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

[Tim Deegan <tim@xxxxxxx>]

At 16:06 +0000 on 14 Mar (1363277205), Jan Beulich wrote:
> >>> On 14.03.13 at 16:16, Suravee Suthikulanit 
> >>> <suravee.suthikulpanit@xxxxxxx> wrote:
> > 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().
> 
> One thing you may want to try is remove the call from REG_C
> read to rtc_timer_update() again - on a second thought it may
> be wrong to do it here, as (other than check_update_timer()
> and alarm_timer_update()) the function doesn't change with
> RTC_PF getting cleared (i.e. I may have wrongly added the call
> in analogy to the other two).

The call is needed because of the new code in rtc_periodic_interrupt()
that disables the timer if a second passes with no RTC_C read to clear
the RTC_PF flag. 

I suspect that getting rid of that (i.e. going back to running the timer
all the time if the guest asks for interrupts) will fix it. 

> I would expect the issue to be that create_periodic_time()
> pointlessly destroys and then recreates an identical rate timer.
> 
> I'm puzzled though that some Windows versions depend on
> the RTC to maintain their wall clock time...

XP was a while ago. :)  It uses the RTC in other surprising ways too --
e.g. doesn't stop the timer when updating the CMOS wallclock, just
writes the fields one at a time.

Tim.

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