Re: [KVM PATCH v10] kvm: add support for irqfd

Gregory Haskins
2009-06-14T12:41:19Z

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Michael S. Tsirkin wrote:
> On Wed, May 20, 2009 at 10:30:49AM -0400, Gregory Haskins wrote:
>
> ...
>
>
>> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
>> +static int
>> +kvm_assign_irqfd(struct kvm *kvm, int fd, int gsi)
>> +{
>> + struct _irqfd *irqfd;
>> + struct file *file = NULL;
>> + int ret;
>> +
>> + irqfd = kzalloc(sizeof(*irqfd), GFP_KERNEL);
>> + if (!irqfd)
>> + return -ENOMEM;
>> +
>> + irqfd->kvm = kvm;
>> + irqfd->gsi = gsi;
>> + INIT_LIST_HEAD(&irqfd->list);
>> + INIT_WORK(&irqfd->work, irqfd_inject);
>> +
>> + /*
>> + * Embed the file* lifetime in the irqfd.
>> + */
>> + file = fget(fd);
>> + if (IS_ERR(file)) {
>> + ret = PTR_ERR(file);
>> + goto fail;
>> + }
>> +
>> + /*
>> + * Install our own custom wake-up handling so we are notified via
>> + * a callback whenever someone signals the underlying eventfd
>> + */
>> + init_waitqueue_func_entry(&irqfd->wait, irqfd_wakeup);
>> + init_poll_funcptr(&irqfd->pt, irqfd_ptable_queue_proc);
>> +
>> + ret = file->f_op->poll(file, &irqfd->pt);
>> + if (ret < 0)
>> + goto fail;
>> +
>> + irqfd->file = file;
>> +
>> + mutex_lock(&kvm->lock);
>> + list_add_tail(&irqfd->list, &kvm->irqfds);
>> + mutex_unlock(&kvm->lock);
>> +
>> + return 0;
>> +
>> +fail:
>> + if (irqfd->wqh)
>> + remove_wait_queue(irqfd->wqh, &irqfd->wait);
>> +
>> + if (file && !IS_ERR(file))
>> + fput(file);
>> +
>> + kfree(irqfd);
>> + return ret;
>> +}
>>
>
> It seems that this lets the guest assign an unlimited number of fds
> to the same gsi, potentially using up all of kernel memory.
>
> Since we don't need multiple fds assigned to the same gsi (instead,
> multiple processes can write to the same eventfd to trigger an
> interrupt) let's simply check that no fd is yet assigned to this gsi.
>

I think Avi asked for this specific feature during review which is the
reason why its there today. However, I agree that it would probably be
a good idea to put an upper limit on the number of supported aliases
that can be registered. Will fix.

Thanks Michael,

-Greg

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