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RE: [xen-devel] System time monotonicity

To: <dan.magenheimer@xxxxxxxxxx>, "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [xen-devel] System time monotonicity
From: "Tian, Kevin" <kevin.tian@xxxxxxxxx>
Date: Wed, 9 Apr 2008 09:16:07 +0800
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Thread-topic: [xen-devel] System time monotonicity
>From: Dan Magenheimer
>Sent: 2008年4月9日 1:40
>But I also observe that all of the hvm platform timer (pit,
>hpet, and pmtimer) code is built on top of the physical TSC
>plus the vmx/svm tsc_offset which doesn't seem to be affected
>by the Xen TSC synchronization.  True?

For cpus on same system bus driven by one crystal, TSC drift among
cpus may be just dozen of cycles after boot time sync, which is 
negligible enough compared to migration overhead and thus it's unlikely
to have HVM guest to observe a non-monotonic behavior after resume.

The issue comes with cpus running on different frequency, like driven
by multiple crystals or on-demand frequency change which affects TSC
too. HVM guest can be configured to avoid migrating among cpus with
different TSC freq, like limiting its cpu affinity to cpus on same system
bus. Or you have to configure HVM guest to not trust TSC...


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