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

[Gleb Natapov <gleb@redhat.com>]

On Wed, May 26, 2010 at 10:09:52PM +0200, Jan Kiszka wrote:
> Blue Swirl wrote:
> > On Tue, May 25, 2010 at 9:44 PM, Jan Kiszka <jan.kiszka@web.de> wrote:
> >> Anthony Liguori wrote:
> >>> On 05/25/2010 02:09 PM, Blue Swirl wrote:
> >>>> On Mon, May 24, 2010 at 8:13 PM, Jan Kiszka<jan.kiszka@web.de>  wrote:
> >>>>
> >>>>> From: Jan Kiszka<jan.kiszka@siemens.com>
> >>>>>
> >>>>> This allows to communicate potential IRQ coalescing during delivery from
> >>>>> the sink back to the source. Targets that support IRQ coalescing
> >>>>> workarounds need to register handlers that return the appropriate
> >>>>> QEMU_IRQ_* code, and they have to propergate the code across all IRQ
> >>>>> redirections. If the IRQ source receives a QEMU_IRQ_COALESCED, it can
> >>>>> apply its workaround. If multiple sinks exist, the source may only
> >>>>> consider an IRQ coalesced if all other sinks either report
> >>>>> QEMU_IRQ_COALESCED as well or QEMU_IRQ_MASKED.
> >>>>>
> >>>> No real devices are interested whether any of their output lines are
> >>>> even connected. This would introduce a new signal type, bidirectional
> >>>> multi-level, which is not correct.
> >>>>
> >>> I don't think it's really an issue of correct, but I wouldn't disagree
> >>> to a suggestion that we ought to introduce a new signal type for this
> >>> type of bidirectional feedback.  Maybe it's qemu_coalesced_irq and has a
> >>> similar interface as qemu_irq.
> >> A separate type would complicate the delivery of the feedback value
> >> across GPIO pins (as Paul requested for the RTC->HPET routing).
> >>
> >>>> I think the real solution to coalescing is put the logic inside one
> >>>> device, in this case APIC because it has the information about irq
> >>>> delivery. APIC could monitor incoming RTC irqs for frequency
> >>>> information and whether they get delivered or not. If not, an internal
> >>>> timer is installed which injects the lost irqs.
> >> That won't fly as the IRQs will already arrive at the APIC with a
> >> sufficiently high jitter. At the bare minimum, you need to tell the
> >> interrupt controller about the fact that a particular IRQ should be
> >> delivered at a specific regular rate. For this, you also need a generic
> >> interface - nothing really "won".
> > 
> > OK, let's simplify: just reinject at next possible chance. No need to
> > monitor or tell anything.
> 
> There are guests that won't like this (I know of one in-house, but
> others may even have more examples), specifically if you end up firing
> multiple IRQs in a row due to a longer backlog. For that reason, the RTC
> spreads the reinjection according to the current rate.
> 
No need to look far for such guests. See commit dd17765b5f77ca.

> And even if the rate did not matter, the APIC woult still have to now
> about the fact that an IRQ is really periodic and does not only appear
> as such for a certain interval. This really does not sound like
> simplifying things or even make them cleaner.
> 
> Jan
> 



--
			Gleb.