[PATCH 1/2] virtio_balloon: add work around for out of spec QEMU

["Michael S. Tsirkin" <mst@redhat.com>]

QEMU implemented the configuration
	VIRTIO_BALLOON_F_REPORTING && ! VIRTIO_BALLOON_F_FREE_PAGE_HINT
incorrectly: it then uses vq3 for reporting, spec says it is always 4.

This is masked by a corresponding bug in driver:
add a work around as I'm going to try and fix the driver bug.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 drivers/virtio/virtio_balloon.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 9a61febbd2f7..7dc3fcd56238 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -597,8 +597,23 @@ static int init_vqs(struct virtio_balloon *vb)
 
 	err = virtio_find_vqs(vb->vdev, VIRTIO_BALLOON_VQ_MAX, vqs,
 			      callbacks, names, NULL);
-	if (err)
-		return err;
+	if (err) {
+		/*
+		 * Try to work around QEMU bug which since 2020 confused vq numbers
+		 * when VIRTIO_BALLOON_F_REPORTING but not
+		 * VIRTIO_BALLOON_F_FREE_PAGE_HINT are offered.
+		 */
+		if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING) &&
+		    !virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
+			names[VIRTIO_BALLOON_VQ_FREE_PAGE] = "reporting_vq";
+			callbacks[VIRTIO_BALLOON_VQ_FREE_PAGE] = balloon_ack;
+			err = virtio_find_vqs(vb->vdev,
+					      VIRTIO_BALLOON_VQ_REPORTING, vqs, callbacks, names, NULL);
+		}
+
+		if (err)
+			return err;
+	}
 
 	vb->inflate_vq = vqs[VIRTIO_BALLOON_VQ_INFLATE];
 	vb->deflate_vq = vqs[VIRTIO_BALLOON_VQ_DEFLATE];
-- 
MST