[PATCH 2/2] vringh: fetch used_idx from vring at vringh_init_iotlb

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

Starting from an used_idx different than 0 is needed in use cases like
virtual machine migration.  Not doing so and letting the caller set an
avail idx different than 0 causes destination device to try to use old
buffers that source driver already recover and are not available
anymore.

While callers like vdpa_sim set avail_idx directly it does not set
used_idx.  Instead of let the caller do the assignment, fetch it from
the guest at initialization like vhost-kernel do.

To perform the same at vring_kernel_init and vring_user_init is left for
the future.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
 drivers/vhost/vringh.c | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
index 33eb941fcf15..0eed825197f2 100644
--- a/drivers/vhost/vringh.c
+++ b/drivers/vhost/vringh.c
@@ -1301,6 +1301,17 @@ static inline int putused_iotlb(const struct vringh *vrh,
 	return 0;
 }
 
+/**
+ * vringh_update_used_idx - fetch used idx from driver's used split vring
+ * @vrh: The vring.
+ *
+ * Returns -errno or 0.
+ */
+static inline int vringh_update_used_idx(struct vringh *vrh)
+{
+	return getu16_iotlb(vrh, &vrh->last_used_idx, &vrh->vring.used->idx);
+}
+
 /**
  * vringh_init_iotlb - initialize a vringh for a ring with IOTLB.
  * @vrh: the vringh to initialize.
@@ -1319,8 +1330,18 @@ int vringh_init_iotlb(struct vringh *vrh, u64 features,
 		      struct vring_avail *avail,
 		      struct vring_used *used)
 {
-	return vringh_init_kern(vrh, features, num, weak_barriers,
-				desc, avail, used);
+	int r = vringh_init_kern(vrh, features, num, weak_barriers, desc,
+				 avail, used);
+
+	if (r != 0)
+		return r;
+
+	/* Consider the ring not initialized */
+	if ((void *)desc == used)
+		return 0;
+
+	return vringh_update_used_idx(vrh);
+
 }
 EXPORT_SYMBOL(vringh_init_iotlb);
 
-- 
2.31.1