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

[Jan Kiszka]

On 2012-09-11 02:49, Maciej W. Rozycki wrote:
> 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.

As we do not clear IRQ14 in IRR of the slave nor do we clear IRQ2 of the
master, the master has a good reason to ask the slave for the vector.

Jan

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