Message-Id: <1308228174-22788-1-git-send-email-bmt@zurich.ibm.com>
Date:	Thu, 16 Jun 2011 14:42:54 +0200
From:	Bernard Metzler <bmt@...ich.ibm.com>
To:	netdev@...r.kernel.org
Cc:	linux-rdma@...r.kernel.org, Bernard Metzler <bmt@...ich.ibm.com>
Subject: [PATCH 14/14] SIWv2: Documentation: siw.txt

---
 Documentation/networking/siw.txt |  156 ++++++++++++++++++++++++++++++++++++++
 1 files changed, 156 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/networking/siw.txt

diff --git a/Documentation/networking/siw.txt b/Documentation/networking/siw.txt
new file mode 100644
index 0000000..805e21b
--- /dev/null
+++ b/Documentation/networking/siw.txt
@@ -0,0 +1,156 @@
+SoftiWARP: Software iWARP kernel driver module.
+
+General
+-------
+SoftiWARP (siw) implements the iWARP protocol suite (MPA/DDP/RDMAP,
+IETF-RFC 5044/5041/5040) completely in software as a Linux kernel module.
+siw runs on top of TCP kernel sockets and exports the Linux kernel ibverbs
+RDMA interface. siw interfaces with the iwcm connection manager.
+
+
+Transmit Path
+-------------
+If a send queue (SQ) work queue element gets posted, siw tries to send
+it directly out of the application context. If the SQ was non-empty,
+SQ processing is done asynchronously by a kernel worker thread. This
+thread gets scheduled if the TCP socket signals new write space to
+be available. If during send operation the socket send space becomes
+exhausted, SQ processing is abandoned until new socket write space
+becomes available.
+
+
+Receive Path
+------------
+All application data is placed into target buffers within softirq
+socket callback. Application notification is asynchronous.
+
+
+User Interface
+--------------
+All user space fast path operations such as posting of work requests and
+reaping of work completions currently involve a isynchronous call into
+the siw kernel module via ib_uverbs interface. Kernel/user-mapped send
+and receive as well as completion queues are not part of the current code.
+In particular, mapped completion queues may improve performance,
+since reaping completion queue entries as well as re-arming
+the completion queue could be done more efficiently.
+
+
+Kernel Client Support
+---------------------
+To guarantee non-blocking fast path operations, for kernel clients
+all work queue elements (send/receive/shared-receive queue) are
+pre-allocated during connection resource setup.
+
+
+Memory Management
+-----------------
+siw currently uses the ib_umem_get() function of the ib_core module
+to pin memory for later use in data transfer operations. Transmit
+and receive memory are checked against correct access permissions only
+in the moment of access by the network input path or before pushing it
+to the TCP socket for transmission.
+ib_umem_get() provides DMA mappings for the requested address space which
+are not used by siw.
+
+
+Module Parameters
+-----------------
+The following siw module parameters are recognized.
+
+loopback_enabled:
+	If set, siw attaches also to the looback device. Checked only
+	during module insertion.
+
+mpa_crc_required:
+	If set, the MPA CRC gets generated and checked both in tx and rx
+	path. Without hardware support, setting this flag will severely
+	hurt throughput. Default setting is 0 (off).
+
+mpa_crc_strict:
+	If set, MPA CRC will not be enabled, even if peer requests
+	it. If the peer requests CRC generation, the connection setup
+	will be aborted. Default setting is 1 (on).
+
+zcopy_tx:
+	If set, payload of non-signalled work requests
+	(such as non-signalled WRITE or SEND as well as all READ
+	responses) are transferred using the TCP sockets
+	sendpage interface. This parameter can be switched on and
+	off dynamically (echo 1 >> /sys/module/siw/parameters/zcopy_tx
+	for enablement, 0 for disabling). System load may benefits from
+	using 0copy data transmission. 0copy is not enabled if
+	mpa_crc_enabled is set. Default setting is 1 (on).
+
+tcp_nodelay:
+	If set, on the TCP socket the TCP_NODELAY option is set.
+	Default setting is 1 (on).
+
+iface_list:
+	Comma separated list of interfaces siw should attach to.
+	If no list is given, siw attaches to all available devices.
+	If a list is given, siw skips those devices not listed.
+	Currently, the list is restricted to 12 entries. If needed,
+	the 'SIW_MAX_IF' #define in siw_main.c can be adaped.
+	This parameter might be usefull to skip devices which are
+	attached to a real RNIC device. Default setting is an empty list.
+
+
+Compile Time Flags:
+-------------------
+-DCHECK_DMA_CAPABILITIES
+	Checks if the device siw wants to attach to provides
+	DMA capabilities. While DMA capabilities are currently not
+	needed (siw works on top of a kernel TCP socket), siw
+	uses ib_umem_get() which performs a (not used) DMA address
+	translation. Writing a siw private memory reservation and
+	pinning routine would solve the issue.
+
+-DSIW_TX_FULLSEGS
+	Experimental, not enabled by default. If set,
+	siw tries not to overrun the socket (not sending until
+	-EAGAIN return), but stops sending if the current segment
+	would not fit into the socket's estimated tx buffer. With that,
+	wire FPDUs may get truncated by the TCP stack far less often.
+	Since this feature manipulates the sock's SOCK_NOSPACE
+	bit, it violates strict layering and is therefore considered
+	proprietary.
+	Since TCP is a byte stream protocol, no guarantee can be given
+	if FPDU's are not fragmented.
+
+
+Debugging SIW:
+--------------
+The siw_debug.h file defines a 'dprint' macro which is used to debug
+siw at runtime. Verbosity of debugging is controlled at compile time
+via setting the 'DPRINT_MASK' to a or'd list of know value as defined
+in siw_debug.h, e.g. '#define DPRINT_MASK (DBG_ON|DBG_CM)' to debug
+errors and connection management. Defining DPRINT_MASK to '0' avoids
+to compile any runtime debugging code.
+
+To track siw's useage of its objects (connection endpoints, tcp sockets,
+protection domains, queue pairs, shared receive queues, completion queues,
+memory registrations, work queue elements), the /sys/class/infiniband/siw*
+directory contains siw interface specific objects, which can be read to
+gather simple statistics:
+
+/sys/class/infiniband/siw*/stats:
+	Summary of allocated WQE's, PD's, QP's, CQ's, SRQ's, MR's, CEP's.
+	WQE statistics are not gathered if 'DPRINT_MASK' is set to '0'
+	(see above).
+
+/sys/class/infiniband/siw*/qp:
+	Summary of allocated queue pairs. If queue pairs are allocated,
+	after reading 'qp' a more detailed status of all queue pairs has
+	been printed to the kernel syslog and can be retrieved via
+	'dmesg' command.
+
+/sys/class/infiniband/siw*/cep:
+	Summary of allocated connection end points. If connection endpoints
+	are allocated, after reading 'cep' a more detailed status of all
+	CEP's is printed to the kernel syslog and can be retrieved via
+	'dmesg' command.
+
+Using the sysfs to gather siw's object allocations is considered a
+tentative aid during further driver development and should disappear
+in a stable version of siw.
-- 
1.5.4.3

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