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

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH 1/2] pci: Add pci_device_get_host_irq
Date: Fri, 1 Jun 2012 16:27:46 +0300

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.

> 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.

> 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?
Maybe we can make the name imply that somehow.

> Any other suggestions?
> Jan
> -- 
> Siemens AG, Corporate Technology, CT T DE IT 1
> Corporate Competence Center Embedded Linux

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