Re: [RFC] Deferred interrupt handling.

Dor Laor
2007-07-18T17:09:37Z

>Alan Cox wrote:
>>> What if we will force the specific device to the end of the list.
>Once
>>> IRQ_NONE was returned by the other devices, we will mask the irq,
>>> forward the irq to the guest, issue a timer for 1msec. Motivation:
>>> 1msec is long enough for the guest to ack the irq + host unmask the
>irq
>>>
>>
>> It makes no difference. The deadlock isn't fixable by timing hacks.
>> Consider the following sequence
>>
>>
>> Guest0 - blocked on I/O
>>
>> IRQ14 from your hardware
>> Block IRQ14
>> Sent to guest (guest is blocked)
>>
>> IRQ14 from hard disk
>> Ignored (as blocked)
>>

But now the timer will pop and the hard disk will get its irq.
The guest will be released right after.

>> Deadlock
>>
>
>IMO the only reasonable solution is to disallow interrupt forwarding
>with shared irqs. If someone later comes up with a bright idea, we can
>implement it. Otherwise the problem will solve itself with hardware
>moving to msi.
>

I though of that but the problem is that we'd like to use it with
current hardware
devices that are shared.
:(