[Qemu-devel] Re: [RFC] KVM: PPC: Add level based interrupt logic

[Avi Kivity <avi@redhat.com>]

  On 08/31/2010 01:07 PM, Alexander Graf wrote:
> KVM on PowerPC used to have completely broken interrupt logic. Usually,
> interrupts work by having a PIC that pulls a line up/down, so the CPU knows
> that an interrupt is active. This line stays active until some action is
> done to the PIC to release the line.
>
> On KVM for PPC, we just checked if there was an interrupt pending and pulled
> a line in the kernel module. We never released it though, hoping that kernel
> space would just declare an interrupt as released when injected - which is
> wrong.
>
> To fix this, we need to completely redesign the interrupt injection logic.
> Whenever an interrupt line gets triggered, we need to notify kernel space
> that the line is up. Whenever it gets released, we do the same. This way
> we can assure that the interrupt state is always known to kernel space.
>
> This fixes random stalls in KVM guests on PowerPC that were waiting for
> an interrupt while everyone else thought they received it already.

This is more or less equivalent to KVM_IRQ_LINE.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.