Re: [Qemu-devel] [PATCH v5 5/6] i8259: fix so that dropping IRQ level always clears the interrupt request

[Jan Kiszka]

On 2012-09-11 06:32, Matthew Ogilvie wrote:
> On Mon, Sep 10, 2012 at 11:09:27AM +0200, Jan Kiszka wrote:
>> On 2012-09-10 10:56, Avi Kivity wrote:
>>> On 09/10/2012 04:27 AM, Matthew Ogilvie wrote:
>>>> Intel's definition of "edge triggered" means: "asserted with a
>>>> low-to-high transition at the time an interrupt is registered and
>>>> then kept high until the interrupt is served via one of the
>>>> EOI mechanisms or goes away unhandled."
>>>>
>>>> So the only difference between edge triggered and level triggered
>>>> is in the leading edge, with no difference in the trailing edge.
>>>
>>> Hard to believe.  So an edge while cpu interrupts are disabled is ignored?
>>
>> No, this is about the PIC, not the CPU interrupt inputs.
>>
>> Matthew, did you verify this on real hardware by reading back the IRR as
>> I suggested?
>>
>> Jan
> 
> I hadn't before, but now that I've checked, it's as expected:
> 
> -----------
> Real hardware [Pentium 4]:
> 
>   cmdRead unmask IRR=4005 mask IRR=4001 sti unmask irq14 IRR=0001 DONE
> 
> [I also see a final IRR=0000 occasionally.  Probably just happened to
> ask it while the timer (IRQ0) line is low (without the new understanding
> of the trailing edge of an edge triggered interrupt, this would
> have been confusing).  I have most IRQ's (including
> timer) masked off.]
> 
> -----------
> Unpatched qemu:
> 
>   cmdRead unmask IRR=4015 mask IRR=4015 sti irq15 unmask IRR=4015 DONE
> 
> [Presumably IRQ4 (0x10 - qemu's serial device model?) had a transient
> edge during initialization, but had been masked off even before I
> masked it off?]
> 
> -----------
> Patched qemu:
> 
>   cmdRead unmask IRR=4005 mask IRR=4001 sti unmask irq14 IRR=0001 DONE
> 

I think this is convincing, maybe worth documenting in the related
changelogs of QEMU and KVM. Avi, doubts remaining?

Jan

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