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

[Blue Swirl <blauwirbel@gmail.com>]

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.