06459cf9d0
Explain how kernel driver RX buffer sizes affect the maximum MTU size in practice. Signed-off-by: Ciara Loftus <ciara.loftus@intel.com> Reviewed-by: Xiaolong Ye <xiaolong.ye@intel.com>
80 lines
3.2 KiB
ReStructuredText
80 lines
3.2 KiB
ReStructuredText
.. SPDX-License-Identifier: BSD-3-Clause
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Copyright(c) 2019 Intel Corporation.
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AF_XDP Poll Mode Driver
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==========================
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AF_XDP is an address family that is optimized for high performance
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packet processing. AF_XDP sockets enable the possibility for XDP program to
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redirect packets to a memory buffer in userspace.
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For the full details behind AF_XDP socket, you can refer to
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`AF_XDP documentation in the Kernel
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<https://www.kernel.org/doc/Documentation/networking/af_xdp.rst>`_.
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This Linux-specific PMD driver creates the AF_XDP socket and binds it to a
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specific netdev queue, it allows a DPDK application to send and receive raw
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packets through the socket which would bypass the kernel network stack.
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Current implementation only supports single queue, multi-queues feature will
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be added later.
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AF_XDP PMD enables need_wakeup flag by default if it is supported. This
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need_wakeup feature is used to support executing application and driver on the
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same core efficiently. This feature not only has a large positive performance
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impact for the one core case, but also does not degrade 2 core performance and
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actually improves it for Tx heavy workloads.
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Options
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-------
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The following options can be provided to set up an af_xdp port in DPDK.
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* ``iface`` - name of the Kernel interface to attach to (required);
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* ``start_queue`` - starting netdev queue id (optional, default 0);
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* ``queue_count`` - total netdev queue number (optional, default 1);
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Prerequisites
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-------------
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This is a Linux-specific PMD, thus the following prerequisites apply:
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* A Linux Kernel (version > v4.18) with XDP sockets configuration enabled;
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* libbpf (within kernel version > v5.1-rc4) with latest af_xdp support installed,
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User can install libbpf via `make install_lib` && `make install_headers` in
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<kernel src tree>/tools/lib/bpf;
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* A Kernel bound interface to attach to;
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* For need_wakeup feature, it requires kernel version later than v5.3-rc1;
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* For PMD zero copy, it requires kernel version later than v5.4-rc1;
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Set up an af_xdp interface
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-----------------------------
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The following example will set up an af_xdp interface in DPDK:
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.. code-block:: console
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--vdev net_af_xdp,iface=ens786f1
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Limitations
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-----------
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- **MTU**
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The MTU of the AF_XDP PMD is limited due to the XDP requirement of one packet
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per page. In the PMD we report the maximum MTU for zero copy to be equal
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to the page size less the frame overhead introduced by AF_XDP (XDP HR = 256)
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and DPDK (frame headroom = 320). With a 4K page size this works out at 3520.
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However in practice this value may be even smaller, due to differences between
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the supported RX buffer sizes of the underlying kernel netdev driver.
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For example, the largest RX buffer size supported by the underlying kernel driver
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which is less than the page size (4096B) may be 3072B. In this case, the maximum
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MTU value will be at most 3072, but likely even smaller than this, once relevant
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headers are accounted for eg. Ethernet and VLAN.
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To determine the actual maximum MTU value of the interface you are using with the
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AF_XDP PMD, consult the documentation for the kernel driver.
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Note: The AF_XDP PMD will fail to initialise if an MTU which violates the driver's
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conditions as above is set prior to launching the application.
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