Re: [Qemu-devel] Re: [PATCH 1/3] Change qemu_set_irq() to return status information.

[Paul Brook]

On Sunday 29 June 2008, Jan Kiszka wrote:
> Paul Brook wrote:
> >> The model takes into account that not all interrupt controller are
> >> capable to detect missed interrupt (it is possible that there is no
> >> interrupt controller at all). In this case irq function should
> >> return one and everything will fall back to how it works now.
> >
> > On a related note the correct way to fix this is "-icount auto".
> > You should make sure that your hacks are not enabled when we have a
> > realistic virtual timebase.
>
> Virtual timebases don't help if the guest has to interact with the real
> life - ie. clocks you cannot tweak. In the end, you will have to bring 
> them in sync again, so this approach basically points in the right
> direction.

That's exactly what -icount auto does. It gradually varies the virtual clock 
rate to keep it in line with the outside world. You get some clock skew, but 
that true whatever you do (including the proposed patch).

In my experience hacks to prevent "coalescing" of interrupts cause as many 
problems as they solve. The guest OS can get awfully confused when it 
compares two different time sources and gets wildly different and conflicting 
answers. I believe VMware has some fairly sophisticated hacks for this, but 
it often still doesn't work in practice.

Paul