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

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

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:

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.

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.

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.

Any other suggestions?

Jan

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