By default, we base the mc146818 RTC on the host clock (CLOCK_REALTIME).
This works fine if only the frequency of the host clock is tuned (e.g.
by NTP) or if it is set to a future time. However, if the host is tuned
backward, e.g. because NTP obtained the correct time after the guest was
already started or the admin decided to tune the local time, we see an
unpleasant effect in the guest: The RTC will stall for the period the
host clock is set back.
This series tries to address the issue more gracefully. By detecting
those warps and providing a callback mechanism to device models, the
RTC is enabled to update its timers and register content immediately.
Tested successfully with a hwclock readout loop in a Linux guest while
fiddling with the host time.
Note that if this kind of RTC adjustment is not wanted, the user is
still free to decouple the RTC from the host clock and base it on the
VM clock - just like before.