FreeBSD differs quite substantially from the XNU kernel used by MacOS because XNU is based on Mach, and it was forked (Edit: from 4.3BSD) in 1988 - before Linux even existed.

The XNU kernel does not have a stable syscall ABI so perhaps it doesn't matter if the syscalls are different because the implementation of libSystem can convert as appropriate in userspace (see also: WINE).

> MacOS because XNU is based on Mach, and it was forked (Edit: from 4.3BSD) in 1988

As another commenter noted: XNU = Mach + FreeBSD.

What you are referring to is what NeXTSTEP was, Mach + BSD4:

* https://en.wikipedia.org/wiki/NeXTSTEP#Unix

By the time Apple got to them it was a few years later, and so they decided to updated that part of the kernel, and also brought in FreeBSD's userland.

>XNU is based on Mach

Partially, XNU is Mach AND the FreeBSD-Kernel:

https://www.youtube.com/watch?v=-7GMHB3Plc8

If you look at the source of a lot of drivers on MacOS a few years ago, they were heavily based on the FreeBSD drivers.

Yes sure they do, just look the presentation.

Could you perhaps relicense your implementation so it could be used outside of Linux?

I don't think I can, I'm using gpl code from other parts of the kernel. I'm not sure I would want to either, I put a lot of work into this and the gpl gives me more of a feeling of ownership.

That said, there's nothing stopping you or anyone else from reworking my code into a (gpl-licensed) FUSE driver. I don't think it's a straightforward task, but it can definitely be done.

I don't think it's a licencing issue. It's implementation as both kernels uses different syscalls and have different architecture.

Syscalls are mostly the same, but indeed, the interface between the kernel and the file systems is very different. However, code which implements that interface on the file system is a relatively small part of the whole thing; most of the code should be reusable.

Historical note: FreeBSD used to support XFS; I believe it was ported from Linux.

It takes a lot of work to get the Linux GPU drivers to build for other operating systems.

True. Although it’s way easier than it used to be, thanks to linuxkpi layer - the piece of FreeBSD kernel which implements various Linux kernel APIs.