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...

Sure, but not in the NT system call table.

> 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...

It's neither, it's an API inside kernel space. They didn't expose it through the NT syscall table, but instead how the linux transtalion layer calls directly into NT kernel functions.

WSL is now a VM. So its really hypervisor that runs the linux syscalls on its virtual processor