Re: [PATCH] raise tsc clocksource rating

Jeremy Fitzhardinge
2007-10-29T22:52:44Z

Ingo Molnar wrote:
> * Zachary Amsden <zach@vmware.com> wrote:
>
>
>> On Mon, 2007-10-29 at 20:10 -0300, Glauber de Oliveira Costa wrote:
>>
>>> From: Glauber de Oliveira Costa <glauber@t60.localdomain>
>>>
>>> tsc is very good time source (when it does not have drifts, does not
>>> change it's frequency, i.e. when it works), so it should have its rating
>>> raised to a value greater than, or equal 400.
>>>
>>> Since it's being a tendency among paravirt clocksources to use values
>>> around 400, we should declare tsc as even better: So we use 500.
>>>
>> Why is the TSC better than a paravirt clocksource? In our case this
>> is definitely inaccurate. Paravirt clocksources should be preferred
>> to TSC, and both must be made available in hardware for platforms
>> which do not support paravirt.
>>
>
> if it's inaccurate why are you exposing it to the guest then? Native
> only uses the TSC if it's safe and accurate to do so.
>

It is used as part of the Xen clocksource as a short term extrapolator,
with correction parameters supplied by the hypervisor. It should never
be used directly.

J