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[Xen-users] Upcoming change in Xen 4.0: handling of the rdtsc instructio

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Subject: [Xen-users] Upcoming change in Xen 4.0: handling of the rdtsc instruction
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Thu, 3 Dec 2009 12:53:26 -0800 (PST)
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Hello Xen users --

There will be a change in Xen 4.0 that will have no effect on most
users but may be visible to some.

In 4.0, the x86 rdtsc ("read timestamp counter") instruction will be
handled differently by Xen and a new VM configuration option
"tsc_mode" has been added to allow user/admin control where necessary.
The default for this option has been chosen intelligently so
few users will even need to be aware of its existence.  But
guest OS's and applications that utilize this instruction
at a very high frequency (>10000/second/processor) may see a
small performance degradation.

If you think you might be affected and you'd like more information
on tsc_mode, some documentation has been checked into the Xen
unstable source tree and can be found here:


If you have any questions, feel free to respond to this

Dan Magenheimer

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