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

[Michael S. Tsirkin]

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 <m...@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