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RE: [Xen-devel] RE: Saving/Restoring IA32_TSC_AUX MSR

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel] RE: Saving/Restoring IA32_TSC_AUX MSR
From: "Nakajima, Jun" <jun.nakajima@xxxxxxxxx>
Date: Fri, 11 Dec 2009 10:38:06 -0800
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Cc: "Dugger, Donald D" <donald.d.dugger@xxxxxxxxx>
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Thread-topic: [Xen-devel] RE: Saving/Restoring IA32_TSC_AUX MSR
Dan Magenheimer wrote on Fri, 11 Dec 2009 at 08:12:00:

>>> Yes, but code which uses fast vgetcpu is expecting
>>> to get physical cpu and physical node number.  Since
>>> an HVM guest OS only has access to virtual cpu and
>>> virtual node number, the information written to TSC_AUX
>>> by a guest OS is misleading and may silently break any
>>> userland code that assumes it is getting physical
>>> information.
>> This is depend on how the node info is virtualized.
>> If the virtual node could reflect the physical
>> node info, what rdtscp returns is valuable to applications.
> If it is possible to ensure that the cpu/node info
> is virtualized so that TSC_AUX always correctly provides the
> information needed by apps, I agree this would be
> valuable.  I don't see how this is possible, but maybe
> you have some creative ideas?

It's possible, and the way guest NUMA supposed to be. We are working on that.

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