RTC: Update interrupt state when interrupts are masked/unmasked

If an interrupt flag is already set when the interrupt becomes enabled,
raise an interrupt immediately, and vice versa if interrupts become
disabled.
Signed-off-by: NYang Zhang <yang.z.zhang@intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/mc146818rtc.c

@@ -221,6 +221,15 @@ static void cmos_ioport_write(void *opaque, uint32_t addr, uint32_t data)
                     rtc_set_time(s);
                 }
             }
+            /* if an interrupt flag is already set when the interrupt
+             * becomes enabled, raise an interrupt immediately.  */
+            if (data & s->cmos_data[RTC_REG_C] & REG_C_MASK) {
+                s->cmos_data[RTC_REG_C] |= REG_C_IRQF;
+                qemu_irq_raise(s->irq);
+            } else {
+                s->cmos_data[RTC_REG_C] &= ~REG_C_IRQF;
+                qemu_irq_lower(s->irq);
+            }
             s->cmos_data[RTC_REG_B] = data;
             periodic_timer_update(s, qemu_get_clock_ns(rtc_clock));
             break;

hw/mc146818rtc_regs.h

@@ -58,5 +58,6 @@
 #define REG_C_IRQF 0x80
 #define REG_C_PF   0x40
 #define REG_C_AF   0x20
+#define REG_C_MASK 0x70
 
 #endif