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

[Matthew Ogilvie <mmogilvi_qemu@miniinfo.net>]

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


          - Matthew