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



On 03/12/2013 11:55 AM, Suravee Suthikulanit wrote:
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

Hi,

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

* AMD NPT performance regression after c/s 24770:7f79475d3de7
Reference:http://marc.info/?l=xen-devel&m=135075376805215


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.

   Paul
Paul,

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).

I believe TSC is usually emulated. Try adding "tsc_mode=2" to your configuration so that RDTSC(P) is never intercepted.

-boris


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