Fix the Xen TOD update when the hypervisor wall clock is nudged.

The "wall clock" in the current code is actually the hypervisor start time.
The time of day is the "start time" plus the hypervisor "uptime".

Large enough bumps in the dom0 clock lead to a hypervisor "bump" which is
implemented as a bump in the start time, not the uptime. The clock.c routines
were reading in the hypervisor start time and then using this as the TOD.
This meant that any hypervisor time bump would cause the FreeBSD DomU to
set its TOD to the hypervisor start time, rather than the actual TOD.

This fix is a bit hacky and some reshuffling should be done later on
to clarify what is going on. I've left the wall clock code alone.
(The code which updates shadow_tv and shadow_tv_version.)
A new routine adds the uptime to the shadow_tv, which is then used to
update the TOD.

I've included some debugging so it is obvious when the clock is nudged.

PR:	135008

sys/i386/xen/clock.c

@@ -87,6 +87,7 @@ __FBSDID("$FreeBSD$");
 #include <machine/xen/xenfunc.h>
 #include <xen/interface/vcpu.h>
 #include <machine/cpu.h>
+#include <machine/xen/xen_clock_util.h>
 
 /*
  * 32-bit time_t's can't reach leap years before 1904 or after 2036, so we
@@ -236,6 +237,15 @@ static void update_wallclock(void)
 
 }
 
+static void
+add_uptime_to_wallclock(void)
+{
+	struct timespec ut;
+
+	xen_fetch_uptime(&ut);
+	timespecadd(&shadow_tv, &ut);
+}
+
 /*
  * Reads a consistent set of time-base values from Xen, into a shadow data
  * area. Must be called with the xtime_lock held for writing.
@@ -331,7 +341,9 @@ clkintr(void *arg)
 	 */
 	
 	if (shadow_tv_version != HYPERVISOR_shared_info->wc_version) {
+		printf("[XEN] hypervisor wallclock nudged; nudging TOD.\n");
 		update_wallclock();
+		add_uptime_to_wallclock();
 		tc_setclock(&shadow_tv);
 	}
 	
@@ -542,6 +554,7 @@ domu_inittodr(time_t base)
 	struct timespec ts;
 
 	update_wallclock();
+	add_uptime_to_wallclock();
 	
 	RTC_LOCK;
 	
@@ -591,6 +604,7 @@ domu_resettodr(void)
 		op.u.settime.system_time = shadow->system_timestamp;
 		HYPERVISOR_dom0_op(&op);
 		update_wallclock();
+		add_uptime_to_wallclock();
 	} else if (independent_wallclock) {
 		/* notyet */
 		;