Re: [PATCH v2] mc146818rtc: fix timer interrupt reinjection

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 10/10/19 2:30 PM, Marcelo Tosatti wrote:
> 
> 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.
> 
> V2: reorganize code (Paolo Bonzini)

^ This line shouldn't be part of git history,
Paolo if possible can you drop it?

> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
> diff --git a/hw/timer/mc146818rtc.c b/hw/timer/mc146818rtc.c
> index 6cb378751b..0e7cf97042 100644
> --- a/hw/timer/mc146818rtc.c
> +++ b/hw/timer/mc146818rtc.c
> @@ -203,24 +203,28 @@ periodic_timer_update(RTCState *s, int64_t current_time, uint32_t old_period)
>   
>       period = rtc_periodic_clock_ticks(s);
>   
> -    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;
> +    }

This is the first change, simplify the if statement with a 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:
> @@ -251,22 +255,19 @@ periodic_timer_update(RTCState *s, int64_t current_time, uint32_t old_period)
>                   rtc_coalesced_timer_update(s);
>               }
>           } else {
> -           /*
> +            /*
>                * no way to compensate the interrupt if LOST_TICK_POLICY_SLEW
>                * is not used, we should make the time progress anyway.
>                */
>               lost_clock = MIN(lost_clock, period);
>           }
> +    }

This is the second change, changing the logic and fixing the bug.

>   
> -        assert(lost_clock >= 0 && 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);

This is part of the 1st change.

I'd rather see this patch split in 2 logical changes...

>   }
>   
>   static void rtc_periodic_timer(void *opaque)
> 
>