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Date: Mon, 17 Jan 2011 17:33:31 +0100
From: Alexander Graf <agraf@suse.de>
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Subject: [Qemu-devel] Re: [PATCH 8/8] ahci: fix !msi interrupts
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To: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Joerg Roedel <Joerg.Roedel@amd.com>, Sebastian Herbszt <herbszt@gmx.de>, qemu-devel Developers <qemu-devel@nongnu.org>, Gerd Hoffmann <kraxel@redhat.com>

Jan Kiszka wrote:
> On 2011-01-17 17:04, Alexander Graf wrote:
>   
>> Jan Kiszka wrote:
>>     
>>> On 2011-01-17 17:00, Alexander Graf wrote:
>>>   
>>>       
>>>> Jan Kiszka wrote:
>>>>     
>>>>         
>>>>> On 2010-12-20 22:13, Alexander Graf wrote:
>>>>>   
>>>>>       
>>>>>           
>>>>>> When not using MSI, receiving an interrupt while the interrupt line is active
>>>>>> pulses the interrupt line. Without this, guests don't realize that a new
>>>>>> interrupt occured.
>>>>>>     
>>>>>>         
>>>>>>             
>>>>> This doesn't look OK. The device model should look at the currently used
>>>>> mode and switch between edge and level triggering accordingly. As it
>>>>> appears like this is what it already does, this change may just paper
>>>>> over the real issue.
>>>>>   
>>>>>       
>>>>>           
>>>> Well, I have this internal abstraction to make edge and level triggered
>>>> interrupt triggering easier. irq_lower is a simple nop for the edge case.
>>>>
>>>>     
>>>>         
>>> I'm concerned about the artificial edge you generate for the level
>>> triggered case. That's not like real hw behaves. If you need it,
>>> something else might still be broken.
>>>   
>>>       
>> Hrm. So worst case we generate a spurious interrupt?
>>
>>     
>
> Worse might also be that unknown issue that force you to inject an IRQ
> here. We don't know. That's probably worst.
>   

Well, IIRC the issue was that usually a level high interrupt line would
simply retrigger an interrupt after enabling the interrupt line in the
APIC again. Unless my memory completely fails on me, that didn't happen,
so I added the manual retrigger on a partial command ACK in ahci code.


Alex