Re: [RFC] Paravirt timer for KVM

Gildas
2007-10-15T10:14:01Z

2007/10/15, Gerd Hoffmann <kraxel-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>:
> Avi Kivity wrote:
> > Gerd Hoffmann wrote:
> >>
> >>> 2) the TSC would have to be used as a clocksource. You don't know the
> >>> frequency which is the first problem with using the TSC but some systems
> >>> have a TSC that changes frequencies.
> >>>
> >> Also note the tsc may stop ticking if the CPU goes sleep in C3, which
> >> IMHO makes the tsc almost useless as clocksource for guests ...
> >
> > But the host knows that, right? So it can update the guest's timebase?
>
> Host should know. Well, I hope. Dunno whenever one really can be sure
> in all cases given all the different CPUs and tsc implementations.
>
> With VT you can attempt to make that invisible to the guest using the
> tsc offset field. Probably svm can do that too (didn't check docs
> though). kvm-lite can't (what is the status btw?). Xen "solves" that
> by not doing power management *evil grin*.
>
> Nevertheless it is probably much easier to go with pv timers (or maybe
> emulate hpet timers).
>
> cheers,
> Gerd

Hell I don't know what is the best technical way to solve this
problem, but as a sysadmin, I'm really "annoyed" when the time starts
to drift madly on the servers as soon as the host is loaded (talking
about esx servers there but I guess this will probably apply to any
existing x86 virtualisation solution).

A simple, reliable solution to get a stable time source is not a must
IMHO, it is a usability requirement.

Cheers,
Gildas