Re: [kvm-devel] [PATCH] raise tsc clocksource rating

Avi Kivity
2007-10-30T04:26:36Z

Dan Hecht wrote:
> Not really. In the case hardware TSC is perfect, the paravirt time
> counter can be implemented directly in terms of hardware TSC; there is
> no loss in optimization. This is done transparently. And virtual TSC
> can be implemented this way too.
>
> The real improvement that a paravirt clocksource offers over the TSC
> clocksource is that the guest does not need to measure the TSC frequency
> itself against some other constant frequency source (which is
> problematic on a virtual machine). Instead, the paravirt clocksource
> queries the hypervisor for the frequency of the counter. As you know,
> with clocksource style kernels, it's important to get this frequency
> correct, or else the guest will have long-term time drift.
>
>

In addition, a paravirt clocksource can compensate for events like vcpu
migration to another host cpu. So I agree: a paravirt clocksource is
always better than or equal to the tsc.

--
Any sufficiently difficult bug is indistinguishable from a feature.