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