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

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

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).

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

...

    J

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel