Re: RTC time could run slow 3s than host time when clock=vm & base=UTC

[Zenghui Yu]

+Cc maintainers and author of [1]

On 2024/9/21 19:33, 贾庆彤 via wrote:
> When start qemu with -rtc base=utc,clock=vm, sometime guest time can
> slower 3s than host.
> 
> There's no problem (also didn't be noticed) as we often start ntp
> service, who will adjust our system time.
> 
> But let's talk about if we havn't enable NTP service(for example system
> just booted)
> 
> After inspect into the code, i found that there are two problem we
> should think about:
> 
> 1） The RTC time accuracy problem
> 
> 1. in the call chains
> rtc_realizefn->rtc_set_date_from_host->qemu_get_timedate->qemu_ref_timedate,
> 
> 
> the qemu_clock_get_ms(clock) / 1000 will lost ~0.999s at max.
> 
> 2. In the call chains cmos_ioport_read->rtc_update_time, the guest_sec =
> guest_nsec / NANOSECONDS_PER_SECOND
> 
> will also lost ~0.999s at max.
> 
> 3. after this[1] merged, in configure_rtc, the rtc_ref_start_datetime =
> qemu_clock_get_ms(QEMU_CLOCK_HOST) / 1000
> 
> will also lost ~0.999s at max.
> 
> Therefore, total about 3s the guest time will behind the host.
> 
> So what precision should we offer? the RTC Hardware's precision is
> second(that means the smallest step is 1 second,
> 
> not the accurate like time error about 8.6 to 1.7 seconds per day),
> 
> 
> 2）The ref start datetime
> 
> After this[1] merged, which intend to solve the -rtc
> base=DATETIME,clock=vm's problem ("It restores determinism
> 
> of guest execution when used with clock=vm and specified base value.").
> 
> "-rtc base=utc,clock=vm"'s ref start date time changed from the host
> time of the current executing code to
> 
> the const time of executing configure_rtc(rtc_ref_start_datetime =
> qemu_clock_get_ms(QEMU_CLOCK_HOST) / 1000;).
> 
> This will add additional time(current approx. 0.4s or other, depend on
> code executing time and how many class_init
> 
>  executed etc.),which start from configure_rtc to rtc device init, into
> guest time.
> 
> Hence, should we try to split -rtc base=utc,clock=vm's ref start date
> time to the original current host time
> 
> qemu_clock_get_ms(QEMU_CLOCK_HOST),not the time(rtc_ref_start_datetime)
> save in configure_rtc,
> 
> to achieve more accurate time?(or other better method)
> 
> 
> [1] https://lists.gnu.org/archive/html/qemu-devel/2018-10/msg03577.html
> 
> 
> .