Re: [RFC PATCH V1 05/12] hvm/dm: Introduce xendevicemodel_set_irq_level DM op

[Julien Grall <julien@xxxxxxx>]

Hi Stefano,

On 11/08/2020 23:48, Stefano Stabellini wrote:
> On Tue, 11 Aug 2020, Julien Grall wrote:
> > >   I vaguely
> > > recall a bug 10+ years ago about this with QEMU on x86 and a line that
> > > could be both active-high and active-low. So QEMU would raise the
> > > interrupt but Xen would actually think that QEMU stopped the interrupt.
> > >
> > > To do this right, we would have to introduce an interface between Xen
> > > and QEMU to propagate the trigger type. Xen would have to tell QEMU when
> > > the guest changed the configuration. That would work, but it would be
> > > better if we can figure out a way to do without it to reduce complexity.
> > Per above, I don't think this is necessary.
> >
> > > Instead, given that QEMU and other emulators don't actually care about
> > > active-high or active-low, if we have a Xen interface that just says
> > > "fire the interrupt" we get away from this kind of troubles. It would
> > > also be more efficient because the total number of hypercalls required
> > > would be lower.
> > I read "fire interrupt" the interrupt as "Please generate an interrupt once".
> > Is it what you definition you expect?
> Yes, that is the idea. It would have to take into account the edge/level
> semantic difference: level would have "start it" and a "stop it".
I am still struggling to see how this can work:
    - At the moment, QEMU is only providing us the line state. How can 
we deduce the type of the interrupt? Would it mean a major modification 
of the QEMU API?
    - Can you provide a rough sketch how this could be implemented in Xen?

Cheers,

--
Julien Grall