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

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Date:	Sun, 14 Jun 2009 08:40:57 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, avi@...hat.com,
	davidel@...ilserver.org, mtosatti@...hat.com
Subject: Re: [KVM PATCH v10] kvm: add support for irqfd

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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