From 5b7c4cabbb65f5c469464da6c5f614cbd7f730f2 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Tue, 21 Feb 2023 18:24:12 -0800 Subject: Merge tag 'net-next-6.3' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next Pull networking updates from Jakub Kicinski: "Core: - Add dedicated kmem_cache for typical/small skb->head, avoid having to access struct page at kfree time, and improve memory use. - Introduce sysctl to set default RPS configuration for new netdevs. - Define Netlink protocol specification format which can be used to describe messages used by each family and auto-generate parsers. Add tools for generating kernel data structures and uAPI headers. - Expose all net/core sysctls inside netns. - Remove 4s sleep in netpoll if carrier is instantly detected on boot. - Add configurable limit of MDB entries per port, and port-vlan. - Continue populating drop reasons throughout the stack. - Retire a handful of legacy Qdiscs and classifiers. Protocols: - Support IPv4 big TCP (TSO frames larger than 64kB). - Add IP_LOCAL_PORT_RANGE socket option, to control local port range on socket by socket basis. - Track and report in procfs number of MPTCP sockets used. - Support mixing IPv4 and IPv6 flows in the in-kernel MPTCP path manager. - IPv6: don't check net.ipv6.route.max_size and rely on garbage collection to free memory (similarly to IPv4). - Support Penultimate Segment Pop (PSP) flavor in SRv6 (RFC8986). - ICMP: add per-rate limit counters. - Add support for user scanning requests in ieee802154. - Remove static WEP support. - Support minimal Wi-Fi 7 Extremely High Throughput (EHT) rate reporting. - WiFi 7 EHT channel puncturing support (client & AP). BPF: - Add a rbtree data structure following the "next-gen data structure" precedent set by recently added linked list, that is, by using kfunc + kptr instead of adding a new BPF map type. - Expose XDP hints via kfuncs with initial support for RX hash and timestamp metadata. - Add BPF_F_NO_TUNNEL_KEY extension to bpf_skb_set_tunnel_key to better support decap on GRE tunnel devices not operating in collect metadata. - Improve x86 JIT's codegen for PROBE_MEM runtime error checks. - Remove the need for trace_printk_lock for bpf_trace_printk and bpf_trace_vprintk helpers. - Extend libbpf's bpf_tracing.h support for tracing arguments of kprobes/uprobes and syscall as a special case. - Significantly reduce the search time for module symbols by livepatch and BPF. - Enable cpumasks to be used as kptrs, which is useful for tracing programs tracking which tasks end up running on which CPUs in different time intervals. - Add support for BPF trampoline on s390x and riscv64. - Add capability to export the XDP features supported by the NIC. - Add __bpf_kfunc tag for marking kernel functions as kfuncs. - Add cgroup.memory=nobpf kernel parameter option to disable BPF memory accounting for container environments. Netfilter: - Remove the CLUSTERIP target. It has been marked as obsolete for years, and we still have WARN splats wrt races of the out-of-band /proc interface installed by this target. - Add 'destroy' commands to nf_tables. They are identical to the existing 'delete' commands, but do not return an error if the referenced object (set, chain, rule...) did not exist. Driver API: - Improve cpumask_local_spread() locality to help NICs set the right IRQ affinity on AMD platforms. - Separate C22 and C45 MDIO bus transactions more clearly. - Introduce new DCB table to control DSCP rewrite on egress. - Support configuration of Physical Layer Collision Avoidance (PLCA) Reconciliation Sublayer (RS) (802.3cg-2019). Modern version of shared medium Ethernet. - Support for MAC Merge layer (IEEE 802.3-2018 clause 99). Allowing preemption of low priority frames by high priority frames. - Add support for controlling MACSec offload using netlink SET. - Rework devlink instance refcounts to allow registration and de-registration under the instance lock. Split the code into multiple files, drop some of the unnecessarily granular locks and factor out common parts of netlink operation handling. - Add TX frame aggregation parameters (for USB drivers). - Add a new attr TCA_EXT_WARN_MSG to report TC (offload) warning messages with notifications for debug. - Allow offloading of UDP NEW connections via act_ct. - Add support for per action HW stats in TC. - Support hardware miss to TC action (continue processing in SW from a specific point in the action chain). - Warn if old Wireless Extension user space interface is used with modern cfg80211/mac80211 drivers. Do not support Wireless Extensions for Wi-Fi 7 devices at all. Everyone should switch to using nl80211 interface instead. - Improve the CAN bit timing configuration. Use extack to return error messages directly to user space, update the SJW handling, including the definition of a new default value that will benefit CAN-FD controllers, by increasing their oscillator tolerance. New hardware / drivers: - Ethernet: - nVidia BlueField-3 support (control traffic driver) - Ethernet support for imx93 SoCs - Motorcomm yt8531 gigabit Ethernet PHY - onsemi NCN26000 10BASE-T1S PHY (with support for PLCA) - Microchip LAN8841 PHY (incl. cable diagnostics and PTP) - Amlogic gxl MDIO mux - WiFi: - RealTek RTL8188EU (rtl8xxxu) - Qualcomm Wi-Fi 7 devices (ath12k) - CAN: - Renesas R-Car V4H Drivers: - Bluetooth: - Set Per Platform Antenna Gain (PPAG) for Intel controllers. - Ethernet NICs: - Intel (1G, igc): - support TSN / Qbv / packet scheduling features of i226 model - Intel (100G, ice): - use GNSS subsystem instead of TTY - multi-buffer XDP support - extend support for GPIO pins to E823 devices - nVidia/Mellanox: - update the shared buffer configuration on PFC commands - implement PTP adjphase function for HW offset control - TC support for Geneve and GRE with VF tunnel offload - more efficient crypto key management method - multi-port eswitch support - Netronome/Corigine: - add DCB IEEE support - support IPsec offloading for NFP3800 - Freescale/NXP (enetc): - support XDP_REDIRECT for XDP non-linear buffers - improve reconfig, avoid link flap and waiting for idle - support MAC Merge layer - Other NICs: - sfc/ef100: add basic devlink support for ef100 - ionic: rx_push mode operation (writing descriptors via MMIO) - bnxt: use the auxiliary bus abstraction for RDMA - r8169: disable ASPM and reset bus in case of tx timeout - cpsw: support QSGMII mode for J721e CPSW9G - cpts: support pulse-per-second output - ngbe: add an mdio bus driver - usbnet: optimize usbnet_bh() by avoiding unnecessary queuing - r8152: handle devices with FW with NCM support - amd-xgbe: support 10Mbps, 2.5GbE speeds and rx-adaptation - virtio-net: support multi buffer XDP - virtio/vsock: replace virtio_vsock_pkt with sk_buff - tsnep: XDP support - Ethernet high-speed switches: - nVidia/Mellanox (mlxsw): - add support for latency TLV (in FW control messages) - Microchip (sparx5): - separate explicit and implicit traffic forwarding rules, make the implicit rules always active - add support for egress DSCP rewrite - IS0 VCAP support (Ingress Classification) - IS2 VCAP filters (protos, L3 addrs, L4 ports, flags, ToS etc.) - ES2 VCAP support (Egress Access Control) - support for Per-Stream Filtering and Policing (802.1Q, 8.6.5.1) - Ethernet embedded switches: - Marvell (mv88e6xxx): - add MAB (port auth) offload support - enable PTP receive for mv88e6390 - NXP (ocelot): - support MAC Merge layer - support for the the vsc7512 internal copper phys - Microchip: - lan9303: convert to PHYLINK - lan966x: support TC flower filter statistics - lan937x: PTP support for KSZ9563/KSZ8563 and LAN937x - lan937x: support Credit Based Shaper configuration - ksz9477: support Energy Efficient Ethernet - other: - qca8k: convert to regmap read/write API, use bulk operations - rswitch: Improve TX timestamp accuracy - Intel WiFi (iwlwifi): - EHT (Wi-Fi 7) rate reporting - STEP equalizer support: transfer some STEP (connection to radio on platforms with integrated wifi) related parameters from the BIOS to the firmware. - Qualcomm 802.11ax WiFi (ath11k): - IPQ5018 support - Fine Timing Measurement (FTM) responder role support - channel 177 support - MediaTek WiFi (mt76): - per-PHY LED support - mt7996: EHT (Wi-Fi 7) support - Wireless Ethernet Dispatch (WED) reset support - switch to using page pool allocator - RealTek WiFi (rtw89): - support new version of Bluetooth co-existance - Mobile: - rmnet: support TX aggregation" * tag 'net-next-6.3' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next: (1872 commits) page_pool: add a comment explaining the fragment counter usage net: ethtool: fix __ethtool_dev_mm_supported() implementation ethtool: pse-pd: Fix double word in comments xsk: add linux/vmalloc.h to xsk.c sefltests: netdevsim: wait for devlink instance after netns removal selftest: fib_tests: Always cleanup before exit net/mlx5e: Align IPsec ASO result memory to be as required by hardware net/mlx5e: TC, Set CT miss to the specific ct action instance net/mlx5e: Rename CHAIN_TO_REG to MAPPED_OBJ_TO_REG net/mlx5: Refactor tc miss handling to a single function net/mlx5: Kconfig: Make tc offload depend on tc skb extension net/sched: flower: Support hardware miss to tc action net/sched: flower: Move filter handle initialization earlier net/sched: cls_api: Support hardware miss to tc action net/sched: Rename user cookie and act cookie sfc: fix builds without CONFIG_RTC_LIB sfc: clean up some inconsistent indentings net/mlx4_en: Introduce flexible array to silence overflow warning net: lan966x: Fix possible deadlock inside PTP net/ulp: Remove redundant ->clone() test in inet_clone_ulp(). ... --- Documentation/networking/af_xdp.rst | 631 ++++++++++++++++++++++++++++++++++++ 1 file changed, 631 insertions(+) create mode 100644 Documentation/networking/af_xdp.rst (limited to 'Documentation/networking/af_xdp.rst') diff --git a/Documentation/networking/af_xdp.rst b/Documentation/networking/af_xdp.rst new file mode 100644 index 000000000..247c6c412 --- /dev/null +++ b/Documentation/networking/af_xdp.rst @@ -0,0 +1,631 @@ +.. SPDX-License-Identifier: GPL-2.0 + +====== +AF_XDP +====== + +Overview +======== + +AF_XDP is an address family that is optimized for high performance +packet processing. + +This document assumes that the reader is familiar with BPF and XDP. If +not, the Cilium project has an excellent reference guide at +http://cilium.readthedocs.io/en/latest/bpf/. + +Using the XDP_REDIRECT action from an XDP program, the program can +redirect ingress frames to other XDP enabled netdevs, using the +bpf_redirect_map() function. AF_XDP sockets enable the possibility for +XDP programs to redirect frames to a memory buffer in a user-space +application. + +An AF_XDP socket (XSK) is created with the normal socket() +syscall. Associated with each XSK are two rings: the RX ring and the +TX ring. A socket can receive packets on the RX ring and it can send +packets on the TX ring. These rings are registered and sized with the +setsockopts XDP_RX_RING and XDP_TX_RING, respectively. It is mandatory +to have at least one of these rings for each socket. An RX or TX +descriptor ring points to a data buffer in a memory area called a +UMEM. RX and TX can share the same UMEM so that a packet does not have +to be copied between RX and TX. Moreover, if a packet needs to be kept +for a while due to a possible retransmit, the descriptor that points +to that packet can be changed to point to another and reused right +away. This again avoids copying data. + +The UMEM consists of a number of equally sized chunks. A descriptor in +one of the rings references a frame by referencing its addr. The addr +is simply an offset within the entire UMEM region. The user space +allocates memory for this UMEM using whatever means it feels is most +appropriate (malloc, mmap, huge pages, etc). This memory area is then +registered with the kernel using the new setsockopt XDP_UMEM_REG. The +UMEM also has two rings: the FILL ring and the COMPLETION ring. The +FILL ring is used by the application to send down addr for the kernel +to fill in with RX packet data. References to these frames will then +appear in the RX ring once each packet has been received. The +COMPLETION ring, on the other hand, contains frame addr that the +kernel has transmitted completely and can now be used again by user +space, for either TX or RX. Thus, the frame addrs appearing in the +COMPLETION ring are addrs that were previously transmitted using the +TX ring. In summary, the RX and FILL rings are used for the RX path +and the TX and COMPLETION rings are used for the TX path. + +The socket is then finally bound with a bind() call to a device and a +specific queue id on that device, and it is not until bind is +completed that traffic starts to flow. + +The UMEM can be shared between processes, if desired. If a process +wants to do this, it simply skips the registration of the UMEM and its +corresponding two rings, sets the XDP_SHARED_UMEM flag in the bind +call and submits the XSK of the process it would like to share UMEM +with as well as its own newly created XSK socket. The new process will +then receive frame addr references in its own RX ring that point to +this shared UMEM. Note that since the ring structures are +single-consumer / single-producer (for performance reasons), the new +process has to create its own socket with associated RX and TX rings, +since it cannot share this with the other process. This is also the +reason that there is only one set of FILL and COMPLETION rings per +UMEM. It is the responsibility of a single process to handle the UMEM. + +How is then packets distributed from an XDP program to the XSKs? There +is a BPF map called XSKMAP (or BPF_MAP_TYPE_XSKMAP in full). The +user-space application can place an XSK at an arbitrary place in this +map. The XDP program can then redirect a packet to a specific index in +this map and at this point XDP validates that the XSK in that map was +indeed bound to that device and ring number. If not, the packet is +dropped. If the map is empty at that index, the packet is also +dropped. This also means that it is currently mandatory to have an XDP +program loaded (and one XSK in the XSKMAP) to be able to get any +traffic to user space through the XSK. + +AF_XDP can operate in two different modes: XDP_SKB and XDP_DRV. If the +driver does not have support for XDP, or XDP_SKB is explicitly chosen +when loading the XDP program, XDP_SKB mode is employed that uses SKBs +together with the generic XDP support and copies out the data to user +space. A fallback mode that works for any network device. On the other +hand, if the driver has support for XDP, it will be used by the AF_XDP +code to provide better performance, but there is still a copy of the +data into user space. + +Concepts +======== + +In order to use an AF_XDP socket, a number of associated objects need +to be setup. These objects and their options are explained in the +following sections. + +For an overview on how AF_XDP works, you can also take a look at the +Linux Plumbers paper from 2018 on the subject: +http://vger.kernel.org/lpc_net2018_talks/lpc18_paper_af_xdp_perf-v2.pdf. Do +NOT consult the paper from 2017 on "AF_PACKET v4", the first attempt +at AF_XDP. Nearly everything changed since then. Jonathan Corbet has +also written an excellent article on LWN, "Accelerating networking +with AF_XDP". It can be found at https://lwn.net/Articles/750845/. + +UMEM +---- + +UMEM is a region of virtual contiguous memory, divided into +equal-sized frames. An UMEM is associated to a netdev and a specific +queue id of that netdev. It is created and configured (chunk size, +headroom, start address and size) by using the XDP_UMEM_REG setsockopt +system call. A UMEM is bound to a netdev and queue id, via the bind() +system call. + +An AF_XDP is socket linked to a single UMEM, but one UMEM can have +multiple AF_XDP sockets. To share an UMEM created via one socket A, +the next socket B can do this by setting the XDP_SHARED_UMEM flag in +struct sockaddr_xdp member sxdp_flags, and passing the file descriptor +of A to struct sockaddr_xdp member sxdp_shared_umem_fd. + +The UMEM has two single-producer/single-consumer rings that are used +to transfer ownership of UMEM frames between the kernel and the +user-space application. + +Rings +----- + +There are a four different kind of rings: FILL, COMPLETION, RX and +TX. All rings are single-producer/single-consumer, so the user-space +application need explicit synchronization of multiple +processes/threads are reading/writing to them. + +The UMEM uses two rings: FILL and COMPLETION. Each socket associated +with the UMEM must have an RX queue, TX queue or both. Say, that there +is a setup with four sockets (all doing TX and RX). Then there will be +one FILL ring, one COMPLETION ring, four TX rings and four RX rings. + +The rings are head(producer)/tail(consumer) based rings. A producer +writes the data ring at the index pointed out by struct xdp_ring +producer member, and increasing the producer index. A consumer reads +the data ring at the index pointed out by struct xdp_ring consumer +member, and increasing the consumer index. + +The rings are configured and created via the _RING setsockopt system +calls and mmapped to user-space using the appropriate offset to mmap() +(XDP_PGOFF_RX_RING, XDP_PGOFF_TX_RING, XDP_UMEM_PGOFF_FILL_RING and +XDP_UMEM_PGOFF_COMPLETION_RING). + +The size of the rings need to be of size power of two. + +UMEM Fill Ring +~~~~~~~~~~~~~~ + +The FILL ring is used to transfer ownership of UMEM frames from +user-space to kernel-space. The UMEM addrs are passed in the ring. As +an example, if the UMEM is 64k and each chunk is 4k, then the UMEM has +16 chunks and can pass addrs between 0 and 64k. + +Frames passed to the kernel are used for the ingress path (RX rings). + +The user application produces UMEM addrs to this ring. Note that, if +running the application with aligned chunk mode, the kernel will mask +the incoming addr. E.g. for a chunk size of 2k, the log2(2048) LSB of +the addr will be masked off, meaning that 2048, 2050 and 3000 refers +to the same chunk. If the user application is run in the unaligned +chunks mode, then the incoming addr will be left untouched. + + +UMEM Completion Ring +~~~~~~~~~~~~~~~~~~~~ + +The COMPLETION Ring is used transfer ownership of UMEM frames from +kernel-space to user-space. Just like the FILL ring, UMEM indices are +used. + +Frames passed from the kernel to user-space are frames that has been +sent (TX ring) and can be used by user-space again. + +The user application consumes UMEM addrs from this ring. + + +RX Ring +~~~~~~~ + +The RX ring is the receiving side of a socket. Each entry in the ring +is a struct xdp_desc descriptor. The descriptor contains UMEM offset +(addr) and the length of the data (len). + +If no frames have been passed to kernel via the FILL ring, no +descriptors will (or can) appear on the RX ring. + +The user application consumes struct xdp_desc descriptors from this +ring. + +TX Ring +~~~~~~~ + +The TX ring is used to send frames. The struct xdp_desc descriptor is +filled (index, length and offset) and passed into the ring. + +To start the transfer a sendmsg() system call is required. This might +be relaxed in the future. + +The user application produces struct xdp_desc descriptors to this +ring. + +Libbpf +====== + +Libbpf is a helper library for eBPF and XDP that makes using these +technologies a lot simpler. It also contains specific helper functions +in tools/lib/bpf/xsk.h for facilitating the use of AF_XDP. It +contains two types of functions: those that can be used to make the +setup of AF_XDP socket easier and ones that can be used in the data +plane to access the rings safely and quickly. To see an example on how +to use this API, please take a look at the sample application in +samples/bpf/xdpsock_usr.c which uses libbpf for both setup and data +plane operations. + +We recommend that you use this library unless you have become a power +user. It will make your program a lot simpler. + +XSKMAP / BPF_MAP_TYPE_XSKMAP +============================ + +On XDP side there is a BPF map type BPF_MAP_TYPE_XSKMAP (XSKMAP) that +is used in conjunction with bpf_redirect_map() to pass the ingress +frame to a socket. + +The user application inserts the socket into the map, via the bpf() +system call. + +Note that if an XDP program tries to redirect to a socket that does +not match the queue configuration and netdev, the frame will be +dropped. E.g. an AF_XDP socket is bound to netdev eth0 and +queue 17. Only the XDP program executing for eth0 and queue 17 will +successfully pass data to the socket. Please refer to the sample +application (samples/bpf/) in for an example. + +Configuration Flags and Socket Options +====================================== + +These are the various configuration flags that can be used to control +and monitor the behavior of AF_XDP sockets. + +XDP_COPY and XDP_ZEROCOPY bind flags +------------------------------------ + +When you bind to a socket, the kernel will first try to use zero-copy +copy. If zero-copy is not supported, it will fall back on using copy +mode, i.e. copying all packets out to user space. But if you would +like to force a certain mode, you can use the following flags. If you +pass the XDP_COPY flag to the bind call, the kernel will force the +socket into copy mode. If it cannot use copy mode, the bind call will +fail with an error. Conversely, the XDP_ZEROCOPY flag will force the +socket into zero-copy mode or fail. + +XDP_SHARED_UMEM bind flag +------------------------- + +This flag enables you to bind multiple sockets to the same UMEM. It +works on the same queue id, between queue ids and between +netdevs/devices. In this mode, each socket has their own RX and TX +rings as usual, but you are going to have one or more FILL and +COMPLETION ring pairs. You have to create one of these pairs per +unique netdev and queue id tuple that you bind to. + +Starting with the case were we would like to share a UMEM between +sockets bound to the same netdev and queue id. The UMEM (tied to the +fist socket created) will only have a single FILL ring and a single +COMPLETION ring as there is only on unique netdev,queue_id tuple that +we have bound to. To use this mode, create the first socket and bind +it in the normal way. Create a second socket and create an RX and a TX +ring, or at least one of them, but no FILL or COMPLETION rings as the +ones from the first socket will be used. In the bind call, set he +XDP_SHARED_UMEM option and provide the initial socket's fd in the +sxdp_shared_umem_fd field. You can attach an arbitrary number of extra +sockets this way. + +What socket will then a packet arrive on? This is decided by the XDP +program. Put all the sockets in the XSK_MAP and just indicate which +index in the array you would like to send each packet to. A simple +round-robin example of distributing packets is shown below: + +.. code-block:: c + + #include + #include "bpf_helpers.h" + + #define MAX_SOCKS 16 + + struct { + __uint(type, BPF_MAP_TYPE_XSKMAP); + __uint(max_entries, MAX_SOCKS); + __uint(key_size, sizeof(int)); + __uint(value_size, sizeof(int)); + } xsks_map SEC(".maps"); + + static unsigned int rr; + + SEC("xdp_sock") int xdp_sock_prog(struct xdp_md *ctx) + { + rr = (rr + 1) & (MAX_SOCKS - 1); + + return bpf_redirect_map(&xsks_map, rr, XDP_DROP); + } + +Note, that since there is only a single set of FILL and COMPLETION +rings, and they are single producer, single consumer rings, you need +to make sure that multiple processes or threads do not use these rings +concurrently. There are no synchronization primitives in the +libbpf code that protects multiple users at this point in time. + +Libbpf uses this mode if you create more than one socket tied to the +same UMEM. However, note that you need to supply the +XSK_LIBBPF_FLAGS__INHIBIT_PROG_LOAD libbpf_flag with the +xsk_socket__create calls and load your own XDP program as there is no +built in one in libbpf that will route the traffic for you. + +The second case is when you share a UMEM between sockets that are +bound to different queue ids and/or netdevs. In this case you have to +create one FILL ring and one COMPLETION ring for each unique +netdev,queue_id pair. Let us say you want to create two sockets bound +to two different queue ids on the same netdev. Create the first socket +and bind it in the normal way. Create a second socket and create an RX +and a TX ring, or at least one of them, and then one FILL and +COMPLETION ring for this socket. Then in the bind call, set he +XDP_SHARED_UMEM option and provide the initial socket's fd in the +sxdp_shared_umem_fd field as you registered the UMEM on that +socket. These two sockets will now share one and the same UMEM. + +There is no need to supply an XDP program like the one in the previous +case where sockets were bound to the same queue id and +device. Instead, use the NIC's packet steering capabilities to steer +the packets to the right queue. In the previous example, there is only +one queue shared among sockets, so the NIC cannot do this steering. It +can only steer between queues. + +In libbpf, you need to use the xsk_socket__create_shared() API as it +takes a reference to a FILL ring and a COMPLETION ring that will be +created for you and bound to the shared UMEM. You can use this +function for all the sockets you create, or you can use it for the +second and following ones and use xsk_socket__create() for the first +one. Both methods yield the same result. + +Note that a UMEM can be shared between sockets on the same queue id +and device, as well as between queues on the same device and between +devices at the same time. + +XDP_USE_NEED_WAKEUP bind flag +----------------------------- + +This option adds support for a new flag called need_wakeup that is +present in the FILL ring and the TX ring, the rings for which user +space is a producer. When this option is set in the bind call, the +need_wakeup flag will be set if the kernel needs to be explicitly +woken up by a syscall to continue processing packets. If the flag is +zero, no syscall is needed. + +If the flag is set on the FILL ring, the application needs to call +poll() to be able to continue to receive packets on the RX ring. This +can happen, for example, when the kernel has detected that there are no +more buffers on the FILL ring and no buffers left on the RX HW ring of +the NIC. In this case, interrupts are turned off as the NIC cannot +receive any packets (as there are no buffers to put them in), and the +need_wakeup flag is set so that user space can put buffers on the +FILL ring and then call poll() so that the kernel driver can put these +buffers on the HW ring and start to receive packets. + +If the flag is set for the TX ring, it means that the application +needs to explicitly notify the kernel to send any packets put on the +TX ring. This can be accomplished either by a poll() call, as in the +RX path, or by calling sendto(). + +An example of how to use this flag can be found in +samples/bpf/xdpsock_user.c. An example with the use of libbpf helpers +would look like this for the TX path: + +.. code-block:: c + + if (xsk_ring_prod__needs_wakeup(&my_tx_ring)) + sendto(xsk_socket__fd(xsk_handle), NULL, 0, MSG_DONTWAIT, NULL, 0); + +I.e., only use the syscall if the flag is set. + +We recommend that you always enable this mode as it usually leads to +better performance especially if you run the application and the +driver on the same core, but also if you use different cores for the +application and the kernel driver, as it reduces the number of +syscalls needed for the TX path. + +XDP_{RX|TX|UMEM_FILL|UMEM_COMPLETION}_RING setsockopts +------------------------------------------------------ + +These setsockopts sets the number of descriptors that the RX, TX, +FILL, and COMPLETION rings respectively should have. It is mandatory +to set the size of at least one of the RX and TX rings. If you set +both, you will be able to both receive and send traffic from your +application, but if you only want to do one of them, you can save +resources by only setting up one of them. Both the FILL ring and the +COMPLETION ring are mandatory as you need to have a UMEM tied to your +socket. But if the XDP_SHARED_UMEM flag is used, any socket after the +first one does not have a UMEM and should in that case not have any +FILL or COMPLETION rings created as the ones from the shared UMEM will +be used. Note, that the rings are single-producer single-consumer, so +do not try to access them from multiple processes at the same +time. See the XDP_SHARED_UMEM section. + +In libbpf, you can create Rx-only and Tx-only sockets by supplying +NULL to the rx and tx arguments, respectively, to the +xsk_socket__create function. + +If you create a Tx-only socket, we recommend that you do not put any +packets on the fill ring. If you do this, drivers might think you are +going to receive something when you in fact will not, and this can +negatively impact performance. + +XDP_UMEM_REG setsockopt +----------------------- + +This setsockopt registers a UMEM to a socket. This is the area that +contain all the buffers that packet can reside in. The call takes a +pointer to the beginning of this area and the size of it. Moreover, it +also has parameter called chunk_size that is the size that the UMEM is +divided into. It can only be 2K or 4K at the moment. If you have an +UMEM area that is 128K and a chunk size of 2K, this means that you +will be able to hold a maximum of 128K / 2K = 64 packets in your UMEM +area and that your largest packet size can be 2K. + +There is also an option to set the headroom of each single buffer in +the UMEM. If you set this to N bytes, it means that the packet will +start N bytes into the buffer leaving the first N bytes for the +application to use. The final option is the flags field, but it will +be dealt with in separate sections for each UMEM flag. + +XDP_STATISTICS getsockopt +------------------------- + +Gets drop statistics of a socket that can be useful for debug +purposes. The supported statistics are shown below: + +.. code-block:: c + + struct xdp_statistics { + __u64 rx_dropped; /* Dropped for reasons other than invalid desc */ + __u64 rx_invalid_descs; /* Dropped due to invalid descriptor */ + __u64 tx_invalid_descs; /* Dropped due to invalid descriptor */ + }; + +XDP_OPTIONS getsockopt +---------------------- + +Gets options from an XDP socket. The only one supported so far is +XDP_OPTIONS_ZEROCOPY which tells you if zero-copy is on or not. + +Usage +===== + +In order to use AF_XDP sockets two parts are needed. The +user-space application and the XDP program. For a complete setup and +usage example, please refer to the sample application. The user-space +side is xdpsock_user.c and the XDP side is part of libbpf. + +The XDP code sample included in tools/lib/bpf/xsk.c is the following: + +.. code-block:: c + + SEC("xdp_sock") int xdp_sock_prog(struct xdp_md *ctx) + { + int index = ctx->rx_queue_index; + + // A set entry here means that the corresponding queue_id + // has an active AF_XDP socket bound to it. + if (bpf_map_lookup_elem(&xsks_map, &index)) + return bpf_redirect_map(&xsks_map, index, 0); + + return XDP_PASS; + } + +A simple but not so performance ring dequeue and enqueue could look +like this: + +.. code-block:: c + + // struct xdp_rxtx_ring { + // __u32 *producer; + // __u32 *consumer; + // struct xdp_desc *desc; + // }; + + // struct xdp_umem_ring { + // __u32 *producer; + // __u32 *consumer; + // __u64 *desc; + // }; + + // typedef struct xdp_rxtx_ring RING; + // typedef struct xdp_umem_ring RING; + + // typedef struct xdp_desc RING_TYPE; + // typedef __u64 RING_TYPE; + + int dequeue_one(RING *ring, RING_TYPE *item) + { + __u32 entries = *ring->producer - *ring->consumer; + + if (entries == 0) + return -1; + + // read-barrier! + + *item = ring->desc[*ring->consumer & (RING_SIZE - 1)]; + (*ring->consumer)++; + return 0; + } + + int enqueue_one(RING *ring, const RING_TYPE *item) + { + u32 free_entries = RING_SIZE - (*ring->producer - *ring->consumer); + + if (free_entries == 0) + return -1; + + ring->desc[*ring->producer & (RING_SIZE - 1)] = *item; + + // write-barrier! + + (*ring->producer)++; + return 0; + } + +But please use the libbpf functions as they are optimized and ready to +use. Will make your life easier. + +Sample application +================== + +There is a xdpsock benchmarking/test application included that +demonstrates how to use AF_XDP sockets with private UMEMs. Say that +you would like your UDP traffic from port 4242 to end up in queue 16, +that we will enable AF_XDP on. Here, we use ethtool for this:: + + ethtool -N p3p2 rx-flow-hash udp4 fn + ethtool -N p3p2 flow-type udp4 src-port 4242 dst-port 4242 \ + action 16 + +Running the rxdrop benchmark in XDP_DRV mode can then be done +using:: + + samples/bpf/xdpsock -i p3p2 -q 16 -r -N + +For XDP_SKB mode, use the switch "-S" instead of "-N" and all options +can be displayed with "-h", as usual. + +This sample application uses libbpf to make the setup and usage of +AF_XDP simpler. If you want to know how the raw uapi of AF_XDP is +really used to make something more advanced, take a look at the libbpf +code in tools/lib/bpf/xsk.[ch]. + +FAQ +======= + +Q: I am not seeing any traffic on the socket. What am I doing wrong? + +A: When a netdev of a physical NIC is initialized, Linux usually + allocates one RX and TX queue pair per core. So on a 8 core system, + queue ids 0 to 7 will be allocated, one per core. In the AF_XDP + bind call or the xsk_socket__create libbpf function call, you + specify a specific queue id to bind to and it is only the traffic + towards that queue you are going to get on you socket. So in the + example above, if you bind to queue 0, you are NOT going to get any + traffic that is distributed to queues 1 through 7. If you are + lucky, you will see the traffic, but usually it will end up on one + of the queues you have not bound to. + + There are a number of ways to solve the problem of getting the + traffic you want to the queue id you bound to. If you want to see + all the traffic, you can force the netdev to only have 1 queue, queue + id 0, and then bind to queue 0. You can use ethtool to do this:: + + sudo ethtool -L combined 1 + + If you want to only see part of the traffic, you can program the + NIC through ethtool to filter out your traffic to a single queue id + that you can bind your XDP socket to. Here is one example in which + UDP traffic to and from port 4242 are sent to queue 2:: + + sudo ethtool -N rx-flow-hash udp4 fn + sudo ethtool -N flow-type udp4 src-port 4242 dst-port \ + 4242 action 2 + + A number of other ways are possible all up to the capabilities of + the NIC you have. + +Q: Can I use the XSKMAP to implement a switch between different umems + in copy mode? + +A: The short answer is no, that is not supported at the moment. The + XSKMAP can only be used to switch traffic coming in on queue id X + to sockets bound to the same queue id X. The XSKMAP can contain + sockets bound to different queue ids, for example X and Y, but only + traffic goming in from queue id Y can be directed to sockets bound + to the same queue id Y. In zero-copy mode, you should use the + switch, or other distribution mechanism, in your NIC to direct + traffic to the correct queue id and socket. + +Q: My packets are sometimes corrupted. What is wrong? + +A: Care has to be taken not to feed the same buffer in the UMEM into + more than one ring at the same time. If you for example feed the + same buffer into the FILL ring and the TX ring at the same time, the + NIC might receive data into the buffer at the same time it is + sending it. This will cause some packets to become corrupted. Same + thing goes for feeding the same buffer into the FILL rings + belonging to different queue ids or netdevs bound with the + XDP_SHARED_UMEM flag. + +Credits +======= + +- Björn Töpel (AF_XDP core) +- Magnus Karlsson (AF_XDP core) +- Alexander Duyck +- Alexei Starovoitov +- Daniel Borkmann +- Jesper Dangaard Brouer +- John Fastabend +- Jonathan Corbet (LWN coverage) +- Michael S. Tsirkin +- Qi Z Zhang +- Willem de Bruijn -- cgit v1.2.3