Re: [RFC 0/4] KVM in-kernel PM Timer implementation

Gleb Natapov
2010-12-14T13:52:28Z

On Tue, Dec 14, 2010 at 03:49:37PM +0200, Avi Kivity wrote:
> On 12/14/2010 03:40 PM, Glauber Costa wrote:
> >>
> >> What is the motivation for this? Are there any important guests that
> >> use the pmtimer?
> >Avi,
> >
> >All older RHEL and Windows, for example, would benefit for this.
>
> They only benefit from it because we don't provide HPET. If we did,
> the guests would use HPET in preference to pmtimer, since HPET is so
> much better than pmtimer (yet still sucks in an absolute sense).
>
> >> If anything I'd expect hpet or the Microsoft synthetic timers to be a
> >> lot more important.
> >
> >True. But also a lot more work.
> >Implementing just the pm timer counter - not the whole of it - in
> >kernel, gives us a lot of gain with not very much effort. Patch is
> >pretty simple, as you can see, and most of it is even code to turn it
> >on/off, etc.
> >
>
> Partial emulation is not something I like since it causes a fuzzy
> kernel/user boundary. In this case, transitioning to userspace when
> interrupts are enabled doesn't look so hot. Are you sure all guests
> that benefit from this don't enable the pmtimer interrupt? What
> about the transition? Will we have a time discontinuity when that
> happens?
>
> What I'd really like to see is this stuff implemented in bytecode,
> unfortunately that's a lot of work which will be very hard to
> upstream.
>
<joke>
Just use ACPI bytecode. It is upstream already.
</joke>

--
Gleb.