Re: question: HPET for multiple VMs

Dor Laor
2008-03-23T09:03:14Z

On Sun, 2008-03-23 at 10:30 +0200, Avi Kivity wrote:
> Anthony Liguori wrote:
> > Avi Kivity wrote:
> >>> And I would like to ask right and wrong to
> >>> implement the functionality in terms of need
> >>> and efficiency (scalability and time accuracy).
> >>>
> >>
> >> I think that for newer kernels we already have the desired accuracy.
> >> We're not always good at exploiting that accuracy; hence the recent
> >> movement of the PIT implementation from userspace to the kernel. But
> >> recent discussion leads me to believe it could have been implemented
> >> with the userspace PIT as well.
> >>
> >
> > What do you think is needed to get the same accuracy in userspace as
> > in kernelspace?
>
> Some mechanism that allows us to implement kvm_inject_pit_timer_irqs()
> and kvm_pit_timer_intr_post(). Specifically, information about whether
> an interrupt was actually processed, and a window for injecting missed
> ticks.
>
> > Better yet, do you think there is a reasonable kvmctl harness we could
> > write to quantify the PIT accuracy?
>
> kvmctl doesn't implement a pit, so no. Of course we can test any
> infrastructure for counting missed interrupts.
>
> >
> > It's easy enough to count timer interrupts and use compare that to an
> > external time source to get some notion of accuracy (on varying
> > frequencies of course). I know you mentioned before that guest CPU
> > consumption also comes into play... I'm not quite sure why though so
> > I'm not sure how to simulate that.
>
> It's not so easy, the code is quite tricky since the cpu processes
> vectors, not interrupt lines. It's also heuristic; if the guest
> programs some random device to share interrupts with the pit, the
> heursitic breaks down. This never happens in practice, though.
>
> Problems show up when both the guest and host are loaded, as then the
> cpu is timesliced instead of being available on demand.
>

I have patches that works for the rtc case with acpi windows, just
cleaning and sending for a review.
The same method should work for the pit as well.

> >
> > The nice thing about the CAP infrastructure is we can always move the
> > PIT back to userspace. I'll happily invest some cycles here as I'm a
> > big fan of getting rid of unneeded kernel code :-)
>
> Yes.