Ruslan Bukin 157654d0c4 Permit supervisor to access user VA space for certain functions only.
This is done by setting SUM (permit Supervisor User Memory access)
bit in sstatus register.

The functions we allow access for are routines in assembly that
explicitly handle crossing the user kernel boundary.

Approved by:	re (kib)
Sponsored by:	DARPA, AFRL
2018-09-05 11:34:58 +00:00
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