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

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

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)

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel