Re: [Qemu-devel] [PATCH 1/2] pci: Add pci_device_get_host_irq

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2012-06-01 16:34, Michael S. Tsirkin wrote:
> On Fri, Jun 01, 2012 at 03:57:01PM +0200, Jan Kiszka wrote:
>> On 2012-06-01 15:27, Michael S. Tsirkin wrote:
>>> On Fri, Jun 01, 2012 at 02:52:56PM +0200, Jan Kiszka wrote:
>>>> On 2012-05-30 22:31, Michael S. Tsirkin wrote:
>>>>>>> So we'll just have PIIX_NUM_PIC_IRQS entries there and use
>>>>>>> irq_count instead of the pic_levels bitmap.
>>>>>>
>>>>>> Just that this affects generic PCI code, not only PIIX-specific things.
>>>>>
>>>>> Yes but it's not a problem - pci_bus_irqs sets the map function and nirqs.
>>>>>
>>>>>> And that we need to save/restore some irq_count field according to the
>>>>>> old semantics.
>>>>>
>>>>> Well, it's a bug: this is redundant info we should not have exposed it.
>>>>>
>>>>> Anyway, let's make the rest work properly and cleanly first, add a FIXME
>>>>> for now, then we'll find a hack making it work for migration.
>>>>
>>>> It remains non-trivial: I got your patch working (a minor init issue),
>>>> but yet without changing the number of IRQs for PIIX3, so keeping the
>>>> irq_count semantics for this host bridge.
>>>>
>>>> Now I'm facing three possibilities of how to proceed:
>>>
>>> They all look OK I think :) Some comments below.
>>>
>>>> 1. Give up on the (currently broken) feature to write a vmstate for
>>>>    older QEMU versions.
>>>>
>>>>    This will allow to declare the irq_count field in vmstate_pcibus
>>>>    unused, and we would have to restore it on vmload step-wise via the
>>>>    PCI devices. It would also allow to change its semantics for PIIX3,
>>>>    mapping directly to PIC IRQs.
>>>
>>> I think that's okay too simply because these things are usually
>>> easy to fix after the fact when the rest of the issues are addressed.
>>
>> Don't get what you mean with "fixed". If we fix the vmstate generation
>> in making it backward-compatible again, we enter option 2. Option 1 is
>> explicitly about giving this up.
> 
> What I really mean is I think I see how 2 can be added without much
> pain. So let's focus on 1 for now and worst case we break migration.

I'd like to avoid planing for this worst case as long as there are also
statements [1] that this is not acceptable for QEMU in general. It
doesn't to create a beautiful architecture initially about which we
already know that it will become more complex than alternatives in the end.

> 
>>>
>>>> 2. Keep writing a legacy irq_count field.
>>>>
>>>>    This will require quite a few new APIs so that host bridges that
>>>>    want to change their nirq can still generate a compatible irq_count
>>>>    vmstate field. Namely:
>>>>     - A function to set up vmstate_irq_count and define a callback that
>>>>       the core will invoke to prepare the vmstate_irq_count before
>>>>       vmsave.
>>>>     - A function to obtain the IRQ mapping /without/ the final host
>>>>       bridge step. This is required so that the callback above can
>>>>       calculate the old state like in the PIIX3 case.
>>>
>>> Does this really need to be so complex? It seems that we just need
>>> pci_get_irq_count(bus, irq) which can use the existing map_irq API, no?
>>> Then invoke that before save.
>>
>> No, because the new map_irq of the PIIX3 bridge will also include the
>> host bridge routing (or masking) according to the PIRQx routoing
>> registers of the PIIX3. Moreover, the fixup of the written legacy
>> irq_count state has to happen in the PCI layer, which therefore has to
>> query the host bridge for fixup information, not the other way around
>> (because the PCI bus vmstate is separate from the PIIX3 host bridge).
>>
>>>
>>>> 3. Keep irq_count and nirq as is, introduce additional map_host_irq.
>>>>
>>>>    This is simpler than 2 and more compatible than 1. It would also
>>>>    allow to introduce the polarity and masking information more
>>>>    smoothly as we won't have to add it to existing map_irq callbacks
>>>>    then.
>>>
>>> So what does it map, and to what?
>>
>> PCI bus IRQ to *host* IRQ. It is supposed to explore the routing of the
>> host bridge between the root bus and the host's interrupt controller
>> (i.e. the step that is currently missing the cached chain).
>>
>>> Maybe we can make the name imply that somehow.
>>
>> Better suggestions for this handler and maybe also the existing map_irq
>> are welcome to make the difference clearer.
>>
>> Jan
> 
> So I won't object to adding a new API but if we do
> it properly this won't help compatibility :(

It will as this API does not touch the parts that affect the vmstate
(ie. semantics of irq_count won't change).

> 
> Let's formulate what these do exactly, this will
> also help us come up with sensible names.
> 
> 1. The difference is that pci bridges route interrupt pins. So it gets
> interrupt pin on device and returns interrupt pin on connector. All
> attributes are standard PCI.  We should remove all mentions of "irq"
> really.
> 
> 
> 2. The pci root (yes it's a host bridge but let's
> not use the term host if we can) routes
> an interrupt pin on device to a host irq. It can also
> do more things like invert polarity.
> 
> So yes we can add 2 to piix but we really should
> remove 1 from it.
> 
> Wrt names - do you object to long names?
> How about route_interrupt_pin for 1
> and route_interrupt_pin_to_irq for 2?

I'm fine with this.

Jan

[1] http://permalink.gmane.org/gmane.comp.emulators.qemu/153357

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux