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

[Matthew Ogilvie]

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.

This bug manifested itself when the guest was Microport UNIX
System V/386 v2.1 (ca. 1987), because it would sometimes mask
off IRQ14 in the slave IMR after it had already been asserted.
The master would still try to deliver an interrupt even though
IRQ2 had dropped again, resulting in a spurious interupt
(IRQ15) and a panicked kernel.

Signed-off-by: Matthew Ogilvie <address@hidden>
---

If you missed the previous thread about this, see
http://www.mail-archive.com/address@hidden/msg129071.html

 hw/i8259.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/hw/i8259.c b/hw/i8259.c
index 6587666..c011787 100644
--- a/hw/i8259.c
+++ b/hw/i8259.c
@@ -157,6 +157,7 @@ static void pic_set_irq(void *opaque, int irq, int level)
             }
             s->last_irr |= mask;
         } else {
+            s->irr &= ~mask;
             s->last_irr &= ~mask;
         }
     }
-- 
1.7.10.2.484.gcd07cc5