vdpa: Allow vDPA to work on big-endian machine

Add .set_vnet_le() function that always returns success, assuming that
vDPA h/w always implements LE data format. Otherwise, QEMU disables vDPA and
outputs the message:
"backend does not support LE vnet headers; falling back on userspace virtio"

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>
Signed-off-by: Konstantin Shkolnyy <kshk@linux.ibm.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

net/vhost-vdpa.c

@@ -262,6 +262,18 @@ static bool vhost_vdpa_has_ufo(NetClientState *nc)
 
 }
 
+/*
+ * FIXME: vhost_vdpa doesn't have an API to "set h/w endianness". But it's
+ * reasonable to assume that h/w is LE by default, because LE is what
+ * virtio 1.0 and later ask for. So, this function just says "yes, the h/w is
+ * LE". Otherwise, on a BE machine, higher-level code would mistakely think
+ * the h/w is BE and can't support VDPA for a virtio 1.0 client.
+ */
+static int vhost_vdpa_set_vnet_le(NetClientState *nc, bool enable)
+{
+    return 0;
+}
+
 static bool vhost_vdpa_check_peer_type(NetClientState *nc, ObjectClass *oc,
                                        Error **errp)
 {
@@ -429,6 +441,7 @@ static NetClientInfo net_vhost_vdpa_info = {
         .cleanup = vhost_vdpa_cleanup,
         .has_vnet_hdr = vhost_vdpa_has_vnet_hdr,
         .has_ufo = vhost_vdpa_has_ufo,
+        .set_vnet_le = vhost_vdpa_set_vnet_le,
         .check_peer_type = vhost_vdpa_check_peer_type,
         .set_steering_ebpf = vhost_vdpa_set_steering_ebpf,
 };