Re: kvm & dyntick

Ingo Molnar
2007-01-14T08:39:54Z

* Avi Kivity <avi@qumranet.com> wrote:

> >( for this to work on my system i have added a 'hyper' clocksource
> > hypercall API for KVM guests to use - this is needed instead of the
> > running-to-slowly TSC. )
> >
>
> What's the problem with the TSC? The only issue I'm aware of is that
> the tsc might go backwards if the vcpu is migrated to another host cpu
> (easily fixed).

this is not a problem of KVM - this is a problem of this laptop: its TSC
stops when going idle. So the TSC is fundamentally unusable for reliable
timekeeping - and qemu doesnt offer pmtimer emulation to fall back to,
so the dyntick kernel refused to go high-res under KVM. Once i added the
hyper clocksource to fall back to, it could utilize the PIT and the
lapic for clock-events.

Ingo