Re: [RFC] Paravirt timer for KVM

[Hollis Blanchard <hollisb-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

On Fri, 2007-10-12 at 15:02 -0500, Anthony Liguori wrote:
> Hollis Blanchard wrote:
> > On Fri, 2007-10-12 at 13:08 -0300, Glauber de Oliveira Costa wrote:
> >   
> >> +config KVM_CLOCK
> >> +       bool "KVM paravirtualized clock"
> >> +       depends on PARAVIRT && GENERIC_CLOCKEVENTS
> >> +       help
> >> +         Turning on this option will allow you to run a paravirtualized clock
> >> +         when running over the KVM hypervisor. Instead of relying on a PIT
> >> +         (or probably other) emulation by the underlying device model, the host
> >> +         provides the guest with timing infrastructure, as time of day, and
> >> +         timer expiration.
> >>     
> >
> > I must have missed earlier discussion on this topic, so I'm left
> > wondering... what's the point? What's wrong with PIT (et al) emulation?
> >   
> 
> There are three separate reasons, that I know of, to have a PV timer.
> 
> 1) the PIT is periodic.  a PV timer can offer a one shot timer which 
> enables dynticks.

Obviously people have figured out how to do dynticks on real x86
hardware, so I don't accept this reason. :)

> 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.  A PV time source gives you more 
> stable clocksource (although as in glommer's patch, when the TSC can be 
> used, it's better to use it).

As I understand it, the TSC is based on CPU frequency, which changes
with power management. Architectural bug.

However, PV time still doesn't help here:
      * The TSC is _user_ accessible, so PV time support in the guest
        kernel doesn't solve the problem.
      * It looks like external agents can perform out-of-kernel
        frequency scaling on x86 (at least I see options for it on IBM
        blades). So there must already exist some mechanism for a kernel
        to be informed that the TSC frequency has been changed.

> 3) a PV clock can support stolen time calculation which there really 
> isn't a concept of with emulation.

This is true, and I know other platforms support this functionality. I
think it's mostly useful for process time accounting. Is that actually
supported in this patch?

-- 
Hollis Blanchard
IBM Linux Technology Center


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