Fix time going back in Xen case

when backward time correction happens to get bigger than the tick duration.

* kern/mach_clock.c (clock_interrupt): When `tickdelta' is bigger than `usec',
truncate it to `usec-1'.

1 files changed, 10 insertions, 2 deletions

diff --git a/kern/mach_clock.c b/kern/mach_clock.c
index dba2320a..c6e2d9df 100644
--- a/kern/mach_clock.c
+++ b/kern/mach_clock.c
@@ -232,8 +232,16 @@ void clock_interrupt(
 		int	delta;
 
 		if (timedelta < 0) {
-		    delta = usec - tickdelta;
-		    timedelta += tickdelta;
+		    if (usec > tickdelta) {
+			delta = usec - tickdelta;
+			timedelta += tickdelta;
+		    } else {
+			/* Not enough time has passed, defer overflowing
+			 * correction for later, keep only one microsecond
+			 * delta */
+			delta = 1;
+			timedelta += usec - 1;
+		    }
 		}
 		else {
 		    delta = usec + tickdelta;