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

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Message-ID: <469E43D5.70802@qumranet.com>
Date:	Wed, 18 Jul 2007 19:46:13 +0300
From:	Avi Kivity <avi@...ranet.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	Dor Laor <dor.laor@...ranet.com>, 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)
>
> 	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.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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