Re: [Xen-devel] [PATCH][HVM] fix PCI interrupt routing via ACPI

["Dave Lively" <dave.lively@xxxxxxxxx>]

On 10/13/06, Wang, Winston L <winston.l.wang@xxxxxxxxx> wrote:

I think (b) is a short term solution, if we upgrade south bridge from PIIX3/4 to newer ICHx, we could have 8 PIC mode PCI IRQ router and 24 IOAPIC IRQ in APIC mode. I think IPF platform already support 24 IOAPIC  IRQs now.

This may be the safer route, since we know guest OSes are accustomed to ICHx
these days.

I would like David separated the  patch, one for PIC mode PCI IRQ routing _ADR address fix, this one is good, another one for  cutting PIC  mode support, this one may need more discussion.

Unfortunately, applying only the _ADR fix will break your existing ACPI-compliant
guests (more than they were broken before).  The LNKx devices are defined in the
ACPI tables regardless of PIC/APIC mode.  In APIC mode, because the PRTA
routing table returned from _PRT does *not* refer to the LNKx devices, the guest OS
will disable each LNKx device by calling its _DIS method.  Because the _DIS method's
write of PIRx is now routed to the right place (PIIX3 config space - due to the _ADR fix),
this write now really disables the PCI INTx, preventing any interrupts from the PCI INTx
lines.

Without the _ADR fix, ACPI guests are finding they can't redirect the PCI LNKx interrupts,
and nicely falling back to using the existing IRQ assignment.  (So they're broken, but in a
way that many guests seem to be able to work around.)

Dave

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