samples/bpf: sample application and documentation for AF_XDP sockets

This is a sample application for AF_XDP sockets. The application
supports three different modes of operation: rxdrop, txonly and l2fwd.

To show-case a simple round-robin load-balancing between a set of
sockets in an xskmap, set the RR_LB compile time define option to 1 in
"xdpsock.h".

v2: The entries variable was calculated twice in {umem,xq}_nb_avail.

Co-authored-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
index 5e31770..8e0c7fb 100644
--- a/samples/bpf/Makefile
+++ b/samples/bpf/Makefile
@@ -45,6 +45,7 @@
 hostprogs-y += syscall_tp
 hostprogs-y += cpustat
 hostprogs-y += xdp_adjust_tail
+hostprogs-y += xdpsock
 
 # Libbpf dependencies
 LIBBPF := ../../tools/lib/bpf/bpf.o ../../tools/lib/bpf/nlattr.o
@@ -98,6 +99,7 @@
 syscall_tp-objs := bpf_load.o $(LIBBPF) syscall_tp_user.o
 cpustat-objs := bpf_load.o $(LIBBPF) cpustat_user.o
 xdp_adjust_tail-objs := bpf_load.o $(LIBBPF) xdp_adjust_tail_user.o
+xdpsock-objs := bpf_load.o $(LIBBPF) xdpsock_user.o
 
 # Tell kbuild to always build the programs
 always := $(hostprogs-y)
@@ -151,6 +153,7 @@
 always += syscall_tp_kern.o
 always += cpustat_kern.o
 always += xdp_adjust_tail_kern.o
+always += xdpsock_kern.o
 
 HOSTCFLAGS += -I$(objtree)/usr/include
 HOSTCFLAGS += -I$(srctree)/tools/lib/
@@ -197,6 +200,7 @@
 HOSTLOADLIBES_syscall_tp += -lelf
 HOSTLOADLIBES_cpustat += -lelf
 HOSTLOADLIBES_xdp_adjust_tail += -lelf
+HOSTLOADLIBES_xdpsock += -lelf -pthread
 
 # Allows pointing LLC/CLANG to a LLVM backend with bpf support, redefine on cmdline:
 #  make samples/bpf/ LLC=~/git/llvm/build/bin/llc CLANG=~/git/llvm/build/bin/clang