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

Avi Kivity
2010-12-14T13:49:41Z

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.

--
error compiling committee.c: too many arguments to function