Re: [PATCH 1/5] Add a global synchronization point for pvclock

Avi Kivity
2010-04-20T09:35:19Z

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.