> I would assume most of them? I'd be surprised if distros like Debian, Ubuntu, Fedora, etc. would ship non-mainline kernel features like that.

It's best not to assume with these things.
With my stock Debian Stable kernel, Proton says this:

fsync: up and running.

And when I disable fsync, it says this:

esync: up and running.

> But it sounds like fsync got you performance pretty close to what ntsync can do, but esync was quite a bit behind both?

No, esync and fsync trade blows in performance. Here are some measurements taken by Kron4ek, who maintains somewhat widely used Wine/Proton builds:

https://web.archive.org/web/20250315200334/https://flightles...

https://web.archive.org/web/20250315200424/https://flightles...

https://web.archive.org/web/20250315200419/https://flightles...

> With vanilla being quite a bit behind esync?

Yes, vanilla Wine has historically fallen behind all of them, of course.

> Also, jeez, fsync, what a terrible name. fsync is a syscall that has to do with filesystem data. So confusing.

We can agree on this. :)

> if Valve didn’t enable that on their build then I don’t have it.

The Proton build is Valve's build. It supports both fsync and esync, the latter of which does not require a kernel patch. If you're gaming on Linux with Steam, you're probably already using it.

https://github.com/ValveSoftware/Proton/?tab=readme-ov-file#...

The common gaming-focused Wine/Proton builds can also use esync (eventfd-based synchronization). IIRC, it doesn't need a patched kernel.

The point being that these massive speed gains will probably not be seen by most people as you suggest, because most Linux gamers already have access to either esync or fsync.