Interesting, thanks for sharing. I did some digging on the linux-kernel mailing list and it seems that after the first patch series in Dec ‘21 and the LPC talk there was no further work on this. Too bad if true. Also I wasn’t able to easily find which hardware supports this today. I did see a reference to a RISC-V project that is adding a similar feature.
It could enable a full user space uring without the need of kernel side polling.