Mostly Linux, but also Windows when I had a VFIO passthrough setup. I don't think it's even possible to set it up with Virtualbox to have decent enough performance.

You can also setup VFIO in VirtualBox/KVM. We haven't polished it yet though. You can check the video here to see GPU virtualization in action:

https://www.cyberus-technology.de/products/hypervisor (Don't mind the English, we are not native speakers. :)

I'm really curious about this, is it gpu para-virtualization or actual VFIO requiring built in support from the gpu hardware?

We have used this with recent Intel GPUs that support SR-IOV. This is what you see in the video on the cyberus website. Intel hasn't managed to upstream the drivers for this yet and you have to piece together things, which is very unpleasant. But we are there to help, if someone wants to use this in a professional setting.

We used to have support for Intel GVT-g GPU virtualization as well, which was more of a software solution. This doesn't work with modern Intel GPUs anymore.

I wonder if it would be possible to use DXVK here for windows guests to play those unruly games. That is have a pass through direct x driver in windows that sends all the commands to dxvk which either sends the image back to the vm or renders directly on the screen.

Thank you for the info. I use AMD consumer GPUs, none of them unfortunately support SR-IOV afaik, there are some developments from google around virtio-gpu(DRM native context) I've been following, I was hoping this was something similar.