My 2 cents for this topic, although I'm still trying to figure out the whole
picture of the patch and discussion thread.

The ACPI MADT table gives the relationship between IOAPIC and gsi, while DSDT
table's _PRT gives the relationship between PCI devices and GSI.

Two way provided in ACPI _PRT table. If the source filed in _PRT entry does not
refert to any device, it means a GSI. I didn't find any hint in ACPI spec on the
polarity/level should be configured. Currently kernel assume the polarity/level
is fixed as low/level, which I suspect is according to PCI spec.

If the _PRT table present the interrupt as PCI Interrupt Link Device, that means
the interrupt attribute is configurable, OSPM need figure out the polarity/level
information through _CRS/_PRS method in these objectes.

For method 1 (i.e. source filed does not refer to device), maybe we can assume
the attribute is fixed and Keir's suggestion will work, while I suspect if Xen
can do anyting for second type.

Quickly check Xiantao's patch, I suspect if it will work for 2nd situation.

BTW, I don't know know any system which use 2nd type when working in APIC mode.

--jyh