[RFC DOCUMENT 03/12] kubevirt-and-kvm: Add Hotplug page

[Andrea Bolognani <abologna@redhat.com>]

https://gitlab.com/abologna/kubevirt-and-kvm/-/blob/master/Hotplug.md

# Hotplug

In Kubernetes, pods are defined to be immutable, so it's not possible
to perform hotplug of devices in the same way as with the traditional
virtualization stack.

This limitation is a result of KubeVirt's guiding principle of
integrating with Kubernetes as much as possible and making VMs appear
the same as containers from the user's point of view.

There have been several attempts at lifting this restriction in
Kubernetes over the years, but they have all been unsuccessful so
far.

## Existing hotplug support

When working with containers, changing the amount of resources
associated with a pod will result in it being destroyed and a new
pod with the updated resource allocation being created in its place.

This works fine for containers, which are designed to be clonable and
disposable, but when it comes to VMs they usually can't be destroyed
on a whim and running multiple instances in parallel is generally not
wise even when possible.

## Possible workarounds

Until a proper hotplug API makes its way into Kubernetes, one
possible way to implement hotplug could be to perform migration to a
container compliant with the new allocation request, and only then
perform the QEMU-level hotplug operation.