Re: [PATCH v11 5/7] virtio_balloon: introduce migration primitives to balloon pages

Rafael Aquini [off-list ref] writes:

On Thu, Nov 08, 2012 at 09:32:18AM +1030, Rusty Russell wrote:

quoted

The first one can be delayed, the second one can be delayed if the host
didn't ask for VIRTIO_BALLOON_F_MUST_TELL_HOST (qemu doesn't).

We could implement a proper request queue for these, and return -EAGAIN
if the queue fills.  Though in practice, it's not important (it might
help performance).

I liked the idea. Give me the directions to accomplish it and I'll give it a try
for sure.

OK, let's get this applied first, but here are some pointers:

Here's the current callback function when the host has processed the
buffers we put in the queue:

 static void balloon_ack(struct virtqueue *vq)
 {
	struct virtio_balloon *vb = vq->vdev->priv;

	wake_up(&vb->acked);
 }

It's almost a noop: here's how we use it to make our queues synchronous:

 static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
 {
	struct scatterlist sg;
	unsigned int len;

	sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns);

	/* We should always be able to add one buffer to an empty queue. */
	if (virtqueue_add_buf(vq, &sg, 1, 0, vb, GFP_KERNEL) < 0)
		BUG();
	virtqueue_kick(vq);

	/* When host has read buffer, this completes via balloon_ack */
	wait_event(vb->acked, virtqueue_get_buf(vq, &len));
 }

And we set up the callback when we create the virtqueue:

	vq_callback_t *callbacks[] = { balloon_ack, balloon_ack, stats_request };
...
	err = vb->vdev->config->find_vqs(vb->vdev, nvqs, vqs, callbacks, names);

So off the top of my head it should be as simple as changing tell_host()
to only wait if the virtqueue_add_buf() fails (ie. queue is full).

Hmm, though you will want to synchronize the inflate and deflate queues:
if we tell the host we're giving a page up we want it to have seen that
before we tell it we're using it again...

Cheers,
Rusty.