sys: add safe_read(9)

[kib]

Summary

amd64: extract uiomove_mem() from memrw()


The MD function with MI interface to provide a way to read arbitrary
(canonical) KVA.  amd64 only for now.

Tested by:      aokblast

[kib]

Slightly different version, avoid the thread-private flag.

[markj]

sys/amd64/amd64/trap.c

797 ↗

(On Diff #178476)

Maybe it should be formulated as if (__predict_false(!usermode && ...?

sys/sys/systm.h

557

559

[kib]

sys/amd64/amd64/trap.c

797 ↗

(On Diff #178476)

I believe that the first check for tf_rip makes the shortest path through the condition: userspace must work hard to fault with this specific %rip, and kernel most likely would not fault on safe_read_read with other conditions false.

[kib]

Edit the comment.

[kib]

Complete overhaul. Stop modifying page fault handler at all. Reuse code from mem.c.