@olivierlambert That makes complete sense
I should mention that even those platforms that have Nested Virtualization, the performance is usually AWFUL, but could be useful for certain things (mostly testing and experiments though). A "Custom DNS Server" or something of that nature ("network service", some type of container running an "agent", etc).
I see why people think they want it, and it makes logical sense, but I've spelled all that out to essentially say "You think you want it, but be careful what you wish for."
I'd love to have a handful or 2 Windows guest VMs with Docker Desktop or HyperV to keep projects better separated and secure, but do I "need" that? not really. There are portability, security and privacy elements that appeal to the way I try to organize guest VMs for certain tasks, but to really make nesting useful, there is an implicit amount of wasted resources, too. Like if I have a nested XCP-ng or TrueNAS guest VM, that's a large amount of resources allocated to a VM that I'm probably not using all of it at once.
That said, I use the "Copy VM" feature all the time, as I have built out certain settings and configurations in a sort of "appliance templates" way - so having a ready-made environment that I could use GPUs to passthrough for something like an AI model I'm "training and tuning" or a similar passthrough type use case that I can just copy quickly and tweak 2-3 things in a few seconds to get everything going is what I'd like to be able to use it for.
So I find the idea of nested convenient and novel, but I've learned its not a necessity. Most server platforms (or even desktop motherboards) end up with wasted / inaccessible slots when using "powerful enough for AI or Gaming" GPUs, and then there are the often difficult-to-satify power requirements, too. So I find that most of the kinds of things I want to try to do with hardware / hardware emulation is usually best done on its on standalone system, anyway.
But maybe one day, single slot GPUs will be more affordable, and virtualization will be a standard feature for a hardware platform, rather than such a heavy lift.
Are there any specific CPU/mobo combinations that tend to have "the most success" with Nested Virtualization for Hypervisors like XCP-ng? It seems that is the biggest factor for NV, as of today (but maybe I'm wrong and it doesn't matter as much as I think it does).