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

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "Nakajima, Jun" <jun.nakajima@xxxxxxxxx>, Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: RE: [Xen-devel] RE: Saving/Restoring IA32_TSC_AUX MSR
From: "Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>
Date: Sat, 12 Dec 2009 07:44:33 +0800
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Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "Dugger, Donald D" <donald.d.dugger@xxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>
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Thread-topic: [Xen-devel] RE: Saving/Restoring IA32_TSC_AUX MSR
Dan Magenheimer wrote:
>>> If this is true the only safe use of TSC_AUX is for
>>> its originally designed intent: To determine if two
>>> successive rdtscp instructions were or were not
>>> executed on the same processor.  Since this cannot
>>> be guaranteed in a VM, that's a reasonable argument
>>> that TSC_AUX shouldn't be exposed at all (meaning the
>>> rdtscp bit in cpuid should be turned off by Xen).
>> This should work if you bind (i.e. pin) each vcpu to each
>> CPU, as I suggested.
> Yes, it does.  If there were a reasonable way for an
> application to check "am I running on a VM for which
> each vcpu has been pinned?" this might be a reasonable
> constraint as, if the app isn't, it could fail or at least
> log a message.  But if the app will randomly fail
> (or perform horribly) depending on whether the
> underlying VM is pinned or not (which might even
> change across a migration or if a sysadmin is
> "tuning" his data center), I don't think
> enterprise customers would appreciate that.

        If later guest NUMA is implemented, both APP and 
Hypervisor/Guest are NUMA awared. APP could get benefit
>From the information of node/processor which is got from
RDTSCP. But how to implement guest NUMA is another story,
either we can use pin, or something other creative idea.

Best Regards,
-- Dongxiao
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