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

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

[-- Attachment #1: Type: text/plain, Size: 2232 bytes --]

On 2011-06-05 20:00, Alexander Graf wrote:
> 
> On 05.06.2011, at 19:56, Jan Kiszka wrote:
> 
>> 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).
> 
> Hrm, ok :). I guess the main reason we don't see major issues is that
> 
>   1) people don't use iothread too often yet - is it even enabled by default?

Nope (unless you use qemu-kvm.git next).

>   2) the decrementor interrupt happens in-kernel, so timer interrupts still arrive properly

Means PPC periodically returns to user space?

Jan


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 259 bytes --]