This of course has implications on how you setup Docker on a Windows machine, each way having pros and cons.
[0] https://blogs.vmware.com/workstation/2020/01/vmware-workstat...
[1] https://docs.microsoft.com/en-us/virtualization/api/hypervis...
Hypervisors that need more capability are going to have to do some crazy stuff to stay compatible. One option is to save and restore the whole hypervisor state whenever it runs. Another option is to be some sort of boot loader, seizing the hypervisor capability before Windows can get to it.
That'll be some tough code to debug.
In other words, you can't implement a hypervisor more advanced than Hyper-V.
If you instead want to be on the outside, with Hyper-V on the inside, then you can't just write a driver. You have to implement a boot loader. You also have to implement nested virtualization, even if you otherwise had no need to do so.
Yeah that's the difference between a type 1 and type 2 hypervisor. A type 1 runs on the bare metal, a type 2 runs via drivers underneath an existing OS. Since Hyper-V is a type 1 (like ESXi) you can't use a type 2 hypervisor on the root VM to escape being under Hyper-V you either have to do some sort of nesting or disable Hyper-V from loading and reboot.
Our hypervisor is far more demanding than ESXi, KVM, and Hyper-V. It needs to interact with low-level Intel processor details in a way that is not supported by any other hypervisor. It won't run correctly if nested inside any other hypervisor. If we supported running under another hypervisor, we would lose important functionality.
If it becomes impossible or impractical to disable Hyper-V, we'll need to do something strange and annoying. Perhaps we could load the driver very early in boot, before Hyper-V loads. Booting as an OS ("type 1", ugh) is an option too, but maintaining that and using it is a real pain. Probably we'd drop Windows host support before we did that.