Jonathan Stone writes:
[...]
 > The point was to look at sharing that same vmspace/pmap (and avoid the
 > pmap_activate() costs) for processes like nfs_iod(), yet give them
 > back `a vmspace of their own' if/when they return to userland.
 > 
 > That was a noticeable-to-users win on NFS servers, back when SunOS did
 > it. seems like it'd be the same for us too. yes?

Yes, that provides a noticable performance improvement when
one has processes stuck in the kernel for a while (nfsiod).
The sun3 and sun3x pmap modules use the "empty context" trick,
which allows delaying the actual allocation of an MMU context
until a process touches a user-space address.  Just FYI.

Gordon