linux-kernel - RE: [kvm-devel] [RFC] Deferred interrupt handling.

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Message-ID: <64F9B87B6B770947A9F8391472E032160CC16D06@ehost011-8.exch011.intermedia.net>
Date:	Wed, 18 Jul 2007 10:09:37 -0700
From:	"Dor Laor" <dor.laor@...ranet.com>
To:	"Avi Kivity" <avi@...ranet.com>,
	"Alan Cox" <alan@...rguk.ukuu.org.uk>
Cc:	<kvm-devel@...ts.sourceforge.net>, <linux-kernel@...r.kernel.org>
Subject: RE: [kvm-devel] [RFC] Deferred interrupt handling.

>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.
:(
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