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. Jan Kiszka (3): qemu-timer: Consolidate qemu_get_clock and qemu_get_clock_ns qemu-timer: Introduce warp callback mc146818rtc: Handle host clock warps hw/mc146818rtc.c | 17 ++++++++++++ qemu-timer.c | 77 ++++++++++++++++++++++++++++++++++++++++++++---------- qemu-timer.h | 5 +++ 3 files changed, 85 insertions(+), 14 deletions(-)
