[Qemu-devel] [PATCH v2 0/3] Let RTC follow backward jumps of host clock immediately

[Jan Kiszka]

Just noticed that this issue is still unfixed because my series was
somehow forgotten. So I've rebased it over current master, refactored it
to use the generic Notifier infrastructure and renamed it to "clock
reset notifier" to avoid confusion with icount related "warping". Please
review / apply before 0.15-rc0, it fixes a relevant issue.

Original series description:

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. We identified that one prominent guest affected
by this is Windows which relies on the periodic RTC interrupt for time
keeping.

This series address the issue 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 both with
hwclock in a Linux guest and by monitoring the Windows clock 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):
  notifier: Pass data argument to callback
  qemu-timer: Introduce clock reset notifier
  mc146818rtc: Handle host clock resets

 hw/fw_cfg.c      |    2 +-
 hw/mc146818rtc.c |   20 ++++++++++++++++++++
 input.c          |    2 +-
 migration.c      |   12 ++++++------
 notify.c         |    4 ++--
 notify.h         |    4 ++--
 qemu-timer.c     |   29 ++++++++++++++++++++++++++++-
 qemu-timer.h     |    5 +++++
 ui/sdl.c         |    2 +-
 ui/spice-core.c  |    2 +-
 ui/spice-input.c |    4 ++--
 ui/vnc.c         |    4 ++--
 usb-linux.c      |    2 +-
 vl.c             |    4 ++--
 xen-all.c        |    2 +-
 15 files changed, 75 insertions(+), 23 deletions(-)