He was involved with Red Dog (the modified Windows host that powers Azure).
He's not involved with CBL-Mariner team at all to my awareness. Mariner is mostly about solving a supply-chain problem at Microsoft... we have a ton of internal teams all using different flavors of Linux and packages have historically come from all over the place. With CBL-Mariner we are basically trying to unify on that and own the package build and distribution portion as well. There isn't much reason for a kernel designer to be involved in that as its a well-understood problem (and entirely different domain) and we already have internal upstream Linux kernel contributors (which is how produce -azure supported kernels).
BSD is just a supported kernel ABI.
Edit: I just found these docs, so I suppose it’s a bit more complicated that I’m stating, I.e. there are portions of the FreeBSD kernel directly in macOS https://developer.apple.com/library/archive/documentation/Da...
"XNU was a hybrid kernel derived from version 2.5 of the Mach kernel developed at Carnegie Mellon University, which incorporated the bulk of the 4.3BSD kernel modified to run atop Mach primitives..."
I'm pretty sure that had to be in reference to STREAMS, because the original UNIX I/O model certainly is not byte-oriented. So basically, entirely irrelevant.
"Unix is a junk OS designed by a committee of PhDs." -Dave Cutler
Xenix is not in the index, which really detracts from the motivations behind the story.
IIRC their TCP/IP stack was FreeBSD's at least for a while
Thus considerable portion of Operating systems at one point or another used BSD4.3-derived network stack to detriment of network API evolution (sometimes updated with later code), some over time developing their own implementation.
DEC eventually shut this down, which prompted his departure for Microsoft. This is unfortunate for DEC, as they eventually poured the company into their Alpha RISC processor, which did not live as long as DEC hoped. Prism might have been a superior design.
At this time, Microsoft was maintaining a UNIX kernel in their Xenix product, so they knew a good kernel engineer when they met one. Microsoft was the leading UNIX vendor in the early 80's.
Cutler famously disparaged the UNIX kernel (his notable saying was "Get a byte, get a byte, get a byte byte byte" to the tune of the finale of Rossini's William Tell Overture).
Microsoft dumped their Xenix onto SCO about this time.
What is more interesting to me was Cutler's involvement with Azure. He must have had some sway over CBL-Mariner, Microsoft's RPM-based Linux distribution.
Much of Cutler's earlier work is documented in the "Showstoppers" book:
https://www.amazon.com/Show-Stopper-Breakneck-Generation-Mic...
The book doesn't really delve into the Xenix decisions, if I remember correctly.
Without Cutler, Microsoft would likely have ended up on a BSD kernel, as Apple did.