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

[Avi Kivity]

On 09/10/2012 04:29 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.
> 
> 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.
> diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c
> index adba28f..5cbba99 100644
> --- a/arch/x86/kvm/i8254.c
> +++ b/arch/x86/kvm/i8254.c
> @@ -302,8 +302,12 @@ static void pit_do_work(struct kthread_work *work)
>       }
>       spin_unlock(&ps->inject_lock);
>       if (inject) {
> -             kvm_set_irq(kvm, kvm->arch.vpit->irq_source_id, 0, 1);
> +             /* Clear previous interrupt, then create a rising
> +              * edge to request another interupt, and leave it at
> +              * level=1 until time to inject another one.
> +              */
>               kvm_set_irq(kvm, kvm->arch.vpit->irq_source_id, 0, 0);
> +             kvm_set_irq(kvm, kvm->arch.vpit->irq_source_id, 0, 1);
>  
>               /*

I thought I understood this, now I'm not sure.  How can this be correct?
 Real hardware doesn't act like this.

What if the PIT is disabled after this?  You're injecting a spurious
interrupt then.

-- 
error compiling committee.c: too many arguments to function