RE: [Xen-devel] [RFC] Physical hot-add cpus and TSC

["Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>]

>-----Original Message-----
>From: Jeremy Fitzhardinge [mailto:jeremy@xxxxxxxx]
>Sent: Wednesday, June 02, 2010 1:07 AM
>To: Dan Magenheimer
>Cc: Jiang, Yunhong; Keir Fraser; Xen-Devel (xen-devel@xxxxxxxxxxxxxxxxxxx); Ian
>Pratt
>Subject: Re: [Xen-devel] [RFC] Physical hot-add cpus and TSC
>
>On 05/31/2010 05:30 PM, Dan Magenheimer wrote:
>>> BTW, I notice one more thing, when system booting w/o hotplug, the warp
>>> is 0. However, after I return back after weekend, I noticed the warp is
>>> 182. Because I did the hotplug action before getting the warp, I'm not
>>> sure if it's caused by the hotplug action, or the system TSC will drift
>>> very slowly.
>>>  (XEN) TSC marked as reliable, warp = 182 (count=2)
>>>
>> Hmmm... I'm much more worried about this case and would
>> like to understand this better.  If this is reproducible
>> on real-world QPI systems, and there is no way to a priori
>> determine that "this is a system where even though the
>> Invariant TSC bit is set, this system may drift", then
>> there is no way Invariant TSC can be exposed to a guest.
>>
>
>Some crappy BIOSes will attempt to hide the time taken by a SMI by
>save/restoring tsc over the call.  Could something like that be
>happening here?
>
>One of the nicest upcoming tsc-related architectural changes is that the
>cpus will expose both the underlying base tsc counter, and the offset
>used to compute rdtsc; a wrtsc will just end up adjusting that offset
>without affecting the underlying counter, making it easy to tell when
>people are trying to play games with the tsc (and also making the
>process of adjusting the tsc one of determining the offset, independent
>of trying to place games with updating a racing tsc).
>

Because that data is collected after a weekend, so I'm not sure if anything 
happen to the system (for example, someone may hot-add a CPU but I'm unware of 
it and didnt check it). I will retry it this weekend. At least after running 
for half day this morning, I didn't find such issue again.

Per my understanding, the TSC should be stable, a lot of effort has been made 
so that TSC is reliable in the system.

--jyh

>> /me can hear Jeremy biting his tongue hard to avoid
>> saying "I told you so". ;-)
>>
>
>...
>
>    J

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