Re: [Qemu-devel] [RFC PATCH] qemu spapr-pci: added IRQ list to PCIBus

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 14/05/12 11:58, David Gibson wrote:
> On Sat, May 12, 2012 at 05:29:53PM +1000, Alexey Kardashevskiy wrote:
>> There is a need for a mechanism to obtain an IRQ line number to
>> initialize End-Of-Interrupt handler.
>>
>> There is another proposed solution (commit
>> b7790763828b732059ad24ba0e64ce327563fe1a "pci: Add callbacks
>> to support retrieving and updating interrupts") which adds pci_get_irq
>> callback to every PCI bus to allow an external caller to calculate
>> IRQ number from IRQ line (ABDD).
>>
>> However it seems to be too complicated as it affects all PCI buses
>> while the only user of it is VFIO-PCI so this could be done simpler
>> by an array of 4 IRQs (lines A, B, C, D) in struct PCIBus which
>> every platform would initialize in its own way.
> 
> I think you need to pin down the definition of what's going on here a
> bit better.  Not all platforms have a concept of global IRQ number,
> and the usual qemu_irq model supports that.  So for this function who
> is it that is defining the number space in which pci_get_irq() is
> returning values.


The idea is that legacy interrupt (INTx) is caught by a host driver (for example, vfio-pci). A
driver disables it and transfers to a guest. In order to enable it back, a host driver has to make
sure that the interrupt has been handled by a guest. It is done by an End-Of-Interrupt (EOI) message
sent by a guest. Then QEMU forwards the message to a host driver which enables INTx back.

At the moment this mechanism - EOI callbacks - operates with global IRQ numbers, both on x86 (acpi)
and power (xics). So pci_get_irq returns only global numbers which PCIBus receives from the calling
code somehow (platform specific code knows how to initialize them, a bus cannot resolve it itself
anyway). And this is not dynamically changing information as PCI _domain_ hotplug does not exist (am
I right? :) ).

If we do not want pci_get_irq to return global IRQ numbers than it makes more sense to keep using
pins (A,B,C,D) plus some bus ID and not introduce any new numbers at all.

How can PCIBus::pci_get_irq() _callbacks_ be useful?



-- 
Alexey