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Subject: Re: [PATCH v22 2/3] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ

On 01/18/2018 12:44 AM, Michael S. Tsirkin wrote:
On Wed, Jan 17, 2018 at 01:10:11PM +0800, Wei Wang wrote:

+	struct scatterlist sg;
+	unsigned int unused;
+	int err;
+	sg_init_one(&sg, addr, sizeof(uint32_t));
This passes a guest-endian value to host. This is a problem:
should always pass LE values.

I think the endianness is handled when virtqueue_add_outbuf():

desc[i].addr = cpu_to_virtio64(_vq->vdev, addr);


+	/*
+	 * This handles the cornercase that the vq happens to be full when
+	 * adding a cmd id. Rarely happen in practice.
+	 */
+	while (!vq->num_free)
+		virtqueue_get_buf(vq, &unused);
I dislike this busy-waiting. It's a hint after all -
why not just retry later - hopefully after getting an

Alternatively, stop adding more entries when we have a single
ring entry left, making sure we have space for the command.

I think the second one looks good. Thanks.

+			queue_work(system_freezable_wq,
+				   &vb->update_balloon_size_work);
+		spin_unlock_irqrestore(&vb->stop_update_lock, flags);
+	}
+	virtio_cread(vb->vdev, struct virtio_balloon_config,
+		     free_page_report_cmd_id, &cmd_id);
You want virtio_cread_feature, don't access the new field
if the feature has not been negotiated.

Right. We probably need to put all the following cmd id related things under the feature check,

How about

if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_VQ)) {

+		WRITE_ONCE(vb->report_free_page, false);
+	} else if (cmd_id != vb->start_cmd_id) {
+		/*
+		 * Host requests to start the reporting by sending a new cmd
+		 * id.
+		 */
+		WRITE_ONCE(vb->report_free_page, true);
I don't know why we bother with WRITE_ONCE here.  The point of
report_free_page being used lockless is that that it's not a big deal if
it's wrong occasionally, right?

Actually the main reason is that "vb->report_free_page" is a value shared by two threads: Written by the config_change here, and read by the worker thread that reports the free pages.

Alternatively, we could let the two sides access to the shared variable with "volatile" pointers.

+		vb->start_cmd_id = cmd_id;
+		queue_work(vb->balloon_wq, &vb->report_free_page_work);
It seems that if a command was already queued (with a different id),
this will result in new command id being sent to host twice, which will
likely confuse the host.

I think that case won't happen, because
- the host sends a cmd id to the guest via the config, while the guest acks back the received cmd id via the virtqueue; - the guest ack back a cmd id only when a new cmd id is received from the host, that is the above check:

if (cmd_id != vb->start_cmd_id) { --> the driver only queues the reporting work only when a new cmd id is received
* Host requests to start the reporting by sending a
                         * new cmd id.
                        WRITE_ONCE(vb->report_free_page, true);
                        vb->start_cmd_id = cmd_id;
queue_work(vb->balloon_wq, &vb->report_free_page_work);

So the same cmd id wouldn't queue the reporting work twice.

+	}
  static void update_balloon_size(struct virtio_balloon *vb)
  	u32 actual = vb->num_pages;
@@ -417,40 +513,113 @@ static void update_balloon_size_func(struct work_struct *work)
static int init_vqs(struct virtio_balloon *vb)
-	struct virtqueue *vqs[3];
-	vq_callback_t *callbacks[] = { balloon_ack, balloon_ack, stats_request };
-	static const char * const names[] = { "inflate", "deflate", "stats" };
-	int err, nvqs;
+	struct virtqueue **vqs;
+	vq_callback_t **callbacks;
+	const char **names;
+	struct scatterlist sg;
+	int i, nvqs, err = -ENOMEM;
+	/* Inflateq and deflateq are used unconditionally */
+	nvqs = 2;
+	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ))
+		nvqs++;
+	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_VQ))
+		nvqs++;
+	/* Allocate space for find_vqs parameters */
+	vqs = kcalloc(nvqs, sizeof(*vqs), GFP_KERNEL);
+	if (!vqs)
+		goto err_vq;
+	callbacks = kmalloc_array(nvqs, sizeof(*callbacks), GFP_KERNEL);
+	if (!callbacks)
+		goto err_callback;
+	names = kmalloc_array(nvqs, sizeof(*names), GFP_KERNEL);
+	if (!names)
+		goto err_names;
Why not just keep these 3 arrays on stack? they aren't large.

Sounds good. Here is the new implementation:

static int init_vqs(struct virtio_balloon *vb)
        struct virtqueue *vqs[4];
        vq_callback_t *callbacks[4];
        const char *names[4];
        struct scatterlist sg;
        int ret;

         * Inflateq and deflateq are used unconditionally. stats_vq and
* free_page_vq uses names[2] and names[3], respectively. The names[]
         * will be NULL if the related feature is not enabled, which will
         * cause no allocation for the corresponding virtqueue in find_vqs.
        callbacks[0] = balloon_ack;
        names[0] = "inflate";
        callbacks[1] = balloon_ack;
        names[1] = "deflate";
        names[2] = NULL;
        names[3] = NULL;

        if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
                names[2] = "stats";
                callbacks[2] = stats_request;
        if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_VQ)) {
                names[3] = "free_page_vq";
                callbacks[3] = NULL;

ret = vb->vdev->config->find_vqs(vb->vdev, 4, vqs, callbacks, names,
                                         NULL, NULL);
        if (ret)
                return ret;

        vb->inflate_vq = vqs[0];
        vb->deflate_vq = vqs[1];

        if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
                vb->stats_vq = vqs[2];
* Prime this virtqueue with one buffer so the hypervisor can
                 * use it to signal us later (it can't be broken yet!).
                sg_init_one(&sg, vb->stats, sizeof(vb->stats));
                ret = virtqueue_add_outbuf(vb->stats_vq, &sg, 1, vb,
                if (ret) {
dev_warn(&vb->vdev->dev, "%s: add stat_vq failed\n",
                        return ret;

        if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_VQ))
                vb->free_page_vq = vqs[3];

        return 0;

Btw, the QEMU side doesn't have an option to disable STATS_VQ currently, we may need to add that later.


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