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 ============
 MSG_ZEROCOPY
 ============
 
 Intro
 =====
 
 The MSG_ZEROCOPY flag enables copy avoidance for socket send calls.
 The feature is currently implemented for TCP and UDP sockets.
 
 
 Opportunity and Caveats
 -----------------------
 
 Copying large buffers between user process and kernel can be
 expensive. Linux supports various interfaces that eschew copying,
 such as sendpage and splice. The MSG_ZEROCOPY flag extends the
 underlying copy avoidance mechanism to common socket send calls.
 
 Copy avoidance is not a free lunch. As implemented, with page pinning,
 it replaces per byte copy cost with page accounting and completion
 notification overhead. As a result, MSG_ZEROCOPY is generally only
 effective at writes over around 10 KB.
 
 Page pinning also changes system call semantics. It temporarily shares
 the buffer between process and network stack. Unlike with copying, the
 process cannot immediately overwrite the buffer after system call
 return without possibly modifying the data in flight. Kernel integrity
 is not affected, but a buggy program can possibly corrupt its own data
 stream.
 
 The kernel returns a notification when it is safe to modify data.
 Converting an existing application to MSG_ZEROCOPY is not always as
 trivial as just passing the flag, then.
 
 
 More Info
 ---------
 
 Much of this document was derived from a longer paper presented at
 netdev 2.1. For more in-depth information see that paper and talk,
 the excellent reporting over at LWN.net or read the original code.
 
   paper, slides, video
     https://netdevconf.org/2.1/session.html?debruijn
 
   LWN article
     https://lwn.net/Articles/726917/
 
   patchset
     [PATCH net-next v4 0/9] socket sendmsg MSG_ZEROCOPY
     https://lore.kernel.org/netdev/20170803202945.70750-1-willemdebruijn.kernel@gmail.com
 
 
 Interface
 =========
 
 Passing the MSG_ZEROCOPY flag is the most obvious step to enable copy
 avoidance, but not the only one.
 
 Socket Setup
 ------------
 
 The kernel is permissive when applications pass undefined flags to the
 send system call. By default it simply ignores these. To avoid enabling
 copy avoidance mode for legacy processes that accidentally already pass
 this flag, a process must first signal intent by setting a socket option:
 
 ::
 
         if (setsockopt(fd, SOL_SOCKET, SO_ZEROCOPY, &one, sizeof(one)))
                 error(1, errno, "setsockopt zerocopy");
 
 Transmission
 ------------
 
 The change to send (or sendto, sendmsg, sendmmsg) itself is trivial.
 Pass the new flag.
 
 ::
 
         ret = send(fd, buf, sizeof(buf), MSG_ZEROCOPY);
 
 A zerocopy failure will return -1 with errno ENOBUFS. This happens if
 the socket option was not set, the socket exceeds its optmem limit or
 the user exceeds its ulimit on locked pages.
 
 
 Mixing copy avoidance and copying
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Many workloads have a mixture of large and small buffers. Because copy
 avoidance is more expensive than copying for small packets, the
 feature is implemented as a flag. It is safe to mix calls with the flag
 with those without.
 
 
 Notifications
 -------------
 
 The kernel has to notify the process when it is safe to reuse a
 previously passed buffer. It queues completion notifications on the
 socket error queue, akin to the transmit timestamping interface.
 
 The notification itself is a simple scalar value. Each socket
 maintains an internal unsigned 32-bit counter. Each send call with
 MSG_ZEROCOPY that successfully sends data increments the counter. The
 counter is not incremented on failure or if called with length zero.
 The counter counts system call invocations, not bytes. It wraps after
 UINT_MAX calls.
 
 
 Notification Reception
 ~~~~~~~~~~~~~~~~~~~~~~
 
 The below snippet demonstrates the API. In the simplest case, each
 send syscall is followed by a poll and recvmsg on the error queue.
 
 Reading from the error queue is always a non-blocking operation. The
 poll call is there to block until an error is outstanding. It will set
 POLLERR in its output flags. That flag does not have to be set in the
 events field. Errors are signaled unconditionally.
 
 ::
 
         pfd.fd = fd;
         pfd.events = 0;
         if (poll(&pfd, 1, -1) != 1 || pfd.revents & POLLERR == 0)
                 error(1, errno, "poll");
 
         ret = recvmsg(fd, &msg, MSG_ERRQUEUE);
         if (ret == -1)
                 error(1, errno, "recvmsg");
 
         read_notification(msg);
 
 The example is for demonstration purpose only. In practice, it is more
 efficient to not wait for notifications, but read without blocking
 every couple of send calls.
 
 Notifications can be processed out of order with other operations on
 the socket. A socket that has an error queued would normally block
 other operations until the error is read. Zerocopy notifications have
 a zero error code, however, to not block send and recv calls.
 
 
 Notification Batching
 ~~~~~~~~~~~~~~~~~~~~~
 
 Multiple outstanding packets can be read at once using the recvmmsg
 call. This is often not needed. In each message the kernel returns not
 a single value, but a range. It coalesces consecutive notifications
 while one is outstanding for reception on the error queue.
 
 When a new notification is about to be queued, it checks whether the
 new value extends the range of the notification at the tail of the
 queue. If so, it drops the new notification packet and instead increases
 the range upper value of the outstanding notification.
 
 For protocols that acknowledge data in-order, like TCP, each
 notification can be squashed into the previous one, so that no more
 than one notification is outstanding at any one point.
 
 Ordered delivery is the common case, but not guaranteed. Notifications
 may arrive out of order on retransmission and socket teardown.
 
 
 Notification Parsing
 ~~~~~~~~~~~~~~~~~~~~
 
 The below snippet demonstrates how to parse the control message: the
 read_notification() call in the previous snippet. A notification
 is encoded in the standard error format, sock_extended_err.
 
 The level and type fields in the control data are protocol family
 specific, IP_RECVERR or IPV6_RECVERR.
 
 Error origin is the new type SO_EE_ORIGIN_ZEROCOPY. ee_errno is zero,
 as explained before, to avoid blocking read and write system calls on
 the socket.
 
 The 32-bit notification range is encoded as [ee_info, ee_data]. This
 range is inclusive. Other fields in the struct must be treated as
 undefined, bar for ee_code, as discussed below.
 
 ::
 
         struct sock_extended_err *serr;
         struct cmsghdr *cm;
 
         cm = CMSG_FIRSTHDR(msg);
         if (cm->cmsg_level != SOL_IP &&
             cm->cmsg_type != IP_RECVERR)
                 error(1, 0, "cmsg");
 
         serr = (void *) CMSG_DATA(cm);
         if (serr->ee_errno != 0 ||
             serr->ee_origin != SO_EE_ORIGIN_ZEROCOPY)
                 error(1, 0, "serr");
 
         printf("completed: %u..%u\n", serr->ee_info, serr->ee_data);
 
 
 Deferred copies
 ~~~~~~~~~~~~~~~
 
 Passing flag MSG_ZEROCOPY is a hint to the kernel to apply copy
 avoidance, and a contract that the kernel will queue a completion
 notification. It is not a guarantee that the copy is elided.
 
 Copy avoidance is not always feasible. Devices that do not support
 scatter-gather I/O cannot send packets made up of kernel generated
 protocol headers plus zerocopy user data. A packet may need to be
 converted to a private copy of data deep in the stack, say to compute
 a checksum.
 
 In all these cases, the kernel returns a completion notification when
 it releases its hold on the shared pages. That notification may arrive
 before the (copied) data is fully transmitted. A zerocopy completion
 notification is not a transmit completion notification, therefore.
 
 Deferred copies can be more expensive than a copy immediately in the
 system call, if the data is no longer warm in the cache. The process
 also incurs notification processing cost for no benefit. For this
 reason, the kernel signals if data was completed with a copy, by
 setting flag SO_EE_CODE_ZEROCOPY_COPIED in field ee_code on return.
 A process may use this signal to stop passing flag MSG_ZEROCOPY on
 subsequent requests on the same socket.
 
 
 Implementation
 ==============
 
 Loopback
 --------
 
 Data sent to local sockets can be queued indefinitely if the receive
 process does not read its socket. Unbound notification latency is not
 acceptable. For this reason all packets generated with MSG_ZEROCOPY
 that are looped to a local socket will incur a deferred copy. This
 includes looping onto packet sockets (e.g., tcpdump) and tun devices.
 
 
 Testing
 =======
 
 More realistic example code can be found in the kernel source under
 tools/testing/selftests/net/msg_zerocopy.c.
 
 Be cognizant of the loopback constraint. The test can be run between
 a pair of hosts. But if run between a local pair of processes, for
 instance when run with msg_zerocopy.sh between a veth pair across
 namespaces, the test will not show any improvement. For testing, the
 loopback restriction can be temporarily relaxed by making
 skb_orphan_frags_rx identical to skb_orphan_frags.

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