Re: RFC: kvmclock / tsc server side fix

[Zachary Amsden <zamsden@redhat.com>]

On 05/17/2010 05:36 AM, Glauber Costa wrote:
> On Fri, May 14, 2010 at 04:07:43PM -1000, Zachary Amsden wrote:
>    
>> I believe this fixes the root cause of the kvmclock warp.  It's
>> quite a plausible phenomenon, and explains why it was so easy to
>> produce.
>>
>>      
> You mean this is the case for both SMP and UP, or just UP as we talked
> before?
>    

It's possible on both SMP and UP, guest and host.  It is impossible on 
UP host unless special circumstances come into play (one of my patches 
created these circumstances).

> I don't get the role of upscale in your patch. Frequency changes are
> already handled by the cpufreq notifier.
>    

The only purpose of upscale is to downscale the measurement of delta 
used for counting stats if CPU frequency was raised since last 
observed.  This is because moving to a faster TSC rate means we might 
have counted some cycles at the wrong rate while the rate was in 
transition.  It doesn't much matter, as the delta for which "overrun" is 
logged was computed wrong anyway.

I'll clean up my patches and resend as a series today.

Zach