RE: [xen-devel] System time monotonicity

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

>From: Dan Magenheimer [mailto:dan.magenheimer@xxxxxxxxxx] 
>Sent: 2008年4月9日 9:55
>
>> Or you have to configure HVM guest to not trust TSC...
>
>Yes, that's what I'm thinking... like Linux, Xen could/should
>build virtual platform timers on a physical clocksource other
>than tsc if all of the potential vcpu->pcpu mappings are not
>on sync'd-TSC-pcpus.

virtual platform timers are only one area. The most important is
TSC itself which is used frequently by guest to calculate relative
offset...

>
>I assume this problem is worse with multi-socket Hypertransport
>and future Intel QPI boxes?  Or is TSC (and frequency changing)
>synchronized for such systems?

For same crystal case, Intel processors with VT-x support all have 
TSC constant feature which is not bound to frequency change and 
can be detected by CPUID. But for multiple crystals case, Xen may
need tackle affinity then.

Thanks,
Kevin

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