Re: Clock jumped 50 minutes in dom0 caused incorrect 2008 R2 domU time

[Mark Adams <mark@campbell-lange.net>]

On Tue, Jan 04, 2011 at 05:28:02PM +0000, Gianni Tedesco wrote:
> On Tue, 2011-01-04 at 17:09 +0000, Ian Campbell wrote:
> > Which ever one of you started it: Please don't top post.
> > 
> > > On Sat, Oct 09, 2010 at 10:15:20AM +0800, wei song wrote:
> > > >  added timer_mode =2  and tsc_mode = 1 and viridian=1 into your configure
> > > > file.
> > 
> > On Mon, 2010-10-11 at 11:10 +0100, Mark Adams wrote:
> > > I can try this, but can you please confirm what these options do?
> > 
> > viridian=1 turns off support for the Hyper-V virtualisation
>                    ^^^ on ;)
> 
> > compatibility layer. Not many of these are actually supported but AFAIK
> > one or two are and can affect the behaviour of Win2008 (although you
> > would hope it was for the better!)
> > 
> > According to xmexample.hvm: tsc_mode : TSC mode (0=default, 1=native
> > TSC, 2=never emulate, 3=pvrdtscp).
> 
> >From xen/include/asm-x86/time.h:
> /*
>  *  PV TSC emulation modes:
>  *    0 = guest rdtsc/p executed natively when monotonicity can be guaranteed
>  *         and emulated otherwise (with frequency scaled if necessary)
>  *    1 = guest rdtsc/p always emulated at 1GHz (kernel and user)
>  *    2 = guest rdtsc always executed natively (no monotonicity/frequency
>  *         guarantees); guest rdtscp emulated at native frequency if
>  *         unsupported by h/w, else executed natively
>  *    3 = same as 2, except xen manages TSC_AUX register so guest can
>  *         determine when a restore/migration has occurred and assumes
>  *         guest obtains/uses pvclock-like mechanism to adjust for
>  *         monotonicity and frequency changes
>  */
> 
> > 
> > Timer mode is apparently "0=delay virtual time when ticks are missed;
> > 1=virtual time is always wallclock time".
> 
> It's all explained in xen/include/public/hvm/params.h
> 
> AIUI, timer_mode=0 means if timer ticks were missed then they all get
> re-injected in to the guest when next run. IOW if 5 ticks were missed
> you get 5 IRQ's in succession and the clock time is incremented stepwise
> with them. I think it should never be used unless the OS simply counts
> ticks to figure out the time (which they don't).
> 
> 1 is default and means the same except virtual time is always up to date
> when re-injecting missed ticks.
> 
> 3 i'm not sure about but sounds like some variant of 4?
> 
> 4 is where missed ticks delivered in one 'late tick'
> 
> Gianni
> 
> 

All, Thanks for input. Do you agree that the -best- setting to maintain
stable time for 2008 R2 hvm guests is to use timer_mode = 2, tsc_mode =
1 and viridian = 1 ?

Best Regards,
Mark

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