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>
diff --git a/hw/isa/lpc_ich9.c b/hw/isa/lpc_ich9.c
index 9c47a2f..bce487a 100644
--- a/hw/isa/lpc_ich9.c
+++ b/hw/isa/lpc_ich9.c
@@ -304,6 +304,21 @@
route.irq = -1;
}
} else {
+ /*
+ * Strictly speaking, this is wrong. The PIRQ should be routed
+ * to *both* the I/O APIC and the PIC, on different pins. The
+ * I/O APIC has a fixed mapping to IRQ16-23, while the PIC is
+ * routed according to the PIRQx_ROUT configuration. But QEMU
+ * doesn't (yet) cope with the concept of pin numbers differing
+ * between PIC and I/O APIC, and neither does the in-kernel KVM
+ * irqchip support. So we route to the I/O APIC *only* if the
+ * routing to the PIC is disabled in the PIRQx_ROUT settings.
+ *
+ * This seems to work even if we boot a Linux guest with 'noapic'
+ * to make it use the legacy PIC, and then kexec directly into a
+ * new kernel which uses the I/O APIC. The new kernel explicitly
+ * disables the PIRQ routing even though it doesn't need to care.
+ */
route.irq = ich9_pirq_to_gsi(pirq_pin);
}
