Rafael Aquini <aquini@xxxxxxxxxx> writes:
> On Thu, Nov 08, 2012 at 09:32:18AM +1030, Rusty Russell wrote:
>> 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.
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