[Qemu-devel] Re: Lost interrupts with upstream KVM

[Jan Kiszka]

Gleb Natapov wrote:
> On Fri, May 29, 2009 at 06:46:47PM +0200, Jan Kiszka wrote:
>> Gleb Natapov wrote:
>>> On Fri, May 29, 2009 at 04:52:41PM +0200, Jan Kiszka wrote:
>>>> Gleb Natapov wrote:
>>>>> On Fri, May 29, 2009 at 10:23:24AM +0200, Jan Kiszka wrote:
>>>>>> Hi Gleb,
>>>>>>
>>>>>> with latest kernel modules, namely beginning with 6bc0a1a235 (Remove
>>>>>> irq_pending bitmap), I'm loosing interrupts with upstream's KVM support.
>>>>>> After some bisecting, hair-pulling and a bit meditation I added a
>>>>>> WARN_ON(kvm_cpu_has_interrupt(vcpu)) to kvm_vcpu_ioctl_interrupt, and it
>>>>>> actually triggered right before the guest got stuck.
>>>>>>
>>>>>> This didn't trigger with qemu-kvm (and -no-kvm-irqchip) yet but, on the
>>>>>> other hand, I currently do not see a potential bug in upstream's
>>>>>> kvm_arch_pre_run. Could you have a look if you can reproduce,
>>>>>> specifically if this isn't a KVM kernel issue in the end?
>>>>>>
>>>>> In kvm_cpu_exec() after calling kvm_arch_pre_run() env->exit_request is
>>>>> tested and function can exit without calling kvm_vcpu_ioctl(KVM_RUN).
>>>>> Can you check if this what happens in your case?
>>>> This path is executed quite frequently here. No obvious correlation with
>>>> the lost IRQ.
>>>>
>>> If kvm_arch_pre_run() injected interrupt kvm_vcpu_ioctl(KVM_RUN) have to
>>> be executed before injecting another interrupt, so if on the fist call
>>> of kvm_cpu_exec() kvm_arch_pre_run() injected interrupt, but
>>> kvm_vcpu_ioctl(KVM_RUN) was not executed because of env->exit_request
>>> and on the next kvm_cpu_exec() other interrupt is injected the previous
>>> one will be lost.
>> ...and kvm_run->ready_for_interrupt_injection is not updated either in
>> that case, right? That makes be wonder if KVM_INTERRUPT shouldn't better
>> return an error in case the queue is full already.
>>
> If kvm_vcpu_ioctl(KVM_RUN) is called, but exit happens before interrupt
> is injected kvm_run->ready_for_interrupt_injection should be update to
> reflect that fact.

Yes, but in this case it isn't called if IIUC. So that is the problem
upstream KVM faces?

Then again: What do you think is the proper long-term fix? Only
adjusting upstream KVM (required anyway) or also making the kernel
support more robust against this pattern?

Jan

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