What WASM runtime does ICP use? Is it homegrown?
wasmtime. Occasionally there are features that we would like to have available that would require maintaining a fork, but so far the benefit wasn’t deemed worth the effort
wondering what features we are talking about. aren’t the maintainers of wasmtime open for contributions? 
The latest discussion was around wasm64 stability and/or the component model IIRC. There the matter is more about when the features get stabilized. The other thing I remember is something about interrupting calls that are in progress after n instructions, which would introduce a significant overhead for everyone even though only very few use cases would profit from it
See also Virtual Canister - #9 by berestovskyy .