@Chuckz Yeah it would be a nice feature to see. I think the issue though is how much work it takes when it's not something anyone should be using in production. It's really just a heavy homelab feature.
I want it to work, don't get me wrong, but no big org should be doing nested virt, it's just not a good idea and even Hyper-V recommends against it.
For now, I think you are right except for Windows-centric shops. Going forward, there is no doubt that running Windows will be only on Hyper-V unless third party hypervisors can maintain support for the increasing number of features in Windows that rely on NV support. For example, important security features in Windows 11 such as core isolation do work on my Windows 11 guests, I suspect also because of lack of NV support in Xen. I also think over time this NV feature will become important also for other platforms that depend more on Linux than Windows does.
Has anyone here seen Windows 11 core isolation working on XCP-ng? One can check on a Windows 11 XCP-ng guest by looking at Windows Security -> Device Security -> Core Isolation -> Core Isolation Details in the Windows guest. I bet in every case it reports that it does not work. When I try to enable it, it successfully enables it and notifies me I need to reboot for the new setting to take effect, but when I reboot the core isolation feature is disabled again.
Apparently Windows virtualization, while important, is not important enough for deep-pocketed customers to push for this feature in XCP-ng and Xen upstream. The question I raise is, can a group of XCP-ng users, perhaps working in their home labs, get the ball rolling in upstream Xen without deep-pocket customers asking them to add the NV feature to Xen? I am hoping yes, because I think the Xen upstream developers really want to add this feature (Vates too, because it is a big negative for XCP-ng compared to other options that do support these Windows features that depend on NV).
But the Xen developers do not have the time to work on NV without deep-pocket customers asking for the feature. We can greatly improve the probability that the Xen developers will work on NV if we can do some of the work for them. I think there are some things we can do to help the Xen developers support NV. This is what I am proposing.