[PATCH 1/2] vhost: Get vring base from vq, not svq

["Eugenio Pérez" <eperezma@redhat.com>]

The SVQ vring used idx usually match with the guest visible one, as long
as all the guest buffers (GPA) maps to exactly one buffer within qemu's
VA. However, as we can see in virtqueue_map_desc, a single guest buffer
could map to many buffers in SVQ vring.

The solution is to stop using the device's used idx and check for the
last avail idx. Since we cannot report in-flight descriptors with vdpa,
let's rewind all of them.

Fixes: 6d0b22266633 ("vdpa: Adapt vhost_vdpa_get_vring_base to SVQ")
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
 hw/virtio/vhost-vdpa.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
index 795ed5a049..18820498b3 100644
--- a/hw/virtio/vhost-vdpa.c
+++ b/hw/virtio/vhost-vdpa.c
@@ -1194,11 +1194,10 @@ static int vhost_vdpa_get_vring_base(struct vhost_dev *dev,
                                        struct vhost_vring_state *ring)
 {
     struct vhost_vdpa *v = dev->opaque;
-    int vdpa_idx = ring->index - dev->vq_index;
     int ret;
 
     if (v->shadow_vqs_enabled) {
-        VhostShadowVirtqueue *svq = g_ptr_array_index(v->shadow_vqs, vdpa_idx);
+        VirtQueue *vq = virtio_get_queue(dev->vdev, ring->index);
 
         /*
          * Setting base as last used idx, so destination will see as available
@@ -1208,7 +1207,10 @@ static int vhost_vdpa_get_vring_base(struct vhost_dev *dev,
          * TODO: This is ok for networking, but other kinds of devices might
          * have problems with these retransmissions.
          */
-        ring->num = svq->last_used_idx;
+        while (virtqueue_rewind(vq, 1)) {
+            continue;
+        }
+        ring->num = virtio_queue_get_last_avail_idx(dev->vdev, ring->index);
         return 0;
     }
 
-- 
2.31.1