It's VP, so not actually a port but a wrapper like WINE. That said, on my Nvidia it runs fairly well - probably the best I've seen of a VP game. I get minor stutters when it streams in new areas, not sure if that's there on the Windows one but is a common sign of a D3D -> OGL layer (CSGO does the same).
That said, it's been stable and great framerates, works with Steam controller too, much better job than VP did with Witcher 2.