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Re: [Xen-devel] [PATCH] x86, cpuidle: remove assertion on X86_FEATURE_TS

To: "Tian, Kevin" <kevin.tian@xxxxxxxxx>, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] x86, cpuidle: remove assertion on X86_FEATURE_TSC_RELIABLE
From: Keir Fraser <keir.xen@xxxxxxxxx>
Date: Tue, 17 May 2011 08:58:16 +0100
Cc: xen devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
Delivery-date: Tue, 17 May 2011 01:00:51 -0700
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Thread-topic: [Xen-devel] [PATCH] x86, cpuidle: remove assertion on X86_FEATURE_TSC_RELIABLE
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On 17/05/2011 01:50, "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:

>> IIRC, this is not true on a multi-socket motherboard.  Even though each
>> socket
>> has NONSTOP_TSC, they are using different crystals, correct?
> it's true that sockets may use different crystals, and NONSTOP_TSC has nothing
> to say
> synchronization among sockets/cores. So it really depends on how you define a
> 'reliable': 
> is it reliable enough to be a Xen time source, or reliable enough to
> passthrough to the
> guest? I'll need to check current assumption and your previous discussions
> first before 
> saying anything inappropriate. :-)

Yes, Dan is right, RELIABLE_TSC means something more than just NONSTOP_TSC
and CONSTANT_TSC. It means that:
 1. TSCs do not stop in deep sleep (NONSTOP_TSC)
 2. TSCs do not change rate with core frequency (CONSTANT_TSC)
 3. Further, that all TSCs system wide run at the same rate at all times, in
perfect sync (not represented by any other cpu feature flag).

 -- Keir

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