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

[Alexander Graf]

On 01.09.2010, at 13:45, Avi Kivity wrote:

> 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.

Makes sense. Well, I guess it doesn't hurt to have the interface as is and only 
implement it for PPC for now.


Alex