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+ The ib_ipoib driver is an implementation of the IP over InfiniBand
+ protocol as specified by RFC 4391 and 4392, issued by the IETF ipoib
+ working group. It is a "native" implementation in the sense of
+ setting the interface type to ARPHRD_INFINIBAND and the hardware
+ address length to 20 (earlier proprietary implementations
+ masqueraded to the kernel as ethernet interfaces).
+Partitions and P_Keys
+ When the IPoIB driver is loaded, it creates one interface for each
+ port using the P_Key at index 0. To create an interface with a
+ different P_Key, write the desired P_Key into the main interface's
+ /sys/class/net/<intf name>/create_child file. For example:
+ echo 0x8001 > /sys/class/net/ib0/create_child
+ This will create an interface named ib0.8001 with P_Key 0x8001. To
+ remove a subinterface, use the "delete_child" file:
+ echo 0x8001 > /sys/class/net/ib0/delete_child
+ The P_Key for any interface is given by the "pkey" file, and the
+ main interface for a subinterface is in "parent."
+ Child interface create/delete can also be done using IPoIB's
+ rtnl_link_ops, where childs created using either way behave the same.
+Datagram vs Connected modes
+ The IPoIB driver supports two modes of operation: datagram and
+ connected. The mode is set and read through an interface's
+ /sys/class/net/<intf name>/mode file.
+ In datagram mode, the IB UD (Unreliable Datagram) transport is used
+ and so the interface MTU has is equal to the IB L2 MTU minus the
+ IPoIB encapsulation header (4 bytes). For example, in a typical IB
+ fabric with a 2K MTU, the IPoIB MTU will be 2048 - 4 = 2044 bytes.
+ In connected mode, the IB RC (Reliable Connected) transport is used.
+ Connected mode takes advantage of the connected nature of the IB
+ transport and allows an MTU up to the maximal IP packet size of 64K,
+ which reduces the number of IP packets needed for handling large UDP
+ datagrams, TCP segments, etc and increases the performance for large
+ messages.
+ In connected mode, the interface's UD QP is still used for multicast
+ and communication with peers that don't support connected mode. In
+ this case, RX emulation of ICMP PMTU packets is used to cause the
+ networking stack to use the smaller UD MTU for these neighbours.
+Stateless offloads
+ If the IB HW supports IPoIB stateless offloads, IPoIB advertises
+ TCP/IP checksum and/or Large Send (LSO) offloading capability to the
+ network stack.
+ Large Receive (LRO) offloading is also implemented and may be turned
+ on/off using ethtool calls. Currently LRO is supported only for
+ checksum offload capable devices.
+ Stateless offloads are supported only in datagram mode.
+Interrupt moderation
+ If the underlying IB device supports CQ event moderation, one can
+ use ethtool to set interrupt mitigation parameters and thus reduce
+ the overhead incurred by handling interrupts. The main code path of
+ IPoIB doesn't use events for TX completion signaling so only RX
+ moderation is supported.
+Debugging Information
+ By compiling the IPoIB driver with CONFIG_INFINIBAND_IPOIB_DEBUG set
+ to 'y', tracing messages are compiled into the driver. They are
+ turned on by setting the module parameters debug_level and
+ mcast_debug_level to 1. These parameters can be controlled at
+ runtime through files in /sys/module/ib_ipoib/.
+ CONFIG_INFINIBAND_IPOIB_DEBUG also enables files in the debugfs
+ virtual filesystem. By mounting this filesystem, for example with
+ mount -t debugfs none /sys/kernel/debug
+ it is possible to get statistics about multicast groups from the
+ files /sys/kernel/debug/ipoib/ib0_mcg and so on.
+ The performance impact of this option is negligible, so it
+ is safe to enable this option with debug_level set to 0 for normal
+ operation.
+ CONFIG_INFINIBAND_IPOIB_DEBUG_DATA enables even more debug output in
+ the data path when data_debug_level is set to 1. However, even with
+ the output disabled, enabling this configuration option will affect
+ performance, because it adds tests to the fast path.
+ Transmission of IP over InfiniBand (IPoIB) (RFC 4391)
+ http://ietf.org/rfc/rfc4391.txt
+ IP over InfiniBand (IPoIB) Architecture (RFC 4392)
+ http://ietf.org/rfc/rfc4392.txt
+ IP over InfiniBand: Connected Mode (RFC 4755)
+ http://ietf.org/rfc/rfc4755.txt