On 2012-06-01 17:30, Michael S. Tsirkin wrote: > 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?
Yes, 2 would allow us to maintain irq_count in different ways as well, specifically using it calculate shared output IRQ lines before reporting them to the PIC generically. This approach has both a charming and an ugly face. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux
