Re: [Xen-devel] [PATCH] Add the USE_PLATFORM_TIMER flag to the FADT, to make w2k3 use the PMTIMER

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

At 17:41 +0100 on 03 Jun (1275586899), Dan Magenheimer wrote:
> So we're guessing that Windows is recognizing that raw TSC
> is skewed and (like Xen and Solaris) providing some per-CPU
> algorithm to correct for it.  Then, sadly, Xen changes
> the vcpu-to-pcpu mapping and really confuses the correction
> algorithm.

Something like that, yes, though the TSC offset code is supposed to mask
that at least to the TSC-sync precision that Xen can manage.  Or
possibly just the jitter added by scheduling and descheduling adds up
over time, because the guest only ever speeds up the stragglers -- I've
seen Xen's own system time do this on some older boxes, though not to
nearly the same extent.

> What tsc_mode is in use when this problem is encountered?

IIRC this was HVMPTM_no_delay_for_missed_ticks (on Xen 3.4 so no
emulated RDTSC).

> I'd think if tsc is emulated, there shouldn't be a problem
> (and since pmtimer is emulated anyway, shouldn't be any slower).

That sounds plausible.

Cheers,

Tim.

> > -----Original Message-----
> > From: Tim Deegan [mailto:Tim.Deegan@xxxxxxxxxx]
> > Sent: Thursday, June 03, 2010 2:58 AM
> > To: Dan Magenheimer
> > Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
> > Subject: Re: [Xen-devel] [PATCH] Add the USE_PLATFORM_TIMER flag to the
> > FADT, to make w2k3 use the PMTIMER
> > 
> > Hi,
> > 
> > At 18:46 +0100 on 02 Jun (1275504378), Dan Magenheimer wrote:
> > > Could you say more about this?  Which default timesource is
> > > drifting?  Aren't all HVM platform timers emulated on top
> > > of Xen system time?  Does w2k3 use rdtsc directly... under
> > > what circumstances?
> > 
> > w2k3 will use RDTSC for its fine-grained time source (the one that's
> > exposed as a performance counter) even if the ACPI PMTIMER is
> > available,
> > unless it's encouraged by this ACPI flag or the "/PMTIMER" boot.ini
> > switch.  W2k3sp2 and later Windowses use the PMTIMER by default,
> > presumably because of the let's-not-go-into-it-again difficulties with
> > using RDSTC as a time source.
> > 
> > http://blogs.technet.com/b/perfguru/archive/2008/02/18/explanation-for-
> > the-usepmtimer-switch-in-the-boot-ini.aspx
> > 
> > Under stress testing, we found that this time source could drift (by
> > several percent) relative to real time, where the PMTIMER-based one did
> > not.
> > 
> > Yes, HVM timers are all based on the same source, except the RTC, which
> > is still implemented as a recurring timer event.  I suspect that the
> > drift is in the way Windows interprets the TSC values.  It could be,
> > for
> > example, that it's warping forward to avoid cross-CPU
> > time-going-backward issues.
> > 
> > Cheers,
> > 
> > Tim.
> > 
> > > > -----Original Message-----
> > > > From: Tim Deegan [mailto:Tim.Deegan@xxxxxxxxxx]
> > > > Sent: Wednesday, June 02, 2010 4:19 AM
> > > > To: xen-devel@xxxxxxxxxxxxxxxxxxx
> > > > Subject: [Xen-devel] [PATCH] Add the USE_PLATFORM_TIMER flag to the
> > > > FADT, to make w2k3 use the PMTIMER
> > > >
> > > > because the default timesource (TSC) drifts under load.
> > > >
> > > > The flag is only defined in ACPI 3.0, and we provide ACPI 2.0
> > tables,
> > > > but Windows seems happy enough to obey it anyway.
> > > >
> > > > Signed-off-by: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
> > > >
> > > >
> > 
> > --
> > Tim Deegan <Tim.Deegan@xxxxxxxxxx>
> > Principal Software Engineer, XenServer Engineering
> > Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

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

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