Also, VFIO mdev can be used to virtualize graphics cards to virtual machines, both nVidia and Intel. Note this is not device assignment (that's "plain" VFIO); it's the host driver exposing a partly pass-through, partly emulated device that virtualizes the host device. mdev's design was initially a Red Hat-nVidia collaboration with input from Intel (I had a small role from the Red Hat side, mostly coming up with the name :-)) and implemented by nVidia, and is used in production with both Windows and Linux guests.
And in that case too, the ioctl layer is almost entirely maintained across the boundaries like the scheme I'm saying. You need to do that or you almost certainly get a weird impedance mismatch between guest and host that destroys perf.
And Intel needs something like this either way because their hardware arch is different, and their GPU is heavily integrated with the main IOMMU.
Linus has said that he doesn't want discussions without code for the reasons I've stated.
All the complaints I've heard about how WSL didn't do the right thing make it sound like there is no right way, despite passthrough graphics drivers for otherwise totally proprietary stacks on hypervisor stacks from companies with blatant GPL violations ending up mainlined with praise (cough VMWare). And despite the fact that the only way that's been proven to virtualize graphics cards without 2stage MMUs is to replicate the host's ioctl layer into the guest. There's a reason why Virgl isn't used by anyone serious about perf, it's the fundamentally wrong architecture.