ark1668ed-bsp/linux/Documentation/filesystems/9p.rst

.. SPDX-License-Identifier: GPL-2.0

=======================================
v9fs: Plan 9 Resource Sharing for Linux
=======================================

About
=====

v9fs is a Unix implementation of the Plan 9 9p remote filesystem protocol.

This software was originally developed by Ron Minnich <rminnich@sandia.gov>
and Maya Gokhale.  Additional development by Greg Watson
<gwatson@lanl.gov> and most recently Eric Van Hensbergen
<ericvh@gmail.com>, Latchesar Ionkov <lucho@ionkov.net> and Russ Cox
<rsc@swtch.com>.

The best detailed explanation of the Linux implementation and applications of
the 9p client is available in the form of a USENIX paper:

   https://www.usenix.org/events/usenix05/tech/freenix/hensbergen.html

Other applications are described in the following papers:

	* XCPU & Clustering
	  http://xcpu.org/papers/xcpu-talk.pdf
	* KVMFS: control file system for KVM
	  http://xcpu.org/papers/kvmfs.pdf
	* CellFS: A New Programming Model for the Cell BE
	  http://xcpu.org/papers/cellfs-talk.pdf
	* PROSE I/O: Using 9p to enable Application Partitions
	  http://plan9.escet.urjc.es/iwp9/cready/PROSE_iwp9_2006.pdf
	* VirtFS: A Virtualization Aware File System pass-through
	  https://kernel.org/doc/ols/2010/ols2010-pages-109-120.pdf

Usage
=====

For remote file server::

	mount -t 9p 10.10.1.2 /mnt/9

For Plan 9 From User Space applications (http://swtch.com/plan9)::

	mount -t 9p `namespace`/acme /mnt/9 -o trans=unix,uname=$USER

For server running on QEMU host with virtio transport::

	mount -t 9p -o trans=virtio <mount_tag> /mnt/9

where mount_tag is the tag generated by the server to each of the exported
mount points. Each 9P export is seen by the client as a virtio device with an
associated "mount_tag" property. Available mount tags can be
seen by reading /sys/bus/virtio/drivers/9pnet_virtio/virtio<n>/mount_tag files.

USBG Usage
==========

To mount a 9p FS on a USB Host accessible via the gadget at runtime::

	mount -t 9p -o trans=usbg,aname=/path/to/fs <device> /mnt/9

To mount a 9p FS on a USB Host accessible via the gadget as root filesystem::

	root=<device> rootfstype=9p rootflags=trans=usbg,cache=loose,uname=root,access=0,dfltuid=0,dfltgid=0,aname=/path/to/rootfs

where <device> is the tag associated by the usb gadget transport.
It is defined by the configfs instance name.

USBG Example
============

The USB host exports a filesystem, while the gadget on the USB device
side makes it mountable.

Diod (9pfs server) and the forwarder are on the development host, where
the root filesystem is actually stored. The gadget is initialized during
boot (or later) on the embedded board. Then the forwarder will find it
on the USB bus and start forwarding requests.

In this case the 9p requests come from the device and are handled by the
host. The reason is that USB device ports are normally not available on
PCs, so a connection in the other direction would not work.

When using the usbg transport, for now there is no native usb host
service capable to handle the requests from the gadget driver. For
this we have to use the extra python tool p9_fwd.py from tools/usb.

Just start the 9pfs capable network server like diod/nfs-ganesha e.g.::

        $ diod -f -n -d 0 -S -l 0.0.0.0:9999 -e $PWD

Optionaly scan your bus if there are more then one usbg gadgets to find their path::

        $ python $kernel_dir/tools/usb/p9_fwd.py list

        Bus | Addr | Manufacturer     | Product          | ID        | Path
        --- | ---- | ---------------- | ---------------- | --------- | ----
          2 |   67 | unknown          | unknown          | 1d6b:0109 | 2-1.1.2
          2 |   68 | unknown          | unknown          | 1d6b:0109 | 2-1.1.3

Then start the python transport::

        $ python $kernel_dir/tools/usb/p9_fwd.py --path 2-1.1.2 connect -p 9999

After that the gadget driver can be used as described above.

One use-case is to use it as an alternative to NFS root booting during
the development of embedded Linux devices.

Options
=======

  ============= ===============================================================
  trans=name	select an alternative transport.  Valid options are
  		currently:

			========  ============================================
			unix 	  specifying a named pipe mount point
			tcp	  specifying a normal TCP/IP connection
			fd   	  used passed file descriptors for connection
                                  (see rfdno and wfdno)
			virtio	  connect to the next virtio channel available
				  (from QEMU with trans_virtio module)
			rdma	  connect to a specified RDMA channel
			usbg	  connect to a specified usb gadget channel
			========  ============================================

  uname=name	user name to attempt mount as on the remote server.  The
  		server may override or ignore this value.  Certain user
		names may require authentication.

  aname=name	aname specifies the file tree to access when the server is
  		offering several exported file systems.

  cache=mode	specifies a caching policy.  By default, no caches are used.
		The mode can be specified as a bitmask or by using one of the
		preexisting common 'shortcuts'.
		The bitmask is described below: (unspecified bits are reserved)

			==========  ====================================================
			0b00000000  all caches disabled, mmap disabled
			0b00000001  file caches enabled
			0b00000010  meta-data caches enabled
			0b00000100  writeback behavior (as opposed to writethrough)
			0b00001000  loose caches (no explicit consistency with server)
			0b10000000  fscache enabled for persistent caching
			==========  ====================================================

		The current shortcuts and their associated bitmask are:

			=========   ====================================================
			none        0b00000000 (no caching)
			readahead   0b00000001 (only read-ahead file caching)
			mmap        0b00000101 (read-ahead + writeback file cache)
			loose       0b00001111 (non-coherent file and meta-data caches)
			fscache     0b10001111 (persistent loose cache)
			=========   ====================================================

		NOTE: only these shortcuts are tested modes of operation at the
		moment, so using other combinations of bit-patterns is not
		known to work.  Work on better cache support is in progress.

		IMPORTANT: loose caches (and by extension at the moment fscache)
		do not necessarily validate cached values on the server.  In other
		words changes on the server are not guaranteed to be reflected
		on the client system.  Only use this mode of operation if you
		have an exclusive mount and the server will modify the filesystem
		underneath you.

  debug=n	specifies debug level.  The debug level is a bitmask.

			=====   ================================
			0x01    display verbose error messages
			0x02    developer debug (DEBUG_CURRENT)
			0x04    display 9p trace
			0x08    display VFS trace
			0x10    display Marshalling debug
			0x20    display RPC debug
			0x40    display transport debug
			0x80    display allocation debug
			0x100   display protocol message debug
			0x200   display Fid debug
			0x400   display packet debug
			0x800   display fscache tracing debug
			=====   ================================

  rfdno=n	the file descriptor for reading with trans=fd

  wfdno=n	the file descriptor for writing with trans=fd

  msize=n	the number of bytes to use for 9p packet payload

  port=n	port to connect to on the remote server

  noextend	force legacy mode (no 9p2000.u or 9p2000.L semantics)

  version=name	Select 9P protocol version. Valid options are:

			========        ==============================
			9p2000          Legacy mode (same as noextend)
			9p2000.u        Use 9P2000.u protocol
			9p2000.L        Use 9P2000.L protocol
			========        ==============================

  dfltuid	attempt to mount as a particular uid

  dfltgid	attempt to mount with a particular gid

  afid		security channel - used by Plan 9 authentication protocols

  nodevmap	do not map special files - represent them as normal files.
  		This can be used to share devices/named pipes/sockets between
		hosts.  This functionality will be expanded in later versions.

  directio	bypass page cache on all read/write operations

  ignoreqv	ignore qid.version==0 as a marker to ignore cache

  noxattr	do not offer xattr functions on this mount.

  access	there are four access modes.
			user
				if a user tries to access a file on v9fs
			        filesystem for the first time, v9fs sends an
			        attach command (Tattach) for that user.
				This is the default mode.
			<uid>
				allows only user with uid=<uid> to access
				the files on the mounted filesystem
			any
				v9fs does single attach and performs all
				operations as one user
			clien
				 ACL based access check on the 9p client
			         side for access validation

  cachetag	cache tag to use the specified persistent cache.
		cache tags for existing cache sessions can be listed at
		/sys/fs/9p/caches. (applies only to cache=fscache)
  ============= ===============================================================

Behavior
========

This section aims at describing 9p 'quirks' that can be different
from a local filesystem behaviors.

 - Setting O_NONBLOCK on a file will make client reads return as early
   as the server returns some data instead of trying to fill the read
   buffer with the requested amount of bytes or end of file is reached.

Resources
=========

Protocol specifications are maintained on github:
http://ericvh.github.com/9p-rfc/

9p client and server implementations are listed on
http://9p.cat-v.org/implementations

A 9p2000.L server is being developed by LLNL and can be found
at http://code.google.com/p/diod/

There are user and developer mailing lists available through the v9fs project
on sourceforge (http://sourceforge.net/projects/v9fs).

News and other information is maintained on a Wiki.
(http://sf.net/apps/mediawiki/v9fs/index.php).

Bug reports are best issued via the mailing list.

For more information on the Plan 9 Operating System check out
http://plan9.bell-labs.com/plan9

For information on Plan 9 from User Space (Plan 9 applications and libraries
ported to Linux/BSD/OSX/etc) check out https://9fans.github.io/plan9port/