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Well, for as long as the driver is ran in ring0, you have context switching. The water is wet, there are no tricks around that even if you are an Apple.

Where do they say their network stack is userspace? I watched the video but the last 10 minutes and I missed it.

Page 105 of their slides.

The userspace networking portion of the talk starts at 46:00.

To clarify, the whole networking stack isn't userspace. They still have the classical BSD networking stack. I think the userspace networking stack is used automatically by system libraries like URLSessions and this new Network.framework library. They introduced their userspace networking stack during last year's WWDC: https://developer.apple.com/videos/play/wwdc2017/707

Ah, good. Thank you.

It's only for datagrams, apparently.

My understanding is that TCP is also partially user-space, but with some kernel management (such as communicating the closing the TCP connections when processes terminate)