Re: [Xen-devel] Clock jumped 50 minutes in dom0 caused incorrect 2008 R2 domU time

[Gianni Tedesco <gianni.tedesco@xxxxxxxxxx>]

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


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