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

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>, "Nakajima, Jun" <jun.nakajima@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel] RE: Saving/Restoring IA32_TSC_AUX MSR
From: "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>
Date: Fri, 11 Dec 2009 16:43:46 +0800
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Thread-topic: [Xen-devel] RE: Saving/Restoring IA32_TSC_AUX MSR
Keir Fraser wrote:
> On 11/12/2009 02:00, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx>
> wrote: 
>> I expect that Keir will not support putting TSC_AUX
>> in the context switch code unless it is absolutely
>> necessary, as it is certainly expensive to read and
>> write to TSC_AUX and this cost will add to every
>> context switch of every VM even though very few will
>> actually use rdtscp/TSC_AUX.
> Well, you'd make it dependent on the guest using TSC_AUX, I suppose.
> I think that's going to be pretty rare.
>> So I think we need to decide first about approach (1),
>> the full faithful implementation of rdtscp.
> The question has to be: what win do we get for faithful
> virtualisation of RDTSCP in a virtualised environment? Supporting CPU
> instructions just because they're there is not a useful effort.

As I know, RDTSCP can used to implment fast vgetcpu in newer Linux kernel.  
Current node and cpu info is saved in the MSR, and applications or libraries 
can get this info at ring3 through this instruction. If enable this instruction 
for vmx non-root mode,  it should benefit these kernels I think. 

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