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

To: "Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>, "Nakajima, Jun" <jun.nakajima@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Xen-devel] RE: Saving/Restoring IA32_TSC_AUX MSR
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Fri, 11 Dec 2009 08:12:00 -0800 (PST)
Cc: "Dugger, Donald D" <donald.d.dugger@xxxxxxxxx>
Delivery-date: Fri, 11 Dec 2009 08:13:14 -0800
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> > 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?

> According to the current PVRDTSC logic, write_rdtscp_aux()
> is called in each scheduling ( schedule()->
> update_vcpu_system_time()->__update_vcpu_system_time()->
> write_rdtscp_aux() ), which is more frequent than
> __context_switch(). 

OK, I see.  Then I am OK with moving the call to write_rdtscp_aux()


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