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

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

At 17:09 +0000 on 04 Jan (1294160969), 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
> compatibility layer.

Surely it turns it _on_, no?  It does indeed help with Windows guests,
in particular avoiding the vexing "STOP 101" bluescreen when Windows
thinks one CPU hasn't seen timer interrupts for too long.

> 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).
> 
> Timer mode is apparently "0=delay virtual time when ticks are missed;
> 1=virtual time is always wallclock time".
> 
> To be honest I'm not entirely sure that those last two actually mean in
> practice. Hopefully someone who understands this stuff will weigh in.

Timer modes describe how guest-visible time and timer interrupts are
updated when a VCPU is rescheduled:  (2 is 'no-missed-ticks-pending')

 *  delay_for_missed_ticks (default):
 *   Do not advance a vcpu's time beyond the correct delivery time for
 *   interrupts that have been missed due to preemption. Deliver missed
 *   interrupts when the vcpu is rescheduled and advance the vcpu's
 *   virtual
 *   time stepwise for each one.
 *  no_delay_for_missed_ticks:
 *   As above, missed interrupts are delivered, but guest time always
 *   tracks
 *   wallclock (i.e., real) time while doing so.
 *  no_missed_ticks_pending:
 *   No missed interrupts are held pending. Instead, to ensure ticks are
 *   delivered at some non-zero rate, if we detect missed ticks then the
 *   internal tick alarm is not disabled if the VCPU is preempted during
 *   the
 *   next tick period.
 *  one_missed_tick_pending:
 *   Missed interrupts are collapsed together and delivered as one 'late
 *   tick'.
 *   Guest time always tracks wallclock (i.e., real) time.

TBH, though, trying to figure out exactly how that interacts with a
multi-processor OS that's trying to work backwards from timer values and
interupts to a consistent wallclock time is, let's say, tricky.
Mostly it's superstition of the "known to work best with OS foo"
variety rather than deep understanding. 

Cheers,

Tim.

-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Xen Platform Team
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

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