hw/isa/ich9: Add comment on imperfect emulation of PIC vs. I/O APIC routing

hw/isa/ich9: Add comment on imperfect emulation of PIC vs. I/O APIC routing

As noted in the comment, the PCI INTx lines are supposed to be routed
to *both* the PIC and the I/O APIC. It's just that we don't cope with
the concept of an IRQ being asserted to two *different* pins on the
two irqchips.

So we have this hack of routing to I/O APIC only if the PIRQ routing to
the PIC is disabled. Which seems to work well enough, even when I try
hard to break it with kexec. But should be explicitly documented and
understood.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Message-Id: <112a09643b8191c4eae7d92fa247a861ab90a9ee.camel@infradead.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>