Patch "virtio_balloon: init 1st buffer in stats vq" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

virtio_balloon: init 1st buffer in stats vq

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
virtio_balloon-init-1st-buffer-in-stats-vq.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Ladi Prosek <[email protected]>
Date: Thu, 23 Mar 2017 08:04:18 +0100
Subject: virtio_balloon: init 1st buffer in stats vq

From: Ladi Prosek <[email protected]>

commit fc8653228c8588a120f6b5dad6983b7b61ff669e upstream.

When init_vqs runs, virtio_balloon.stats is either uninitialized or contains stale values. The host updates its state with garbage data because it has no way of knowing that this is just a marker buffer used for signaling.

This patch updates the stats before pushing the initial buffer.

Alternative fixes:
* Push an empty buffer in init_vqs. Not easily done with the current virtio implementation and violates the spec "Driver MUST supply the same subset of statistics in all buffers submitted to the statsq".
* Push a buffer with invalid tags in init_vqs. Violates the same spec clause, plus "invalid tag" is not really defined.

Note: the spec says:
When using the legacy interface, the device SHOULD ignore all values in the first buffer in the statsq supplied by the driver after device initialization. Note: Historically, drivers supplied an uninitialized buffer in the first buffer.

Unfortunately QEMU does not seem to implement the recommendation even for the legacy interface.

---
 drivers/virtio/virtio_balloon.c |    2 ++
 1 file changed, 2 insertions(+)

@@ -416,6 +416,8 @@ static int init_vqs(struct virtio_balloo
 		 * Prime this virtqueue with one buffer so the hypervisor can
 		 * use it to signal us later (it can't be broken yet!).
 		 */
+		update_balloon_stats(vb);
+
 		sg_init_one(&sg, vb->stats, sizeof vb->stats);
 		if (virtqueue_add_outbuf(vb->stats_vq, &sg, 1, vb, GFP_KERNEL)
 		    < 0)

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/virtio_balloon-init-1st-buffer-in-stats-vq.patch queue-4.4/kvm-nvmx-invvpid-handling-improvements.patch