Re: [PATCH] hw/rtc/pl031: Send RTC_CHANGE QMP event

[Peter Maydell]

On Thu, 9 Sept 2021 at 13:24, Eric Auger <eric.auger@redhat.com> wrote:
>
> The PL031 currently is not able to report guest RTC change to the QMP
> monitor as opposed to mc146818 or spapr RTCs. This patch adds the call
> to qapi_event_send_rtc_change() when the Load Register is written. The
> value that is reported corresponds to the difference between the new
> RTC value and the RTC value elapsed since the base.
>
> For instance adding 20s to the guest RTC value will report 20:
> {'timestamp': {'seconds': 1631189494, 'microseconds': 16932},
> 'event': 'RTC_CHANGE', 'data': {'offset': 20}}
>
> Adding another extra 20s to the guest RTC value will report 40:
> {'timestamp': {'seconds': 1631189498, 'microseconds': 9708},
> 'event': 'RTC_CHANGE', 'data': {'offset': 40}}
>
> To compute the offset we need to track the origin tick_offset (the one
> computed at init time). So we need to migrate that field, which is done
> in a dedicated subsection. The migration of this subsection is disabled
> for machine types less or equal than 6.1.
>
> After migration, adding an extra 20s on the destination returns 60:
> {'timestamp': {'seconds': 1631189522, 'microseconds': 13081},
> 'event': 'RTC_CHANGE', 'data': {'offset': 60}}
>
> Signed-off-by: Eric Auger <eric.auger@redhat.com>


> @@ -138,6 +140,7 @@ static void pl031_write(void * opaque, hwaddr offset,
>      switch (offset) {
>      case RTC_LR:
>          s->tick_offset += value - pl031_get_count(s);
> +        qapi_event_send_rtc_change(s->tick_offset - s->original_tick_offset);
>          pl031_set_alarm(s);
>          break;
>      case RTC_MR:

None of the other users of qapi_event_send_rtc_change()
seem to have to track the baseline time like this. Shouldn't
this be doing something involving using qemu_ref_timedate()
as the baseline that it uses to figure out the change value ?
(The other users do this via qemu_timedate_diff() but since we
start with a value-in-seconds we might want to expose some other
API in softmmu/rtc.c.)

thanks
-- PMM