Re: [Qemu-devel] RFC: ioapic polarity vs. qemu os-x guest

[Gabriel L. Somlo]

On Mon, Feb 17, 2014 at 07:06:11PM +0100, Paolo Bonzini wrote:
> Il 17/02/2014 19:01, Gabriel L. Somlo ha scritto:
> >On Mon, Feb 17, 2014 at 12:57:00PM -0500, Gabriel L. Somlo wrote:
> >>On Sun, Feb 16, 2014 at 06:23:11PM +0200, Michael S. Tsirkin wrote:
> >>>Well there is a bigger issue: any interrupt with
> >>>multiple sources is broken.
> >>>
> >>>__kvm_irq_line_state does a logical OR of all sources,
> >>>before XOR with polarity.
> >>>
> >>>This makes no sense if polarity is active low.
> >>
> >>So, do you think something like this would make sense, to address
> >>active-low polarity in __kvm_irq_line_state ?
> >>(this would be independent of the subsequent xor in
> >>kvm_ioapic_set_irq()):
> >
> >-    return !!(*irq_state);
> >+    if (polarity) {
> >+            /* Logical AND for level trig interrupt, active-low */
> >+            return !~(*irq_state);
> 
> This is ~*irq_state == 0, i.e. *irq_state == ~0.
> 
> What if high-order bits of *irq_state are never used?  That is, do
> you need to consider the maximum valid irq_source_id too?

Oh, I think I'm starting to comprehend the problem here. The bits of
"*irq_state" are indexed by "irq_source_id", which is dynamically
assigned by kvm_request_irq_source_id().

So, doing the OR thing when assuming always-active-high makes
sense. Doing AND based on an active-low assumption doesn't make
sense, because there could ALWAYS be 0 bits that just weren't
allocated (yet), and I'm having trouble imagining how I'd keep
track of where the current allocation boundary is in a sane way :)

Which I *think* was Michael's original point...

--Gabriel