Re: [KVM PATCH v4 2/2] kvm: add support for irqfd via eventfd-notification interface

Avi Kivity
2009-05-05T18:11:37Z

Gregory Haskins wrote:
> Avi Kivity wrote:
>
>> Gregory Haskins wrote:
>>
>>
>>> +int
>>> +kvm_irqfd(struct kvm *kvm, int gsi, int flags)
>>> +{
>>> + struct _irqfd *irqfd;
>>> + struct file *file = NULL;
>>> + int fd = -1;
>>> + int ret;
>>> +
>>> + irqfd = kzalloc(sizeof(*irqfd), GFP_KERNEL);
>>> + if (!irqfd)
>>> + return -ENOMEM;
>>> +
>>> + irqfd->kvm = kvm;
>>>
>>>
>> You need to increase the refcount on struct kvm here. Otherwise evil
>> userspace will create an irqfd, close the vm and vcpu fds, and inject
>> an interrupt.
>>
>
> I just reviewed the code in prep for v5, and now I remember why I didnt
> take a reference: I designed it the opposite direction: the vm-fd owns
> a reference to the irqfd, and will decouple the kvm context from the
> eventfd on shutdown (see kvm_irqfd_release()). I still need to spin a
> v5 regardless in order to add the padding as previously discussed. But
> let me know if you still see any holes in light of this alternate object
> lifetime approach I am using.
>

Right, irqfd_release works. But I think refcounting is simpler, since
we already kvm_get_kvm() and kvm_put_kvm(), and you wouldn't need the
irqfd list. On the other hand, I'm not sure you get a callback from
eventfd on close(), so refcounting may not be implementable.

Drat, irqfd_release doesn't work. You reference kvm->lock in
irqfd_inject without taking any locks.

btw, there's still your original idea of creating the eventfd in
userspace and passing it down. That would be workable if we can see a
way to both signal the eventfd and get called back in irq context.
Maybe that's preferable to what we're doing here, but we need to see how
it would work.

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.