Merge branch 'bpf-libbpf-af-xdp'

Magnus Karlsson says:

====================
This patch proposes to add AF_XDP support to libbpf. The main reason
for this is to facilitate writing applications that use AF_XDP by
offering higher-level APIs that hide many of the details of the AF_XDP
uapi. This is in the same vein as libbpf facilitates XDP adoption by
offering easy-to-use higher level interfaces of XDP
functionality. Hopefully this will facilitate adoption of AF_XDP, make
applications using it simpler and smaller, and finally also make it
possible for applications to benefit from optimizations in the AF_XDP
user space access code. Previously, people just copied and pasted the
code from the sample application into their application, which is not
desirable.

The proposed interface is composed of two parts:

* Low-level access interface to the four rings and the packet
* High-level control plane interface for creating and setting up umems
  and AF_XDP sockets. This interface also loads a simple XDP program
  that routes all traffic on a queue up to the AF_XDP socket.

The sample program has been updated to use this new interface and in
that process it lost roughly 300 lines of code. I cannot detect any
performance degradations due to the use of this library instead of the
previous functions that were inlined in the sample application. But I
did measure this on a slower machine and not the Broadwell that we
normally use.

The rings are now called xsk_ring and when a producer operates on
it. It is xsk_ring_prod and for a consumer it is xsk_ring_cons. This
way we can get some compile time error checking that the rings are
used correctly.

Comments and contenplations:

* The current behaviour is that the library loads an XDP program (if
  requested to do so) but the clean up of this program is left to the
  application. It would be possible to implement this cleanup in the
  library, but it would require state to be kept on netdev level,
  which there is none at the moment, and the synchronization of this
  between processes. All this adding complexity. But when we get an
  XDP program per queue id, then it becomes trivial to also remove the
  XDP program when the application exits. This proposal from Jesper,
  Björn and others will also improve the performance of libbpf, since
  most of the XDP program code can be removed when that feature is
  supported.

* In a future release, I am planning on adding a higher level data
  plane interface too. This will be based around recvmsg and sendmsg
  with the use of struct iovec for batching, without the user having
  to know anything about the underlying four rings of an AF_XDP
  socket. There will be one semantic difference though from the
  standard recvmsg and that is that the kernel will fill in the iovecs
  instead of the application. But the rest should be the same as the
  libc versions so that application writers feel at home.

Patch 1: adds AF_XDP support in libbpf
Patch 2: updates the xdpsock sample application to use the libbpf functions
Patch 3: Documentation update to help first time users

Changes v5 to v6:
  * Fixed prog_fd bug found by Xiaolong Ye. Thanks!
Changes v4 to v5:
  * Added a FAQ to the documentation
  * Removed xsk_umem__get_data and renamed xsk_umem__get_dat_raw to
    xsk_umem__get_data
  * Replaced the netlink code with bpf_get_link_xdp_id()
  * Dynamic allocation of the map sizes. They are now sized after
    the max number of queueus on the netdev in question.
Changes v3 to v4:
  * Dropped the pr_*() patch in favor of Yonghong Song's patch set
  * Addressed the review comments of Daniel Borkmann, mainly leaking
    of file descriptors at clean up and making the data plane APIs
    all static inline (with the exception of xsk_umem__get_data that
    uses an internal structure I do not want to expose).
  * Fixed the netlink callback as suggested by Maciej Fijalkowski.
  * Removed an unecessary include in the sample program as spotted by
    Ilia Fillipov.
Changes v2 to v3:
  * Added automatic loading of a simple XDP program that routes all
    traffic on a queue up to the AF_XDP socket. This program loading
    can be disabled.
  * Updated function names to be consistent with the libbpf naming
    convention
  * Moved all code to xsk.[ch]
  * Removed all the XDP program loading code from the sample since
    this is now done by libbpf
  * The initialization functions now return a handle as suggested by
    Alexei
  * const statements added in the API where applicable.
Changes v1 to v2:
  * Fixed cleanup of library state on error.
  * Moved API to initial version
  * Prefixed all public functions by xsk__ instead of xsk_
  * Added comment about changed default ring sizes, batch size and umem
    size in the sample application commit message
  * The library now only creates an Rx or Tx ring if the respective
    parameter is != NULL
====================

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>