um: rtc: use RTC time when calculating the alarm

The kernel realtime and the current RTC time may have a (small) offset.
Should the kernel time be slightly in the future, then the timeout is
zero. This is problematic in time-travel mode, as a zero timeout can be
correctly configured and time never advances.

Replace the kernel realtime read with a read of the actual persistent
RTC clock. Also, for time-travel, calculate the exact nanoseconds needed
for the clock to advance.

Signed-off-by: Benjamin Berg <[e-mail]>
Co-developed-by: Avraham Stern <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Johannes Berg <[e-mail]>

arch/um/drivers/rtc_kern.c

···
 
 static int uml_rtc_alarm_irq_enable(struct device *dev, unsigned int enable)
 {
+	struct timespec64 ts;
 	unsigned long long secs;
 
 	if (!enable && !uml_rtc_alarm_enabled)
···
 
 	uml_rtc_alarm_enabled = enable;
 
-	secs = uml_rtc_alarm_time - ktime_get_real_seconds();
+	read_persistent_clock64(&ts);
+	secs = uml_rtc_alarm_time - ts.tv_sec;
 
 	if (time_travel_mode == TT_MODE_OFF) {
 		if (!enable) {
···
 
 		if (enable)
 			time_travel_add_event_rel(&uml_rtc_alarm_event,
-						  secs * NSEC_PER_SEC);
+						  secs * NSEC_PER_SEC -
+						  ts.tv_nsec);
 	}
 
 	return 0;