> Looks like amd64 is the only architecture that does that guardpage
> thing.  So I think this is fine.

On that topic, it would be nice for at least all 64-bit arches with
USPACE_ALIGN = 0, to always allocate UPAGES + 1 contiguous virtual pages
for the u area, UPAGES physical pages, and make the lowest page's
virtual address transation invalid.

This has been on my todo list for a very long time, but
ENOTENOUGHSPARETIME as usual. Maybe once I'm retired...