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