[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] [PATCH] x86, cpuidle: remove assertion on X86_FEATURE_TSC_RELIABLE

  • To: "Tian, Kevin" <kevin.tian@xxxxxxxxx>, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>
  • 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
  • Domainkey-signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=user-agent:date:subject:from:to:cc:message-id:thread-topic :thread-index:in-reply-to:mime-version:content-type :content-transfer-encoding; b=sx/0n19opyMhqUkwGC1uH41eNQcDHn2iHWWIgUCEl75RIPeiPtdY7fd8vvNt9JwkH5 sYoDP9CEYIkkbxe5oYbd3gYKDFhtUB3jGSLoHcbHTNtWT9e0a50Qd+v0yWobdYSCoIdD 74ICnqLUa9vH8/5CbDnV8urUA/Le3ePLZDpho=
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AcwRkZivueGLm2sbT3yA1MbSrwDN0QCmdk4AAA8u6PQ=
  • Thread-topic: [Xen-devel] [PATCH] x86, cpuidle: remove assertion on X86_FEATURE_TSC_RELIABLE

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

Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.