nbd: add a basic netlink interface

The existing ioctl interface for configuring NBD devices is a bit
cumbersome and hard to extend.  The other problem is we leave a
userspace app sitting in it's syscall until the device disconnects,
which is less than ideal.

This patch introduces a netlink interface for adding and disconnecting
nbd devices.  This has the benefits of being easily extendable without
breaking older userspace applications, and allows us to configure a nbd
device without leaving a userspace app sitting waiting for the device to
disconnect.

With this interface we also gain the ability to configure more devices
than are preallocated at insmod time.  We also have gained the ability
to not specify a particular device and be provided one for us so that
userspace doesn't need to find a free device to configure.
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

include/uapi/linux/nbd-netlink.h

+/*
+ * Copyright (C) 2017 Facebook.  All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public
+ * License v2 as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public
+ * License along with this program; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 021110-1307, USA.
+ */
+#ifndef _UAPILINUX_NBD_NETLINK_H
+#define _UAPILINUX_NBD_NETLINK_H
+
+#define NBD_GENL_FAMILY_NAME	"nbd"
+#define NBD_GENL_VERSION	0x1
+
+/* Configuration policy attributes, used for CONNECT */
+enum {
+	NBD_ATTR_UNSPEC,
+	NBD_ATTR_INDEX,
+	NBD_ATTR_SIZE_BYTES,
+	NBD_ATTR_BLOCK_SIZE_BYTES,
+	NBD_ATTR_TIMEOUT,
+	NBD_ATTR_SERVER_FLAGS,
+	NBD_ATTR_CLIENT_FLAGS,
+	NBD_ATTR_SOCKETS,
+	__NBD_ATTR_MAX,
+};
+#define NBD_ATTR_MAX (__NBD_ATTR_MAX - 1)
+
+/*
+ * This is the format for multiple sockets with NBD_ATTR_SOCKETS
+ *
+ * [NBD_ATTR_SOCKETS]
+ *   [NBD_SOCK_ITEM]
+ *     [NBD_SOCK_FD]
+ *   [NBD_SOCK_ITEM]
+ *     [NBD_SOCK_FD]
+ */
+enum {
+	NBD_SOCK_ITEM_UNSPEC,
+	NBD_SOCK_ITEM,
+	__NBD_SOCK_ITEM_MAX,
+};
+#define NBD_SOCK_ITEM_MAX (__NBD_SOCK_ITEM_MAX - 1)
+
+enum {
+	NBD_SOCK_UNSPEC,
+	NBD_SOCK_FD,
+	__NBD_SOCK_MAX,
+};
+#define NBD_SOCK_MAX (__NBD_SOCK_MAX - 1)
+
+enum {
+	NBD_CMD_UNSPEC,
+	NBD_CMD_CONNECT,
+	NBD_CMD_DISCONNECT,
+	__NBD_CMD_MAX,
+};
+#define NBD_CMD_MAX	(__NBD_CMD_MAX - 1)
+
+#endif /* _UAPILINUX_NBD_NETLINK_H */