1 files changed, 19 insertions, 18 deletions

diff --git a/hw/timer/mc146818rtc.c b/hw/timer/mc146818rtc.c
index 1b8d3d7d4c..82843ed03f 100644
--- a/hw/timer/mc146818rtc.c
+++ b/hw/timer/mc146818rtc.c
@@ -291,26 +291,15 @@ static void check_update_timer(RTCState *s)
 
     /* From the data sheet: "Holding the dividers in reset prevents
      * interrupts from operating, while setting the SET bit allows"
-     * them to occur.  However, it will prevent an alarm interrupt
-     * from occurring, because the time of day is not updated.
+     * them to occur.
      */
     if ((s->cmos_data[RTC_REG_A] & 0x60) == 0x60) {
-        timer_del(s->update_timer);
-        return;
-    }
-    if ((s->cmos_data[RTC_REG_C] & REG_C_UF) &&
-        (s->cmos_data[RTC_REG_B] & REG_B_SET)) {
-        timer_del(s->update_timer);
-        return;
-    }
-    if ((s->cmos_data[RTC_REG_C] & REG_C_UF) &&
-        (s->cmos_data[RTC_REG_C] & REG_C_AF)) {
+        assert((s->cmos_data[RTC_REG_A] & REG_A_UIP) == 0);
         timer_del(s->update_timer);
         return;
     }
 
     guest_nsec = get_guest_rtc_ns(s) % NANOSECONDS_PER_SECOND;
-    /* if UF is clear, reprogram to next second */
     next_update_time = qemu_clock_get_ns(rtc_clock)
         + NANOSECONDS_PER_SECOND - guest_nsec;
 
@@ -321,7 +310,21 @@ static void check_update_timer(RTCState *s)
     s->next_alarm_time = next_update_time +
                          (next_alarm_sec - 1) * NANOSECONDS_PER_SECOND;
 
-    if (s->cmos_data[RTC_REG_C] & REG_C_UF) {
+    /* If update_in_progress latched the UIP bit, we must keep the timer
+     * programmed to the next second, so that UIP is cleared.  Otherwise,
+     * if UF is already set, we might be able to optimize.
+     */
+    if (!(s->cmos_data[RTC_REG_A] & REG_A_UIP) &&
+        (s->cmos_data[RTC_REG_C] & REG_C_UF)) {
+        /* If AF cannot change (i.e. either it is set already, or
+         * SET=1 and then the time is not updated), nothing to do.
+         */
+        if ((s->cmos_data[RTC_REG_B] & REG_B_SET) ||
+            (s->cmos_data[RTC_REG_C] & REG_C_AF)) {
+            timer_del(s->update_timer);
+            return;
+        }
+
         /* UF is set, but AF is clear.  Program the timer to target
          * the alarm time.  */
         next_update_time = s->next_alarm_time;
@@ -727,12 +730,10 @@ static uint64_t cmos_ioport_read(void *opaque, hwaddr addr,
             ret = s->cmos_data[s->cmos_index];
             break;
         case RTC_REG_A:
+            ret = s->cmos_data[s->cmos_index];
             if (update_in_progress(s)) {
-                s->cmos_data[s->cmos_index] |= REG_A_UIP;
-            } else {
-                s->cmos_data[s->cmos_index] &= ~REG_A_UIP;
+                ret |= REG_A_UIP;
             }
-            ret = s->cmos_data[s->cmos_index];
             break;
         case RTC_REG_C:
             ret = s->cmos_data[s->cmos_index];