vhost: Clean up how VhostOpts method vhost_backend_init() fails

vhost_user_backend_init() can fail without setting an error.  Unclean.
Its caller vhost_dev_init() compensates by substituting a generic
error then.  Goes back to commit 28770ff935 "vhost: Distinguish errors
in vhost_backend_init()".

Clean up by moving the generic error from vhost_dev_init() to all the
failure paths that neglect to set an error.

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210720125408.387910-14-armbru@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

hw/virtio/vhost-user.c

@@ -1876,6 +1876,7 @@ static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque,
 
     err = vhost_user_get_features(dev, &features);
     if (err < 0) {
+        error_setg_errno(errp, -err, "vhost_backend_init failed");
         return err;
     }
 
@@ -1885,6 +1886,7 @@ static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque,
         err = vhost_user_get_u64(dev, VHOST_USER_GET_PROTOCOL_FEATURES,
                                  &protocol_features);
         if (err < 0) {
+            error_setg_errno(errp, EPROTO, "vhost_backend_init failed");
             return -EPROTO;
         }
 
@@ -1903,6 +1905,7 @@ static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque,
 
         err = vhost_user_set_protocol_features(dev, dev->protocol_features);
         if (err < 0) {
+            error_setg_errno(errp, EPROTO, "vhost_backend_init failed");
             return -EPROTO;
         }
 
@@ -1911,6 +1914,7 @@ static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque,
             err = vhost_user_get_u64(dev, VHOST_USER_GET_QUEUE_NUM,
                                      &dev->max_queues);
             if (err < 0) {
+                error_setg_errno(errp, EPROTO, "vhost_backend_init failed");
                 return -EPROTO;
             }
         } else {
@@ -1940,6 +1944,7 @@ static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque,
         } else {
             err = vhost_user_get_max_memslots(dev, &ram_slots);
             if (err < 0) {
+                error_setg_errno(errp, EPROTO, "vhost_backend_init failed");
                 return -EPROTO;
             }
 
@@ -1966,6 +1971,7 @@ static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque,
     if (dev->vq_index == 0) {
         err = vhost_setup_slave_channel(dev);
         if (err < 0) {
+            error_setg_errno(errp, EPROTO, "vhost_backend_init failed");
             return -EPROTO;
         }
     }

hw/virtio/vhost.c

@@ -1289,7 +1289,6 @@ int vhost_dev_init(struct vhost_dev *hdev, void *opaque,
                    VhostBackendType backend_type, uint32_t busyloop_timeout,
                    Error **errp)
 {
-    ERRP_GUARD();
     uint64_t features;
     int i, r, n_initialized_vqs = 0;
 
@@ -1301,9 +1300,6 @@ int vhost_dev_init(struct vhost_dev *hdev, void *opaque,
 
     r = hdev->vhost_ops->vhost_backend_init(hdev, opaque, errp);
     if (r < 0) {
-        if (!*errp) {
-            error_setg_errno(errp, -r, "vhost_backend_init failed");
-        }
         goto fail;
     }