[Xen-devel] RE: s_time going backwards on same processor?

["Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx>]

OK, I am definitely recording stime going backwards
observed with the attached patch.  I have recorded dozens
over a few hours.  It appears to have no
obvious pattern between reports, but one thing
is consistent: In all cases, t->tsc_scale.shift
is -1.  I'll try to run some more tests over the
weekend (e.g. with different clocksources... this
is with clocksource=hpet), but thought I'd report
what I have seen.  I'm running on a dual-core single
socket ("Conroe").

Dan

P.S. If you try the patch, ensure you set
the boot parameter "measurestime".

> -----Original Message-----
> From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
> Sent: Wednesday, July 23, 2008 1:06 AM
> To: dan.magenheimer@xxxxxxxxxx; Xen-Devel (E-mail)
> Cc: Dave Winchell
> Subject: Re: s_time going backwards on same processor?
> 
> 
> On 22/7/08 22:58, "Dan Magenheimer" 
> <dan.magenheimer@xxxxxxxxxx> wrote:
> 
> > I *do* know that get_s_time() on different processors
> > can have this behavior and I know it is possible for
> > hvm_get_guest_time() to go backwards (timer_mode=0),
> > but I thought s_time was monotonically non-decreasing
> > on any given processor and that read_platform_stime()
> > is also monotonically non-decreasing.
> >
> > Does dom0 maybe have direct hardware access to the hardware
> > platform timer that xen system time is dependent on?
> 
> No matter what happens to the underlying platform timer, it should be
> impossible for Xen system time to go backwards on any given 
> processor. The
> calibration function never sets the TSC and system timestamps 
> for the next
> time record any earlier than current TSC value and current 
> computed system
> time value. Hence it should be impossible for system time to 
> be computed as
> earlier than that time record.
> 
>  -- Keir
> 
> 
>

Attachment: checkbackwards.patch
Description: Binary data

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