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From: Alexander Graf <agraf@suse.de>
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Date: Tue, 18 Jan 2011 13:05:11 +0100
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Subject: [Qemu-devel] Re: [PATCH 8/8] ahci: fix !msi interrupts
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To: Gerd Hoffmann <kraxel@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Jan Kiszka <jan.kiszka@siemens.com>, Sebastian Herbszt <herbszt@gmx.de>, qemu-devel Developers <qemu-devel@nongnu.org>, Joerg Roedel <Joerg.Roedel@amd.com>


On 18.01.2011, at 10:08, Gerd Hoffmann wrote:

>  Hi,
>=20
>>> Worse might also be that unknown issue that force you to inject an =
IRQ
>>> here. We don't know. That's probably worst.
>>=20
>> 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.
>=20
> edge triggered interrupts wouldn't though.

The code change doesn't change anything for edge triggered interrupts - =
they work fine. Only !msi (=3D=3D level) ones are broken.

>=20
>> 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.
>=20
> 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 =3D 0
<events happen>
ahci irq bits =3D 1 | 2
irq line trigger
guest clears 1
ahci bits =3D 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?


Alex