Messages in this thread | | | Date | Tue, 30 Oct 2007 06:24:56 +0200 | From | Avi Kivity <> | Subject | Re: [kvm-devel] [PATCH] raise tsc clocksource rating |
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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.
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