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

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Jun 01, 2012 at 05:15:42PM +0200, Jan Kiszka wrote:
> 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.

1 and 2 are same really except 2 adds a hack for compatibility, no?