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

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Subject: Re: [Xen-devel] RE: Saving/Restoring IA32_TSC_AUX MSR
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Sun, 13 Dec 2009 10:59:09 -0800
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, "Dugger, Donald D" <donald.d.dugger@xxxxxxxxx>, "Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, "Nakajima, Jun" <jun.nakajima@xxxxxxxxx>, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>
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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.)


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