um: do not send SIGALRM to userspace in time-travel mode

We send a SIGALRM to userspace processes to interrupt them. Really,
doing so is only needed if they are actually executing at the time (to
ensure we return to kernelspace). Unfortunately, we do not have that
information readily available. We can however be sure that this is never
the case when we are in time-travel mode with infinite CPU.

Signed-off-by: Benjamin Berg <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Johannes Berg <[email protected]>

Author: benzea

arch/um/kernel/time.c

@@ -856,11 +856,16 @@ static struct clock_event_device timer_clockevent = {
 
 static irqreturn_t um_timer(int irq, void *dev)
 {
-	if (get_current()->mm != NULL)
-	{
-        /* userspace - relay signal, results in correct userspace timers */
+	/*
+	 * Interrupt the (possibly) running userspace process, technically this
+	 * should only happen if userspace is currently executing.
+	 * With infinite CPU time-travel, we can only get here when userspace
+	 * is not executing. Do not notify there and avoid spurious scheduling.
+	 */
+	if (time_travel_mode != TT_MODE_INFCPU &&
+	    time_travel_mode != TT_MODE_EXTERNAL &&
+	    get_current()->mm)
 		os_alarm_process(get_current()->mm->context.id.pid);
-	}
 
 	(*timer_clockevent.event_handler)(&timer_clockevent);