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SubjectRe: [PATCH 1/5] Add a global synchronization point for pvclock
On 04/20/2010 04:57 AM, Marcelo Tosatti wrote:
>> Marcelo can probably confirm it, but he has a nehalem with an appearently
>> very good tsc source. Even this machine warps.
>> It stops warping if we only write pvclock data structure once and forget it,
>> (which only updated tsc_timestamp once), according to him.
> Yes. So its not as if the guest visible TSCs go out of sync (they don't
> on this machine Glauber mentioned, or even on a multi-core Core 2 Duo),
> but the delta calculation is very hard (if not impossible) to get right.
> The timewarps i've seen were in the 0-200ns range, and very rare (once
> every 10 minutes or so).

Might be due to NMIs or SMIs interrupting the rdtsc(); ktime_get()
operation which establishes the timeline. We could limit it by having a
loop doing rdtsc(); ktime_get(); rdtsc(); and checking for some bound,
but it isn't worthwhile (and will break nested virtualization for
sure). Better to have the option to calibrate kvmclock just once on

Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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