Adding upstream version 6.6.15.upstream/6.6.15

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.. 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
+	  http://goo.gl/3WPDg
+
+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 associated 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.
+
+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
+			========  ============================================
+
+  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/