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
machines with X86_FEATURE_NONSTOP_TRULY_RELIABLE _TSC_HONESTLY.
--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.
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