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

[Avi Kivity <avi@redhat.com>]

  On 09/01/2010 12:38 PM, Alexander Graf wrote:
> On 01.09.2010, at 09:41, Avi Kivity wrote:
>
>> 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.
> My question was if you think the internal C interface is generic enough or if it needs a lot more magic for x86 anyways :).
>

So you noticed I avoided it.  Well, being forced to look, I don't think 
it's worthwhile to try to be generic here.  Both the PIC<->APIC and the 
APIC<->core interfaces are too complicated to be modelled by a single line.

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