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.
> 
> > 
> >>>
> >>>> 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.
> >>
> >> No, because the new map_irq of the PIIX3 bridge will also include the
> >> host bridge routing (or masking) according to the PIRQx routoing
> >> registers of the PIIX3. Moreover, the fixup of the written legacy
> >> irq_count state has to happen in the PCI layer, which therefore has to
> >> query the host bridge for fixup information, not the other way around
> >> (because the PCI bus vmstate is separate from the PIIX3 host bridge).
> >>
> >>>
> >>>> 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?
> >>
> >> PCI bus IRQ to *host* IRQ. It is supposed to explore the routing of the
> >> host bridge between the root bus and the host's interrupt controller
> >> (i.e. the step that is currently missing the cached chain).
> >>
> >>> Maybe we can make the name imply that somehow.
> >>
> >> Better suggestions for this handler and maybe also the existing map_irq
> >> are welcome to make the difference clearer.
> >>
> >> Jan
> > 
> > So I won't object to adding a new API but if we do
> > it properly this won't help compatibility :(
> 
> It will as this API does not touch the parts that affect the vmstate
> (ie. semantics of irq_count won't change).

Yes but irq_count in vmstate is a bug. IMO even if we do
not change anything we should ignore irq_count on
load and recalculate it from what the devices supply.

> > 
> > Let's formulate what these do exactly, this will
> > also help us come up with sensible names.
> > 
> > 1. The difference is that pci bridges route interrupt pins. So it gets
> > interrupt pin on device and returns interrupt pin on connector. All
> > attributes are standard PCI.  We should remove all mentions of "irq"
> > really.
> > 
> > 
> > 2. The pci root (yes it's a host bridge but let's
> > not use the term host if we can) routes
> > an interrupt pin on device to a host irq. It can also
> > do more things like invert polarity.
> > 
> > So yes we can add 2 to piix but we really should
> > remove 1 from it.
> > 
> > Wrt names - do you object to long names?
> > How about route_interrupt_pin for 1
> > and route_interrupt_pin_to_irq for 2?
> 
> I'm fine with this.
> 
> Jan
> 
> [1] http://permalink.gmane.org/gmane.comp.emulators.qemu/153357
> 
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