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Subject[PATCH 3.18 110/124] virtio_balloon: init 1st buffer in stats vq
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3.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ladi Prosek <lprosek@redhat.com>

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.

Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -355,6 +355,8 @@ 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)

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