Re: [PATCH 09/18] Robust TSC compensation

Zachary Amsden
2010-07-13T21:45:51Z

On 07/13/2010 11:42 AM, David S. Ahern wrote:
>
> On 07/13/10 15:15, Zachary Amsden wrote:
>
>
>>> What prevents a vcpu from seeing its TSC go backwards, in case the first
>>> write in the 5 second window is smaller than the victim vcpu's last
>>> visible TSC value ?
>>>
>>>
>> Nothing, unfortunately. However, the TSC would already have to be out
>> of sync in order for the problem to occur. It can never happen in
>> normal circumstances on a stable hardware TSC except in one case;
>> migration. During the CPU state transfer phase of migration, however,
>>
> What about across processor sockets? Aren't CPUs brought up at different
> points such that their TSCs start at different times?
>

Yes, that's called an unsynchronized TSC. In that case, the
compensation does the best it can based on time since the first TSC
write, but it will never be exact.