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Date: Sun, 6 Jun 2010 10:15:36 +0300
From: Gleb Natapov <gleb@redhat.com>
Subject: Re: [Qemu-devel] Re: [RFT][PATCH 07/15] qemu_irq: Add IRQ handlers
	with delivery feedback
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To: Jan Kiszka <jan.kiszka@web.de>
Cc: Blue Swirl <blauwirbel@gmail.com>, qemu-devel@nongnu.org, Juan Quintela <quintela@redhat.com>

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.

--
			Gleb.