xsk: Improve documentation for AF_XDP

Added sections on all the bind flags, libbpf, all the setsockopts and
all the getsockopts. Also updated the document to reflect the latest
features and to correct some spelling errors.

v1 -> v2:
* Updated XDP program with latest BTF map format
* Added one more FAQ entry
* Some minor edits and corrections

v2 -> v3:
* Simplified XDP_SHARED_UMEM example XDP program
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/1571648224-16889-1-git-send-email-magnus.karlsson@intel.com

Documentation/networking/af_xdp.rst

@@ -40,13 +40,13 @@ 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
+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
+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
+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.
 
@@ -91,11 +91,16 @@ Concepts
 ========
 
 In order to use an AF_XDP socket, a number of associated objects need
-to be setup.
+to be setup. These objects and their options are explained in the
+following sections.
 
-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/.
+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
 ----
@@ -113,22 +118,22 @@ 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
+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
+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
+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.
+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
@@ -146,7 +151,7 @@ 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
+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.
@@ -164,8 +169,8 @@ 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 indicies are
+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
@@ -181,7 +186,7 @@ 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
+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
@@ -199,8 +204,24 @@ 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
@@ -216,21 +237,184 @@ 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_ZERO_COPY 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_ZERO_COPY 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, but
+only if they share the same queue id. In this mode, each socket has
+their own RX and TX rings, but the UMEM (tied to the fist socket
+created) only has a single FILL ring and a single COMPLETION
+ring. 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, 0);
+   }
+
+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.
+
+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 if you have a UMEM tied to your socket,
+which is the normal case. 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.
+
+XDP_UMEM_REG setsockopt
+-----------------------
+
+This setsockopt registers a UMEM to a socket. This is the area that
+contain all the buffers that packet can recide 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 there are two parts needed. The
+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::
+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 correspnding queue_id
+       // 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);
@@ -238,7 +422,10 @@ The XDP code sample included in tools/lib/bpf/xsk.c is the following::
        return XDP_PASS;
    }
 
-Naive ring dequeue and enqueue could look like this::
+A simple but not so performance ring dequeue and enqueue could look
+like this:
+
+.. code-block:: c
 
     // struct xdp_rxtx_ring {
     // 	__u32 *producer;
@@ -287,17 +474,16 @@ Naive ring dequeue and enqueue could look like this::
         return 0;
     }
 
-
-For a more optimized version, please refer to the sample application.
+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 both private and shared
-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::
+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 \
@@ -311,13 +497,18 @@ using::
 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,
+   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
@@ -343,9 +534,21 @@ A: When a netdev of a physical NIC is initialized, Linux usually
      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 capabilitites of
+   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 betwen 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.
+
 Credits
 =======