From Windows 8.1 onwards ISA serial IRQs cannot be shared when ACPI Revision
5.0 is used in the FACP table. The reason for this is that if a 2-byte IRQ
Descriptor is used then the interrupt is considered to be high true, edge
sensitive, non-shareable. Since legacy serial ports COM1/3 and COM2/4 share
an IRQ then if more than 2 serial ports are added, Windows indicates a
conflict in Device Manager and these combinations cannot be used together.

Change the 2-byte IRQ Descriptor in the _CRS resource to a 3-byte IRQ
Descriptor indicating that the ISA serial IRQ is low true, edge sensitive and
shareable. This enables all 4 legacy serial ports to be used in Windows without
conflict.

Note that it was agreed during the list discussion that this change does not
require a compatibility property since it is not part of the default machine
configuration and was already broken.

Signed-off-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
---
 hw/char/serial-isa.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/hw/char/serial-isa.c b/hw/char/serial-isa.c
index a4be0492c5..686336f34f 100644
--- a/hw/char/serial-isa.c
+++ b/hw/char/serial-isa.c
@@ -92,7 +92,8 @@ static void serial_isa_build_aml(AcpiDevAmlIf *adev, Aml *scope)
 
     crs = aml_resource_template();
     aml_append(crs, aml_io(AML_DECODE16, isa->iobase, isa->iobase, 0x00, 0x08));
-    aml_append(crs, aml_irq_no_flags(isa->isairq));
+    aml_append(crs, aml_irq(isa->isairq, AML_EDGE, AML_ACTIVE_LOW,
+                            AML_SHARED));
 
     dev = aml_device("COM%d", isa->index + 1);
     aml_append(dev, aml_name_decl("_HID", aml_eisaid("PNP0501")));
-- 
2.43.0