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Date: Tue, 18 Jan 2011 13:58:25 +0100
From: Jan Kiszka <jan.kiszka@siemens.com>
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Subject: [Qemu-devel] Re: [PATCH 8/8] ahci: fix !msi interrupts
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To: Alexander Graf <agraf@suse.de>
Cc: Kevin Wolf <kwolf@redhat.com>, Joerg Roedel <Joerg.Roedel@amd.com>, Sebastian Herbszt <herbszt@gmx.de>, Gerd Hoffmann <kraxel@redhat.com>, qemu-devel Developers <qemu-devel@nongnu.org>

On 2011-01-18 13:05, Alexander Graf wrote:
> 
> On 18.01.2011, at 10:08, Gerd Hoffmann wrote:
> 
>>  Hi,
>>
>>>> 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.
>>
>> edge triggered interrupts wouldn't though.
> 
> The code change doesn't change anything for edge triggered interrupts - they work fine. Only !msi (== level) ones are broken.
> 
>>
>>> 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.
>>
>> That re-trigger smells like you are not clearing the interrupt line where you should.  For starters try calling ahci_check_irq() after guest writes to PORT_IRQ_STAT.
> 
> The problem happened when I had the following:
> 
> ahci irq bits = 0
> <events happen>
> ahci irq bits = 1 | 2
> irq line trigger
> guest clears 1
> ahci bits = 2
> <no irq line trigger, since we're still irq high>
> 
> On normal hardware, the high state of the irq line would simply trigger another interrupt in the guest when it gets ACKed on the LAPIC. Somehow it doesn't get triggered here. I don't see why clearing the interrupt line would help? It's not what real hardware would do, no?
> 

No, real devices would simply leave the line asserted as long as there
is a reason to.

So again my question: With which irqchips do you see this effect, kvm's
in-kernel model and/or qemu's user space model? If there is an issue
with retriggering a CPU interrupt while the source is still asserted,
that probably needs to be fixed.

Jan

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