Re: [Qemu-devel] Re: [RFT][PATCH 07/15] qemu_irq: Add IRQ handlers with delivery feedback

[Gleb Natapov <gleb@redhat.com>]

On Sun, Jun 06, 2010 at 07:39:49AM +0000, Blue Swirl wrote:
> On Sun, Jun 6, 2010 at 7:15 AM, Gleb Natapov <gleb@redhat.com> wrote:
> > On Sat, Jun 05, 2010 at 02:04:01AM +0200, Jan Kiszka wrote:
> >> > I'd like to also support EOI handling. When the guest clears the
> >> > interrupt condtion, the EOI callback would be called. This could occur
> >> > much later than the IRQ delivery time. I'm not sure if we need the
> >> > result code in that case.
> >> >
> >> > If any intermediate device (IOAPIC?) needs to be informed about either
> >> > delivery or EOI also, it could create a proxy message with its
> >> > callbacks in place. But we need then a separate opaque field (in
> >> > addition to payload) to store the original message.
> >> >
> >> > struct IRQMsg {
> >> >  DeviceState *src;
> >> >  void (*delivery_cb)(IRQMsg *msg, int result);
> >> >  void (*eoi_cb)(IRQMsg *msg, int result);
> >> >  void *src_opaque;
> >> >  void *payload;
> >> > };
> >>
> >> Extending the lifetime of IRQMsg objects beyond the delivery call stack
> >> means qemu_malloc/free for every delivery. I think it takes a _very_
> >> appealing reason to justify this. But so far I do not see any use case
> >> for eio_cb at all.
> >>
> > I dislike use of eoi for reinfecting missing interrupts since
> > it eliminates use of internal PIC/APIC queue of not yet delivered
> > interrupts. PIC and APIC has internal queue that can handle two elements:
> > one is delivered, but not yet acked interrupt in isr and another is
> > pending interrupt in irr. Using eoi callback (or ack notifier as it's
> > called inside kernel) interrupt will be considered coalesced even if irr
> > is cleared, but no ack was received for previously delivered interrupt.
> > But ack notifiers actually has another use: device assignment. There is
> > a plan to move device assignment from kernel to userspace and for that
> > ack notifiers will have to be extended to userspace too. If so we can
> > use them to do irq decoalescing as well. I doubt they should be part
> > of IRQMsg though. Why not do what kernel does: have globally registered
> > notifier based on irqchip/pin.
> 
> Because translation at IOAPIC may be lossy, IRQs from many devices
> pointing to the same vector? With IRQMsg you know where a specific
> message came from. The situation is different inside the kernel: it
> manages both translation and registration, whereas in QEMU we could
> only control registration.
Configuring IOAPIC like that is against x86 architecture. OS will not be
able to map from interrupt vector back to device.

--
			Gleb.