Re: [KVM PATCH v2 2/2] kvm: add support for irqfd via eventfd-notification interface

Avi Kivity
2009-04-27T10:48:25Z

Gregory Haskins wrote:
>>> This allows an eventfd to be registered as an irq source with a
>>> guest. Any
>>> signaling operation on the eventfd (via userspace or kernel) will inject
>>> the registered GSI at the next available window.
>>>
>>>
>>> +struct kvm_irqfd {
>>> + __u32 fd;
>>> + __u32 gsi;
>>> +};
>>> +
>>>
>>>
>> I think it's better to have ioctl create and return the fd. This way
>> we aren't tied to eventfd (though it makes a lot of sense to use it).
>>
>
> I dont mind either way, but I am not sure it buys us much as the one
> driving the fd would need to understand if the interface is
> eventfd-esque or something else anyway. Let me know if you still want
> to see this changed.
>

Sure, the interface remains the same (write 8 bytes), but the
implementation can change. For example, we can implement it to work
from interrupt context, once we hack the locking appropriately.

>>> +static void
>>> +irqfd_inject(struct work_struct *work)
>>> +{
>>> + struct _irqfd *irqfd = container_of(work, struct _irqfd, work);
>>> + struct kvm *kvm = irqfd->kvm;
>>> +
>>> + mutex_lock(&kvm->lock);
>>> + kvm_set_irq(kvm, kvm->irqfd.src, irqfd->gsi, 1);
>>>
>>>
>> Need to lower the irq too (though irqfd only supports edge triggered
>> interrupts).
>>
>>
> Should I just do back-to-back 1+0 inside the same lock?
>
>

Yes. Might be nice to add a kvm_toggle_irq(), but let's leave that
until later.

>> One day we'll have lockless injection and we'll want to drop this. I
>> guess if we create the fd ourselves we can make it work, but I don't
>> see how we can do this with eventfd.
>>
>>
>
> Hmm...this is a good point. There probably is no way to use eventfd
> "off the shelf" in a way that doesn't cause this callback to be in a
> critical section. Should we just worry about switching away from
> eventfd when this occurs, or should I implement a custom anon-fd now?
>

I'd just go with eventfd, and switch when it becomes relevant. As long
as the kernel allocates the fd, we're free to do as we like.

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