CPU and RAM Hot-Plugging

Now these operating systems support hot-swappable cpu and memory, and customers have a strong demand for these.

At the same time I think that the reserve resources to support the hot plug that is bad way, because it is a waste of physical resources, it is unreasonable, And directly let vm resize to the bigest What is the difference?. So reserved resources to vm expansion is very unreasonable.

I have two ideas:
1, api --> scheduler -- > if resource is insufficient --> prompts for hot migration --> then horizon calls hot-plug interfaces to implement live-resize
2, api -> scheduler -> resources enough -> call libvirt to resize