Re: [Qemu-devel] [PATCH 1/2] KVM: fix i8259 interrupt high to low transition logic

[Maciej W. Rozycki]

On Sun, 9 Sep 2012, Matthew Ogilvie wrote:

> 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 UNIX kernel.

 That is quite weird actually -- from my experience the spurious vector is 
never sent from a slave (quite understandably -- since the interrupt is 
gone and no other is pending, the master has no reason to select a slave 
to supply a vector and therefore supplies the spurious vector itself) and 
therefore a spurious IRQ7 is always issued regardless of whether the 
discarded request came from a slave or from the master.

 Is there a bug elsewhere then too?  I would have expected a reasonable 
(and especially an old-school) x86 OS to be able to cope with spurious 
8259A interrupts, but then obviously one would expect them on IRQ7 only.

  Maciej