Merge branch 'share-umem'

Magnus Karlsson says:

====================
This patch set extends libbpf and the xdpsock sample program to
demonstrate the shared umem mode (XDP_SHARED_UMEM) as well as Rx-only
and Tx-only sockets. This in order for users to have an example to use
as a blue print and also so that these modes will be exercised more
frequently.

Note that the user needs to supply an XDP program with the
XDP_SHARED_UMEM mode that distributes the packets over the sockets
according to some policy. There is an example supplied with the
xdpsock program, but there is no default one in libbpf similarly to
when XDP_SHARED_UMEM is not used. The reason for this is that I felt
that supplying one that would work for all users in this mode is
futile. There are just tons of ways to distribute packets, so whatever
I come up with and build into libbpf would be wrong in most cases.

This patch has been applied against commit 30ee348 ("Merge branch 'bpf-libbpf-fixes'")

Structure of the patch set:

Patch 1: Adds shared umem support to libbpf
Patch 2: Shared umem support and example XPD program added to xdpsock sample
Patch 3: Adds Rx-only and Tx-only support to libbpf
Patch 4: Uses Rx-only sockets for rxdrop and Tx-only sockets for txpush in
         the xdpsock sample
Patch 5: Add documentation entries for these two features
====================

Signed-off-by: Alexei Starovoitov <[email protected]>

Author: Alexei Starovoitov

Documentation/networking/af_xdp.rst

@@ -295,7 +295,7 @@ round-robin example of distributing packets is shown below:
    {
 	rr = (rr + 1) & (MAX_SOCKS - 1);
 
-	return bpf_redirect_map(&xsks_map, rr, 0);
+	return bpf_redirect_map(&xsks_map, rr, XDP_DROP);
    }
 
 Note, that since there is only a single set of FILL and COMPLETION
@@ -304,6 +304,12 @@ 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.
 
+Libbpf uses this mode if you create more than one socket tied to the
+same umem. However, note that you need to supply the
+XSK_LIBBPF_FLAGS__INHIBIT_PROG_LOAD libbpf_flag with the
+xsk_socket__create calls and load your own XDP program as there is no
+built in one in libbpf that will route the traffic for you.
+
 XDP_USE_NEED_WAKEUP bind flag
 -----------------------------
 
@@ -355,10 +361,22 @@ 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.
+COMPLETION ring are mandatory as you need to have a UMEM tied to your
+socket. 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 as the ones from the shared umem will
+be used. Note, that the rings are single-producer single-consumer, so
+do not try to access them from multiple processes at the same
+time. See the XDP_SHARED_UMEM section.
+
+In libbpf, you can create Rx-only and Tx-only sockets by supplying
+NULL to the rx and tx arguments, respectively, to the
+xsk_socket__create function.
+
+If you create a Tx-only socket, we recommend that you do not put any
+packets on the fill ring. If you do this, drivers might think you are
+going to receive something when you in fact will not, and this can
+negatively impact performance.
 
 XDP_UMEM_REG setsockopt
 -----------------------

samples/bpf/Makefile

@@ -167,6 +167,7 @@ always += xdp_sample_pkts_kern.o
 always += ibumad_kern.o
 always += hbm_out_kern.o
 always += hbm_edt_kern.o
+always += xdpsock_kern.o
 
 ifeq ($(ARCH), arm)
 # Strip all except -D__LINUX_ARM_ARCH__ option needed to handle linux

samples/bpf/xdpsock.h

@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0
+ *
+ * Copyright(c) 2019 Intel Corporation.
+ */
+
+#ifndef XDPSOCK_H_
+#define XDPSOCK_H_
+
+#define MAX_SOCKS 4
+
+#endif /* XDPSOCK_H */

samples/bpf/xdpsock_kern.c

@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/bpf.h>
+#include "bpf_helpers.h"
+#include "xdpsock.h"
+
+/* This XDP program is only needed for the XDP_SHARED_UMEM mode.
+ * If you do not use this mode, libbpf can supply an XDP program for you.
+ */
+
+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, XDP_DROP);
+}