Re: [Qemu-devel] [PATCH] pc: Clean up PIC-to-APIC IRQ path

[Anthony Liguori <anthony@codemonkey.ws>]

On 09/04/2011 10:20 AM, Blue Swirl wrote:
> On Sun, Sep 4, 2011 at 2:37 PM, Anthony Liguori<anthony@codemonkey.ws>  wrote:
>> On 09/04/2011 08:57 AM, Anthony Liguori wrote:
>>>
>>> On 09/04/2011 08:49 AM, Jan Kiszka wrote:
>>>>
>>>> On 2011-09-04 15:41, Anthony Liguori wrote:
>>>>>
>>>>> On 09/04/2011 08:36 AM, Jan Kiszka wrote:
>>>>> Having some sort of global interrupt routing table is just going to add
>>>>> a layer of complexity for very little obvious gain.
>>>>
>>>> It's not yet decided how the problem is solved. A global interrupt
>>>> matrix is just one proposal, another option is to extend the pin model
>>>> in a way that supports routing change notifiers and backward polling.
>>>
>>> If that's all you need, then you really just want notification on socket
>>> changes. Backwards polling can be achieved by just adding state to the
>>> Pin (which I full heartedly support).
>>>
>>> If that's all you're proposing, than I'm entirely happy with it :-)
>>
>> It's not that simple.
>>
>> Routing paths can change because of device changes, not just socket changes.
>
> Yes, that's why callbacks are needed to let the device inform global matrix.
>
>> I think you would need an interface for irq routing.  Something like:
>>
>> struct IrqRouter {
>>     Interface parent;
>>
>>     void (*foreach_output)(IrqRouter *obj,
>>                            void (*fn)(const char *out, void *opaque),
>>                            void *opaque);
>>
>>     void (*foreach_input)(IrqRouter *obj,
>>                           void (*fn)(const char *in, void *opaque),
>>                           void *opaque);
>
> Are the above a way for a parent to tell this device that its inputs
> are changed?

No... there really isn't a notion of parent/children.

If something is interested in the routing path, it needs to listen for 
mapping changes at every IrqRouter point.  I neglected to add an 
interface for registering for mapping changes so there would also need 
to be a notification registration interface here.

>
>>     const char *(*get_mapping)(IrqRouter *obj, const char *in);
>
> This would be useful for the matrix too, the matrix would use this to
> query for initial routing, maybe also for later changes if the change
> callback didn't tell the mapping directly.

I think the main difference is, do you try to solve the problem 
everywhere, or do you just allow for certain paths to be introspected. 
And then who is reasonable for maintaining the full path.

>
>> };
>>
>> You could then implement this interface in I440FX or any other controller
>> where we want to be able to support device passthrough.
>>
>> Representing endpoints as strings means that you can correlate inputs to
>> outputs throughout the chain provided that you understand how inputs/outputs
>> relate to plugs/sockets.
>
> The string format assumes that there is a reliable way to convert Pin
> to string and vice versa. Why can't they be array of pointers to Pins?

Yes, all Pins are devices and all devices have unique names.  You could 
just as easily use Pin *s.

It's not clear to me whether you want to make the strings device names 
or just property names though.

>
> String representation (e.g. "/i440fx/pci-bridge@2,0/serial@3fc:2")
> could be useful for debugging and 'info qtree' ('info irqtree'?)
> though.

IIUC, then this would be (in QOM)

i440fx::piix3::serial[0]::irq

This is the unique name the device would get.  That's because the PIIX3 
and Serial devices are created via composition.  You could indirectly 
reference it multiple ways:

/i440fx/slot[2]/serial[0]/irq
/i440fx/piix3/serial[0]/irq

Regards,

Anthony Liguori