x86/pvclock: Zero last_value on resume

If the guest domain has been suspend/resumed or migrated, then the
system clock backing the pvclock clocksource may revert to a smaller
value (ie, can be non-monotonic across the migration/save-restore).

Make sure we zero last_value in that case so that the domain
continues to see clock updates.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/pvclock.c

@@ -83,6 +83,11 @@ unsigned long pvclock_tsc_khz(struct pvclock_vcpu_time_info *src)
 
 static atomic64_t last_value = ATOMIC64_INIT(0);
 
+void pvclock_resume(void)
+{
+	atomic64_set(&last_value, 0);
+}
+
 cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
 {
 	struct pvclock_shadow_time shadow;