Am 12.01.2011 14:05, Jan Kiszka wrote: > Am 12.01.2011 13:27, Gleb Natapov wrote: >> On Wed, Jan 12, 2011 at 12:26:25PM +0100, Jan Kiszka wrote: >>> Am 23.12.2010 21:45, Zachary Amsden wrote: >>>> On 12/17/2010 04:58 AM, Jan Kiszka wrote: >>>>> 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. >>>>> >>>> >>>> Did you test this with a Windows guest? They rely heavily on RTC, this >>>> is probably a better behavior for that case. I'd be curious if Windows >>>> accepts the RTC register changing underneath it, but based on earlier >>>> versions of Windows Time Service, would be surprised if it did not. >>> >>> I haven't tried with Windows yet. When does it read the RTC and how can >>> I check the outcome? >>> >> Windows relies on timely delivery of RTC interrupts to calculate wall >> clock. If, dues to the stall described above, interrupts will not be >> delivered for some period of time Windows guest may experience time >> drift. > > Ah, now I remember again. Will check the behavior before/after the patch.
Looks good: Without my patches, Windows' clock stops to tick once I reverse the host time by a significant period. With the patches, the guest clock proceeds apparently unaffectedly. So this series should be want-to-have for Windows virtualization as well. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux
