drivers/misc/ntsync.c | 1146 ++++++++++++++
Assuming this doesn't go into futex(2) or some other existing code...
Can you start putting all of this into top-level "windows" directory? I suspect there will be more Windows stuff in the future.
So those who don't care about Windows can turn off just one config option (CONFIG_WINDOWS) and be done with it.
Name it "Linux Subsystem for Windows" for 146% better memes.