[Qemu-devel] Re: [PATCH 8/8] ahci: fix !msi interrupts

[Alexander Graf]

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?


Alex