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

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Subject: RE: [Xen-devel] RE: Saving/Restoring IA32_TSC_AUX MSR
From: "Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>
Date: Mon, 14 Dec 2009 14:33:12 +0800
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Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "Dugger, Donald D" <donald.d.dugger@xxxxxxxxx>, "Nakajima, Jun" <jun.nakajima@xxxxxxxxx>, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
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Thread-topic: [Xen-devel] RE: Saving/Restoring IA32_TSC_AUX MSR
Jeremy Fitzhardinge wrote:
> On 12/13/09 10:06, Dan Magenheimer wrote:
>> I agree there are some cases where the TSC_AUX value
>> set by a guest OS may be useful.  But ensuring that its
>> is always useful (NEVER incorrect) requires too many restrictions,
>> such as pinning. 
> At least with respect to Linux guests [*], this objection to rdtscp is
> moot, because if it isn't present then Linux will fall back to another
> mechanism which is always present.  Guest usermode will get the same
> info, good/bad/misleading/whatever, either way; rdtscp can't make it
> worse.  The only question is whether specifically adding
> rdtscp/TSC_AUX support adds any overall improvement.
> (* I don't know if any other rdtscp-users attempt to put NUMA or other
> physical topology info into TSC_AUX.  If they just stick to
> setting/using the cpu number, then they will get a net win from
> rdtscp.) 

Just have a glance at the open-solaris code, in its mp_startup() function, 
it will write the cpu_id value into the TSC_AUX MSR. Therefore I think 
open-solaris also uses this feature. 


>      J
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