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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