Re: [Xen-devel] Time Skewing on Windows XP

[Tim Deegan <tim@xxxxxxx>]

At 10:16 -0500 on 14 Mar (1363256187), Suravee Suthikulanit wrote:
> On 3/12/2013 3:03 AM, Jan Beulich wrote:
> 
> >While I recall respective reports, the issue should not be present
> >on current -unstable, so if you do have observations like this they
> >will need investigation.
> 
> Jan,
> 
> I finally traced the issue back to the patch that this first happened.  
> This bug started in the patch :
> 
> H86/HVM: assorted RTC emulation adjustment (w/ git commit id 
> 620d5dad54008e40798c4a0c4322aef274c36fa3)
> 
> I believe there are some issues with the changes in rtc_ioport_read in the 
> arch/x86/hvm/rtc.c
> and in the pt_update_irq().

Yes, it looks like the new call to rtc_timer_update() will reset the RTC
timer every time RTC_C is read, even if it's already running.  So each
second will start from the last read of RTC_C, not the start of the last
second.

The attached patch makes the drift better, but doesn't fix it entirely.
I'm not sure that it's the best approach though - it seems like in the
common case we shouldn't be restarting the timer at all.

Tim.

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