xref: /linux/Documentation/networking/af_xdp.rst (revision fc9c7ca5fcbf7fe3bcba87d1ff72f0009071ba86)

.. 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 addresses from Tx
descriptors that the kernel has finished processing and that can now be
used again by user space, for either Tx or Rx. This includes frames whose
transmission has completed as well as frames referenced by invalid Tx
descriptors rejected by the kernel. A completion therefore returns
ownership of a frame to user space, but does not by itself guarantee that
the packet was successfully transmitted.

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 to 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 referenced
by Tx descriptors that the kernel has finished processing and can be
used by user-space again. This includes both frames whose transmission
has completed and frames referenced by invalid Tx descriptors that were
rejected and reclaimed by the kernel. A completion entry does not
guarantee successful packet transmission. 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/testing/selftests/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.

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 <linux/bpf.h>
   #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 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.

SO_BINDTODEVICE setsockopt
--------------------------

This is a generic SOL_SOCKET option that can be used to tie AF_XDP
socket to a particular network interface.  It is useful when a socket
is created by a privileged process and passed to a non-privileged one.
Once the option is set, kernel will refuse attempts to bind that socket
to a different interface.  Updating the value requires CAP_NET_RAW.

XDP_MAX_TX_SKB_BUDGET setsockopt
--------------------------------

This setsockopt sets the maximum number of descriptors that can be handled
and passed to the driver at one send syscall. It is applied in the copy
mode to allow application to tune the per-socket maximum iteration for
better throughput and less frequency of send syscall.
Allowed range is [32, xs->tx->nentries].

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.

Multi-Buffer Support
====================

With multi-buffer support, programs using AF_XDP sockets can receive
and transmit packets consisting of multiple buffers both in copy and
zero-copy mode. For example, a packet can consist of two
frames/buffers, one with the header and the other one with the data,
or a 9K Ethernet jumbo frame can be constructed by chaining together
three 4K frames.

Some definitions:

* A packet consists of one or more frames

* A descriptor in one of the AF_XDP rings always refers to a single
  frame. In the case the packet consists of a single frame, the
  descriptor refers to the whole packet.

To enable multi-buffer support for an AF_XDP socket, use the new bind
flag XDP_USE_SG. If this is not provided, all multi-buffer packets
will be dropped just as before. Note that the XDP program loaded also
needs to be in multi-buffer mode. This can be accomplished by using
"xdp.frags" as the section name of the XDP program used.

To represent a packet consisting of multiple frames, a new flag called
XDP_PKT_CONTD is introduced in the options field of the Rx and Tx
descriptors. If it is true (1) the packet continues with the next
descriptor and if it is false (0) it means this is the last descriptor
of the packet. Why the reverse logic of end-of-packet (eop) flag found
in many NICs? Just to preserve compatibility with non-multi-buffer
applications that have this bit set to false for all packets on Rx,
and the apps set the options field to zero for Tx, as anything else
will be treated as an invalid descriptor.

These are the semantics for producing packets onto AF_XDP Tx ring
consisting of multiple frames:

* When an invalid descriptor is found, the complete packet is treated as
  invalid. The kernel consumes descriptors through the descriptor marking
  the end of the packet and returns all their frame addresses through the
  COMPLETION ring. A standalone invalid descriptor is treated as a
  one-descriptor invalid packet. The descriptor following the end of the
  invalid packet is treated as the start of a new packet. As before, if
  your program is producing invalid descriptors you have a bug that must
  be fixed. Rejected descriptors are reported in the ``tx_invalid_descs``
  statistic.

* Zero length descriptors are treated as invalid descriptors.

* For copy mode, the maximum supported number of frames in a packet is
  equal to CONFIG_MAX_SKB_FRAGS + 1. If it is exceeded, all descriptors
  through the end of the oversized packet are consumed, treated as invalid,
  and their frame addresses are returned through the COMPLETION ring. To
  produce an application that will work on any system regardless of this
  config setting, limit the number of frags to 18, as the minimum value of
  the config is 17.

* For zero-copy mode, the limit is up to what the NIC HW
  supports. Usually at least five on the NICs we have checked. We
  consciously chose to not enforce a rigid limit (such as
  CONFIG_MAX_SKB_FRAGS + 1) for zero-copy mode, as it would have
  resulted in copy actions under the hood to fit into what limit the
  NIC supports. Kind of defeats the purpose of zero-copy mode. How to
  probe for this limit is explained in the "probe for multi-buffer
  support" section.

On the Rx path in copy-mode, the xsk core copies the XDP data into
multiple descriptors, if needed, and sets the XDP_PKT_CONTD flag as
detailed before. Zero-copy mode works the same, though the data is not
copied. When the application gets a descriptor with the XDP_PKT_CONTD
flag set to one, it means that the packet consists of multiple buffers
and it continues with the next buffer in the following
descriptor. When a descriptor with XDP_PKT_CONTD == 0 is received, it
means that this is the last buffer of the packet. AF_XDP guarantees
that only a complete packet (all frames in the packet) is sent to the
application. If there is not enough space in the AF_XDP Rx ring, all
frames of the packet will be dropped.

If application reads a batch of descriptors, using for example the libxdp
interfaces, it is not guaranteed that the batch will end with a full
packet. It might end in the middle of a packet and the rest of the
buffers of that packet will arrive at the beginning of the next batch,
since the libxdp interface does not read the whole ring (unless you
have an enormous batch size or a very small ring size).

An example program each for Rx and Tx multi-buffer support can be found
later in this document.

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 xdp-project at
https://github.com/xdp-project/bpf-examples/tree/main/AF_XDP-example.

The XDP code sample 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.

Usage Multi-Buffer Rx
---------------------

Here is a simple Rx path pseudo-code example (using libxdp interfaces
for simplicity). Error paths have been excluded to keep it short:

.. code-block:: c

    void rx_packets(struct xsk_socket_info *xsk)
    {
        static bool new_packet = true;
        u32 idx_rx = 0, idx_fq = 0;
        static char *pkt;

        int rcvd = xsk_ring_cons__peek(&xsk->rx, opt_batch_size, &idx_rx);

        xsk_ring_prod__reserve(&xsk->umem->fq, rcvd, &idx_fq);

        for (int i = 0; i < rcvd; i++) {
            struct xdp_desc *desc = xsk_ring_cons__rx_desc(&xsk->rx, idx_rx++);
            char *frag = xsk_umem__get_data(xsk->umem->buffer, desc->addr);
            bool eop = !(desc->options & XDP_PKT_CONTD);

            if (new_packet)
                pkt = frag;
            else
                add_frag_to_pkt(pkt, frag);

            if (eop)
                process_pkt(pkt);

            new_packet = eop;

            *xsk_ring_prod__fill_addr(&xsk->umem->fq, idx_fq++) = desc->addr;
        }

        xsk_ring_prod__submit(&xsk->umem->fq, rcvd);
        xsk_ring_cons__release(&xsk->rx, rcvd);
    }

Usage Multi-Buffer Tx
---------------------

Here is an example Tx path pseudo-code (using libxdp interfaces for
simplicity) ignoring that the umem is finite in size, and that we
eventually will run out of packets to send. Also assumes pkts.addr
points to a valid location in the umem.

.. code-block:: c

    void tx_packets(struct xsk_socket_info *xsk, struct pkt *pkts,
                    int batch_size)
    {
        u32 idx, i, pkt_nb = 0;

        xsk_ring_prod__reserve(&xsk->tx, batch_size, &idx);

        for (i = 0; i < batch_size;) {
            u64 addr = pkts[pkt_nb].addr;
            u32 len = pkts[pkt_nb].size;

            do {
                struct xdp_desc *tx_desc;

                tx_desc = xsk_ring_prod__tx_desc(&xsk->tx, idx + i++);
                tx_desc->addr = addr;

                if (len > xsk_frame_size) {
                    tx_desc->len = xsk_frame_size;
                    tx_desc->options = XDP_PKT_CONTD;
                } else {
                    tx_desc->len = len;
                    tx_desc->options = 0;
                    pkt_nb++;
                }
                len -= tx_desc->len;
                addr += xsk_frame_size;

                if (i == batch_size) {
                    /* Remember len, addr, pkt_nb for next iteration.
                     * Skipped for simplicity.
                     */
                    break;
                }
            } while (len);
        }

        xsk_ring_prod__submit(&xsk->tx, i);
    }

Probing for Multi-Buffer Support
--------------------------------

To discover if a driver supports multi-buffer AF_XDP in SKB or DRV
mode, use the XDP_FEATURES feature of netlink in linux/netdev.h to
query for NETDEV_XDP_ACT_RX_SG support. This is the same flag as for
querying for XDP multi-buffer support. If XDP supports multi-buffer in
a driver, then AF_XDP will also support that in SKB and DRV mode.

To discover if a driver supports multi-buffer AF_XDP in zero-copy
mode, use XDP_FEATURES and first check the NETDEV_XDP_ACT_XSK_ZEROCOPY
flag. If it is set, it means that at least zero-copy is supported and
you should go and check the netlink attribute
NETDEV_A_DEV_XDP_ZC_MAX_SEGS in linux/netdev.h. An unsigned integer
value will be returned stating the max number of frags that are
supported by this device in zero-copy mode. These are the possible
return values:

1: Multi-buffer for zero-copy is not supported by this device, as max
   one fragment supported means that multi-buffer is not possible.

>=2: Multi-buffer is supported in zero-copy mode for this device. The
     returned number signifies the max number of frags supported.

For an example on how these are used through libbpf, please take a
look at tools/testing/selftests/bpf/xskxceiver.c.

Multi-Buffer Support for Zero-Copy Drivers
------------------------------------------

Zero-copy drivers usually use the batched APIs for Rx and Tx
processing. Note that the Tx batch API guarantees that it will provide
a batch of Tx descriptors that ends with full packet at the end. This
to facilitate extending a zero-copy driver with multi-buffer support.

Sample application
==================
There is a xdpsock benchmarking/test application that can be found at
https://github.com/xdp-project/bpf-examples/tree/main/AF_XDP-example
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/testing/selftests/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 <interface> 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 <interface> rx-flow-hash udp4 fn
     sudo ethtool -N <interface> 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