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

[Blue Swirl]

On Sun, Jun 6, 2010 at 7:15 AM, Gleb Natapov <address@hidden> 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.