DOOM in the kernel, or fibers in eBPF

Some time ago my coworker who was working on first version of Perforator (https://github.com/yandex/perforator) kept talking about eBPF, so I got curious about its ISA and restrictions. That was around the time I read about DOOM on pregnancy test, so I thought: "what if someone run DOOM inside of Linux kernel in eBPF? Surely, with some restrictions, this should still be possible?"I started to play around with it somewhere in 2024 - first stripping DOOM to bare minimum that is needed for showcase, simplifying code and trying to pass first restrictions that I encountered - function limit, recursion, memory access and checks. I tried to do this by hand, but it was, well, tedious labor. So I changed direction and instead started to do some of the things with LLVM-passes - rewriting memory access, simplifying and rewriting loops etc. But there were just so many corners. And given absence of time and all, I forgot about this project.But couple months ago I thought: "LLMs are quite good nowadays, so why not try again this time with more hands". I tried different approaches on how memory access could be "virtualized", how loops can be made bounded, and in general - how to run unbounded logic on such a machine.When I got DOOM running, I got carried away. Approach was so "generic", so it would be a crime not to try to run more things. And now we have it - lua running on xdp hot path, llama2 working with softfloats, and even cpython, that did not fit initially into 1M verifier budget, is running there now thanks to freplace.There is ton of work to do in order to make programs run faster, to make integration easier, but working examples are already there.