i'm fine with it not being accepted. It just the idea

https://github.com/juju/juju/pull/16330

[Simon Richardson (simonrichardson)]

I believe the correct way to do this is via machine config (or even the system concept?). We shouldn't weaken the constraints modelling. This would require a bit work for the machine config to be put in place between all the layers (cmd -> apiserver -> providers).