cifs: add documentation for smbdirect setup

Document steps to use SMB over RDMA using the linux SMB client and
KSMBD server

Signed-off-by: Meetakshi Setiya <msetiya@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

Documentation/filesystems/smb/index.rst

@@ -8,3 +8,4 @@ CIFS
 
    ksmbd
    cifsroot
+   smbdirect

Documentation/filesystems/smb/smbdirect.rst

@@ -0,0 +1,103 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===========================
+SMB Direct - SMB3 over RDMA
+===========================
+
+This document describes how to set up the Linux SMB client and server to
+use RDMA.
+
+Overview
+========
+The Linux SMB kernel client supports SMB Direct, which is a transport
+scheme for SMB3 that uses RDMA (Remote Direct Memory Access) to provide
+high throughput and low latencies by bypassing the traditional TCP/IP
+stack.
+SMB Direct on the Linux SMB client can be tested against KSMBD - a
+kernel-space SMB server.
+
+Installation
+=============
+- Install an RDMA device. As long as the RDMA device driver is supported
+  by the kernel, it should work. This includes both software emulators (soft
+  RoCE, soft iWARP) and hardware devices (InfiniBand, RoCE, iWARP).
+
+- Install a kernel with SMB Direct support. The first kernel release to
+  support SMB Direct on both the client and server side is 5.15. Therefore,
+  a distribution compatible with kernel 5.15 or later is required.
+
+- Install cifs-utils, which provides the `mount.cifs` command to mount SMB
+  shares.
+
+- Configure the RDMA stack
+
+  Make sure that your kernel configuration has RDMA support enabled. Under
+  Device Drivers -> Infiniband support, update the kernel configuration to
+  enable Infiniband support.
+
+  Enable the appropriate IB HCA support or iWARP adapter support,
+  depending on your hardware.
+
+  If you are using InfiniBand, enable IP-over-InfiniBand support.
+
+  For soft RDMA, enable either the soft iWARP (`RDMA _SIW`) or soft RoCE
+  (`RDMA_RXE`) module. Install the `iproute2` package and use the
+  `rdma link add` command to load the module and create an
+  RDMA interface.
+
+  e.g. if your local ethernet interface is `eth0`, you can use:
+
+    .. code-block:: bash
+
+        sudo rdma link add siw0 type siw netdev eth0
+
+- Enable SMB Direct support for both the server and the client in the kernel
+  configuration.
+
+    Server Setup
+
+    .. code-block:: text
+
+        Network File Systems  --->
+            <M> SMB3 server support
+                [*] Support for SMB Direct protocol
+
+    Client Setup
+
+    .. code-block:: text
+
+        Network File Systems  --->
+            <M> SMB3 and CIFS support (advanced network filesystem)
+                [*] SMB Direct support
+
+- Build and install the kernel. SMB Direct support will be enabled in the
+  cifs.ko and ksmbd.ko modules.
+
+Setup and Usage
+================
+
+- Set up and start a KSMBD server as described in the `KSMBD documentation
+  <https://www.kernel.org/doc/Documentation/filesystems/smb/ksmbd.rst>`_.
+  Also add the "server multi channel support = yes" parameter to ksmbd.conf.
+
+- On the client, mount the share with `rdma` mount option to use SMB Direct
+  (specify a SMB version 3.0 or higher using `vers`).
+
+  For example:
+
+    .. code-block:: bash
+
+        mount -t cifs //server/share /mnt/point -o vers=3.1.1,rdma
+
+- To verify that the mount is using SMB Direct, you can check dmesg for the
+  following log line after mounting:
+
+    .. code-block:: text
+
+        CIFS: VFS: RDMA transport established
+
+  Or, verify `rdma` mount option for the share in `/proc/mounts`:
+
+    .. code-block:: bash
+
+        cat /proc/mounts | grep cifs