[PATCH 3/3] 9p: doc: add v9fsroot description

[Changbin Du <changbin.du@gmail.com>]

This documentation is modified from cifs/cifsroot.rst.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
---
 Documentation/filesystems/v9fs/index.rst    |  1 +
 Documentation/filesystems/v9fs/v9fsroot.rst | 52 +++++++++++++++++++++
 2 files changed, 53 insertions(+)
 create mode 100644 Documentation/filesystems/v9fs/v9fsroot.rst

diff --git a/Documentation/filesystems/v9fs/index.rst b/Documentation/filesystems/v9fs/index.rst
index a1e45b89e2a2..65e1ceb04c9c 100644
--- a/Documentation/filesystems/v9fs/index.rst
+++ b/Documentation/filesystems/v9fs/index.rst
@@ -9,3 +9,4 @@ v9fs
    :numbered:
 
    9p
+   v9fsroot
diff --git a/Documentation/filesystems/v9fs/v9fsroot.rst b/Documentation/filesystems/v9fs/v9fsroot.rst
new file mode 100644
index 000000000000..ce6b3c85e301
--- /dev/null
+++ b/Documentation/filesystems/v9fs/v9fsroot.rst
@@ -0,0 +1,52 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==========================================
+Mounting root file system via v9fs (9p.ko)
+==========================================
+
+:Author: Changbin Du <changbin.du@gmail.com>
+
+The CONFIG_9P_FS_ROOT option enables experimental root file system
+support for v9fs.
+
+It introduces a new kernel command-line option called 'v9fsroot='
+which will tell the kernel to mount the root file system by
+utilizing the 9p protocol.
+
+
+Kernel command line
+===================
+
+::
+
+    root=/dev/v9fs
+
+This is just a virtual device that basically tells the kernel to mount
+the root file system via 9p protocol.
+
+::
+
+    v9fsroot=<path>[,options]
+
+Enables the kernel to mount the root file system via 9p specified in this
+option.
+
+path
+	Could be a remote file server, Plan 9 From User Space applications
+	or mount tag of virtio transport.
+
+options
+	Optional mount options.
+
+Examples
+========
+Test it under QEMU on a kernel built with CONFIG_9P_FS_ROOT and
+CONFIG_IP_PNP options enabled::
+
+    # qemu-system-x86_64 -enable-kvm -cpu host -m 1024 \
+    -virtfs local,path=$rootfs_dir,mount_tag=r,security_model=passthrough,id=r \
+    -kernel /path/to/linux/arch/x86/boot/bzImage -nographic \
+    -append "root=/dev/v9fs v9fsroot=r,trans=virtio rw console=ttyS0 3"
+
+The above example mounts v9fs with tag 'r' as rootfs in qemu guest via
+virtio transport.
-- 
2.30.2