Re: [PATCH] rtc/mc146818rtc: fix QEMU crash when time runs backwards

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Nov 13, 2025 at 11:02:11AM +0800, shenjiatong wrote:
> Loosen restriction on computed lost_tick. When QEMU's rtc_clock is set
> to host, the walltime is used for mc146818rtc which requires computed
> lost_tick to be strictly non-negative. But host walltime could possibly
> run backwards, for example, configured by NTP service. Under such
> circumstances QEMU process could crash unexpectly.

could crash -> crashes on an assert

remove "unexpectly."

> This situation is
> easy to reproduce for Windows 2012/2016 guests.

hmm what does it matter what is the guest?


Pls describe a fix shortly here.


And please follow standard practice to submit changes,
such as read and sign off on the DCO.


> ---
>  hw/rtc/mc146818rtc.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/hw/rtc/mc146818rtc.c b/hw/rtc/mc146818rtc.c
> index 8631386b9f..04185eb6e9 100644
> --- a/hw/rtc/mc146818rtc.c
> +++ b/hw/rtc/mc146818rtc.c
> @@ -173,8 +173,7 @@ static void periodic_timer_update(MC146818RtcState *s, int64_t current_time,
>          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);
> +        lost_clock = MAX(cur_clock - last_periodic_clock, 0);
>      }
>  
>      /*
> -- 
> 2.34.1
> 
> 
>