kernel - Add /dev/upmap and /dev/kpmap and sys/upmap.h (2)

kernel - Add /dev/upmap and /dev/kpmap and sys/upmap.h (2)

* Disallow writing to kpmap.

* Handle cpu read/update races against ts_uptime and ts_realtime by
  making these arrays[2] and using kpmap->upticks to tell userland
  which one is stable.

  The procedure is as follows:

	struct timespec ts_uptime;
	struct timespec ts_realtime;
	int w;

        do {
                w = kpmap->upticks;
                cpu_lfence();
                ts_uptime = kpmap->ts_uptime[w & 1];
                ts_realtime = kpmap->ts_realtime[w & 1];
                cpu_lfence();
                w = kpmap->upticks - w;
        } while (w > 1);

  This procedure avoids having to loop in all standard cases including
  the nominal kernel update / userland read race.  It will only loop if
  the kernel deschedules the user process for > 1 tick during the access
  sequence.