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Re: [Xen-devel] Time Skewing on Windows XP

On 3/12/2013 4:13 AM, Paul Durrant wrote:
-----Original Message-----
From: xen-devel-bounces@xxxxxxxxxxxxx [mailto:xen-devel-
bounces@xxxxxxxxxxxxx] On Behalf Of Suravee Suthikulpanit
Sent: 12 March 2013 04:40
To: George Dunlap; Jan Beulich
Cc: xen-devel@xxxxxxxxxxxxx
Subject: [Xen-devel] Time Skewing on Windows XP


While I was investigating the following issue on Windows XP (both 32-bit and

* AMD NPT performance regression after c/s 24770:7f79475d3de7

On the latest source form xen-unstable, I ran into an issue where the timing
on the HVM guests skewing about 2x slower than the actual wall clock time.
This results in the system time slowing down. This is regardless of the
cpufreq governor scaling. (I tried with both ondemand and performance).

However, I don't see the same behavior on the Win7 HVM guests. Is this a
known issue. I assume that XP and Win7 uses different mechanism for
keeping time (e.g. rdtsc vs. HPET)?

I think XP uses TSC and more recent versions of Windows use PM TIMER. Try 
booting XP with the /USEPMTIMER switch in boot.ini.


That still doesn't help in this case. I still see the WindowsXP guest time skewing. Also, I would not think the normal RDTSC skewing should be this severe (0.5x comparing to the wall-clock time).


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