Well for starters WSL implements Linux system calls, so it definitely has its own system calls!

But even beyond that, I was thinking of one thing they added for performance reasons (see 3rd bullet): https://github.com/Microsoft/WSL/issues/873#issuecomment-425...

However I'm not entirely sure if it's a concrete "syscall" or if it's implemented as a new option through an existing one. (It's not clear to me how they could reasonably fit this into anything that already exists, but it might.) Though I would be mildly surprised if they didn't implement at least one syscall specifically for WSL...