Re: [PATCH] raise tsc clocksource rating

Glauber de Oliveira Costa
2007-10-30T12:01:26Z

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Zachary Amsden escreveu:
> On Tue, 2007-10-30 at 00:02 +0100, Ingo Molnar wrote:
>> * Zachary Amsden <zach@vmware.com> wrote:
>
>>> Not every guest support paravirt, but for correctness, all guests
>>> require TSC, which must be exposed all the way up to userspace, no
>>> matter what the efficiency or accuracy may be.
>> but if there's a perfect TSC available (there is such hardware) then the
>> TSC _is_ the best clocksource. Paravirt now turns it off unconditionally
>> in essence.
>
> No, if no paravirt clocksource is detected, nothing can override the
> perfect TSC hardware clocksource rating of 400. And if a paravirt
> clocksource is detected, it is always better than TSC.

Why always? tsc is the best possible alternative the _platform_ can
provide. So the paravirt clocksource will be at best, as good as tsc.
And if it is the case: why bother with communication mechanisms of all
kinds, calculations, etc, if we can just read the tsc?

Noting again: If the tsc does not arrive accurate to the guest, it will
fail the tsc clocksource tests. So it will be rated zero anyway

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