libbpf: Support shared umems between queues and devices

Add support for shared umems between hardware queues and devices to
the AF_XDP part of libbpf. This so that zero-copy can be achieved in
applications that want to send and receive packets between HW queues
on one device or between different devices/netdevs.

In order to create sockets that share a umem between hardware queues
and devices, a new function has been added called
xsk_socket__create_shared(). It takes the same arguments as
xsk_socket_create() plus references to a fill ring and a completion
ring. So for every socket that share a umem, you need to have one more
set of fill and completion rings. This in order to maintain the
single-producer single-consumer semantics of the rings.

You can create all the sockets via the new xsk_socket__create_shared()
call, or create the first one with xsk_socket__create() and the rest
with xsk_socket__create_shared(). Both methods work.
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Link: https://lore.kernel.org/bpf/1598603189-32145-14-git-send-email-magnus.karlsson@intel.com

tools/lib/bpf/libbpf.map

@@ -306,4 +306,5 @@ LIBBPF_0.2.0 {
 		perf_buffer__buffer_fd;
 		perf_buffer__epoll_fd;
 		perf_buffer__consume_buffer;
+		xsk_socket__create_shared;
 } LIBBPF_0.1.0;

tools/lib/bpf/xsk.h

@@ -234,6 +234,15 @@ LIBBPF_API int xsk_socket__create(struct xsk_socket **xsk,
 				  struct xsk_ring_cons *rx,
 				  struct xsk_ring_prod *tx,
 				  const struct xsk_socket_config *config);
+LIBBPF_API int
+xsk_socket__create_shared(struct xsk_socket **xsk_ptr,
+			  const char *ifname,
+			  __u32 queue_id, struct xsk_umem *umem,
+			  struct xsk_ring_cons *rx,
+			  struct xsk_ring_prod *tx,
+			  struct xsk_ring_prod *fill,
+			  struct xsk_ring_cons *comp,
+			  const struct xsk_socket_config *config);
 
 /* Returns 0 for success and -EBUSY if the umem is still in use. */
 LIBBPF_API int xsk_umem__delete(struct xsk_umem *umem);