Re: [Android-virt] [PATCH v3 1/8] ARM: KVM: Initial skeleton to compile KVM support

[Jan Kiszka <jan.kiszka@web.de>]

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On 2011-06-05 19:54, Alexander Graf wrote:
> 
> On 05.06.2011, at 19:48, Jan Kiszka wrote:
> 
>> On 2011-06-05 19:19, Alexander Graf wrote:
>>>
>>> On 05.06.2011, at 18:33, Avi Kivity wrote:
>>>
>>>> On 06/05/2011 07:30 PM, Alexander Graf wrote:
>>>>>>>
>>>>>>> Could you elaborate what you mean here? I'm not really following. Are
>>>>>>> you suggesting a new arch-generic interface? (Pardon my ignorance).
>>>>>>
>>>>>> Using KVM_IRQ_LINE everywhere except s390, not just in x86 and ARM.
>>>>>
>>>>> An in-kernel MPIC implementation is coming for PPC, so I don't see any reason to switch from something that works now.
>>>>
>>>> Right, this is spilled milk.
>>>>
>>>> Does the ppc qemu implementation raise KVM_INTERRUPT solely from the vcpu thread?
>>>
>>> Well, without iothread it used to obviously. Now that we have an iothread, it calls ioctl(KVM_INTERRUPT) from a separate thread. The code also doesn't forcefully wake up the vcpu thread, so yes, I think here's a chance for at least delaying interrupt delivery. Chances are pretty slim we don't get out of the vcpu thread at all :).
>>
>> There are good chances to run into a deadlock when calling a per-vcpu
>> IOCTL over a foreign context: calling thread holds qemu_mutex and blocks
>> on kvm_mutex inside the kernel, target vcpu is running endless guest
>> loop, holding kvm_mutex, all other qemu threads will sooner or later
>> block on the global lock. That's at least one pattern you can get on x86
>> (we had a few of such bugs in the past).
> 
> Any recommendations? Should we just signal the main thread when we want to inject an interrupt?

Yep. That's also what x86 does (when using user space irqchips).

Jan


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