mc146818rtc: fix timer interrupt reinjection

commit 369b41359af46bded5799c9ef8be2b641d92e043 broke timer interrupt
reinjection when there is no period change by the guest. 

In that case, old_period is 0, which ends up zeroing irq_coalesced
(counter of reinjected interrupts).

The consequence is Windows 7 is unable to synchronize time via NTP.
Easily reproducible by playing a fullscreen video with cirrus and VNC.

Fix by not updating s->irq_coalesced when old_period is 0.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

On 09/10/19 20:40, Marcelo Tosatti wrote:
>              s->period = period;
>              lost_clock += old_irq_coalesced * old_period;
> -            s->irq_coalesced = lost_clock / s->period;
> +            if (old_period) {
> +                s->irq_coalesced = lost_clock / s->period;
> +            }
> +
>              lost_clock %= s->period;
>              if (old_irq_coalesced != s->irq_coalesced ||
>                  old_period != s->period) {

I think none of the code in the "if (s->lost_tick_policy == 
LOST_TICK_POLICY_SLEW) {" matters if old_period == 0 (and lost_clock 
will always be 0).  So perhaps we should place all that big "if" under
the existing "if (old_period)"?

Or even something like:

diff --git a/hw/timer/mc146818rtc.c b/hw/timer/mc146818rtc.c
index 6cb378751b..3337a8da98 100644
--- a/hw/timer/mc146818rtc.c
+++ b/hw/timer/mc146818rtc.c
@@ -202,25 +202,33 @@ periodic_timer_update(RTCState *s, int64_t current_time, uint32_t old_period)
     int64_t cur_clock, next_irq_clock, lost_clock = 0;
 
     period = rtc_periodic_clock_ticks(s);
+    if (old_period && old_period == period) {
+        return;
+    }
 
-    if (period) {
-        /* compute 32 khz clock */
-        cur_clock =
-            muldiv64(current_time, RTC_CLOCK_RATE, NANOSECONDS_PER_SECOND);
+    if (!period) {
+        s->irq_coalesced = 0;
+        timer_del(s->periodic_timer);
+        return;
 
-        /*
-        * if the periodic timer's update is due to period re-configuration,
-        * we should count the clock since last interrupt.
-        */
-        if (old_period) {
-            int64_t last_periodic_clock, next_periodic_clock;
-
-            next_periodic_clock = muldiv64(s->next_periodic_time,
-                                    RTC_CLOCK_RATE, NANOSECONDS_PER_SECOND);
-            last_periodic_clock = next_periodic_clock - old_period;
-            lost_clock = cur_clock - last_periodic_clock;
-            assert(lost_clock >= 0);
-        }
+    }
+
+    /* compute 32 khz clock */
+    cur_clock =
+        muldiv64(current_time, RTC_CLOCK_RATE, NANOSECONDS_PER_SECOND);
+
+    /*
+     * if the periodic timer's update is due to period re-configuration,
+     * we should count the clock since last interrupt.
+     */
+    if (old_period) {
+        int64_t last_periodic_clock, next_periodic_clock;
+
+        next_periodic_clock = muldiv64(s->next_periodic_time,
+                                       RTC_CLOCK_RATE, NANOSECONDS_PER_SECOND);
+        last_periodic_clock = next_periodic_clock - old_period;
+        lost_clock = cur_clock - last_periodic_clock;
+        assert(lost_clock >= 0);
 
         /*
          * s->irq_coalesced can change for two reasons:
@@ -243,13 +251,10 @@ periodic_timer_update(RTCState *s, int64_t current_time, uint32_t old_period)
             lost_clock += old_irq_coalesced * old_period;
             s->irq_coalesced = lost_clock / s->period;
             lost_clock %= s->period;
-            if (old_irq_coalesced != s->irq_coalesced ||
-                old_period != s->period) {
-                DPRINTF_C("cmos: coalesced irqs scaled from %d to %d, "
-                          "period scaled from %d to %d\n", old_irq_coalesced,
-                          s->irq_coalesced, old_period, s->period);
-                rtc_coalesced_timer_update(s);
-            }
+            DPRINTF_C("cmos: coalesced irqs scaled from %d to %d, "
+                      "period scaled from %d to %d\n", old_irq_coalesced,
+                      s->irq_coalesced, old_period, s->period);
+            rtc_coalesced_timer_update(s);
         } else {
            /*
              * no way to compensate the interrupt if LOST_TICK_POLICY_SLEW
@@ -257,16 +262,12 @@ periodic_timer_update(RTCState *s, int64_t current_time, uint32_t old_period)
              */
             lost_clock = MIN(lost_clock, period);
         }
-
         assert(lost_clock >= 0 && lost_clock <= period);
-
-        next_irq_clock = cur_clock + period - lost_clock;
-        s->next_periodic_time = periodic_clock_to_ns(next_irq_clock) + 1;
-        timer_mod(s->periodic_timer, s->next_periodic_time);
-    } else {
-        s->irq_coalesced = 0;
-        timer_del(s->periodic_timer);
     }
+
+    next_irq_clock = cur_clock + period - lost_clock;
+    s->next_periodic_time = periodic_clock_to_ns(next_irq_clock) + 1;
+    timer_mod(s->periodic_timer, s->next_periodic_time);
 }
 
 static void rtc_periodic_timer(void *opaque)


Best read with "diff -b".

Paolo

On Wed, Oct 09, 2019 at 11:13:16PM +0200, Paolo Bonzini wrote:
> On 09/10/19 20:40, Marcelo Tosatti wrote:
> >              s->period = period;
> >              lost_clock += old_irq_coalesced * old_period;
> > -            s->irq_coalesced = lost_clock / s->period;
> > +            if (old_period) {
> > +                s->irq_coalesced = lost_clock / s->period;
> > +            }
> > +
> >              lost_clock %= s->period;
> >              if (old_irq_coalesced != s->irq_coalesced ||
> >                  old_period != s->period) {
> 
> I think none of the code in the "if (s->lost_tick_policy == 
> LOST_TICK_POLICY_SLEW) {" matters if old_period == 0 (and lost_clock 
> will always be 0).  So perhaps we should place all that big "if" under
> the existing "if (old_period)"?
> 
> Or even something like:

This skips reprogramming the timer if the period is equal.

Sending v2 based on your suggestions and patch below.

> 
> diff --git a/hw/timer/mc146818rtc.c b/hw/timer/mc146818rtc.c
> index 6cb378751b..3337a8da98 100644
> --- a/hw/timer/mc146818rtc.c
> +++ b/hw/timer/mc146818rtc.c
> @@ -202,25 +202,33 @@ periodic_timer_update(RTCState *s, int64_t current_time, uint32_t old_period)
>      int64_t cur_clock, next_irq_clock, lost_clock = 0;
>  
>      period = rtc_periodic_clock_ticks(s);
> +    if (old_period && old_period == period) {
> +        return;
> +    }
>  
> -    if (period) {
> -        /* compute 32 khz clock */
> -        cur_clock =
> -            muldiv64(current_time, RTC_CLOCK_RATE, NANOSECONDS_PER_SECOND);
> +    if (!period) {
> +        s->irq_coalesced = 0;
> +        timer_del(s->periodic_timer);
> +        return;
>  
> -        /*
> -        * if the periodic timer's update is due to period re-configuration,
> -        * we should count the clock since last interrupt.
> -        */
> -        if (old_period) {
> -            int64_t last_periodic_clock, next_periodic_clock;
> -
> -            next_periodic_clock = muldiv64(s->next_periodic_time,
> -                                    RTC_CLOCK_RATE, NANOSECONDS_PER_SECOND);
> -            last_periodic_clock = next_periodic_clock - old_period;
> -            lost_clock = cur_clock - last_periodic_clock;
> -            assert(lost_clock >= 0);
> -        }
> +    }
> +
> +    /* compute 32 khz clock */
> +    cur_clock =
> +        muldiv64(current_time, RTC_CLOCK_RATE, NANOSECONDS_PER_SECOND);
> +
> +    /*
> +     * if the periodic timer's update is due to period re-configuration,
> +     * we should count the clock since last interrupt.
> +     */
> +    if (old_period) {
> +        int64_t last_periodic_clock, next_periodic_clock;
> +
> +        next_periodic_clock = muldiv64(s->next_periodic_time,
> +                                       RTC_CLOCK_RATE, NANOSECONDS_PER_SECOND);
> +        last_periodic_clock = next_periodic_clock - old_period;
> +        lost_clock = cur_clock - last_periodic_clock;
> +        assert(lost_clock >= 0);
>  
>          /*
>           * s->irq_coalesced can change for two reasons:
> @@ -243,13 +251,10 @@ periodic_timer_update(RTCState *s, int64_t current_time, uint32_t old_period)
>              lost_clock += old_irq_coalesced * old_period;
>              s->irq_coalesced = lost_clock / s->period;
>              lost_clock %= s->period;
> -            if (old_irq_coalesced != s->irq_coalesced ||
> -                old_period != s->period) {
> -                DPRINTF_C("cmos: coalesced irqs scaled from %d to %d, "
> -                          "period scaled from %d to %d\n", old_irq_coalesced,
> -                          s->irq_coalesced, old_period, s->period);
> -                rtc_coalesced_timer_update(s);
> -            }
> +            DPRINTF_C("cmos: coalesced irqs scaled from %d to %d, "
> +                      "period scaled from %d to %d\n", old_irq_coalesced,
> +                      s->irq_coalesced, old_period, s->period);
> +            rtc_coalesced_timer_update(s);
>          } else {
>             /*
>               * no way to compensate the interrupt if LOST_TICK_POLICY_SLEW
> @@ -257,16 +262,12 @@ periodic_timer_update(RTCState *s, int64_t current_time, uint32_t old_period)
>               */
>              lost_clock = MIN(lost_clock, period);
>          }
> -
>          assert(lost_clock >= 0 && lost_clock <= period);
> -
> -        next_irq_clock = cur_clock + period - lost_clock;
> -        s->next_periodic_time = periodic_clock_to_ns(next_irq_clock) + 1;
> -        timer_mod(s->periodic_timer, s->next_periodic_time);
> -    } else {
> -        s->irq_coalesced = 0;
> -        timer_del(s->periodic_timer);
>      }
> +
> +    next_irq_clock = cur_clock + period - lost_clock;
> +    s->next_periodic_time = periodic_clock_to_ns(next_irq_clock) + 1;
> +    timer_mod(s->periodic_timer, s->next_periodic_time);
>  }
>  
>  static void rtc_periodic_timer(void *opaque)
> 
> 
> Best read with "diff -b".
> 
> Paolo