netdev - [PATCH bpf-next 2/2] xsk: document ability to redirect to any socket bound to the same umem

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Message-ID: <20240205123553.22180-3-magnus.karlsson@gmail.com>
Date: Mon,  5 Feb 2024 13:35:51 +0100
From: Magnus Karlsson <magnus.karlsson@...il.com>
To: magnus.karlsson@...el.com,
	bjorn@...nel.org,
	ast@...nel.org,
	daniel@...earbox.net,
	netdev@...r.kernel.org,
	maciej.fijalkowski@...el.com,
	yuvale@...ware.com
Cc: bpf@...r.kernel.org
Subject: [PATCH bpf-next 2/2] xsk: document ability to redirect to any socket bound to the same umem

From: Magnus Karlsson <magnus.karlsson@...el.com>

Document the ability to redirect to any socket bound to the same umem.

Signed-off-by: Magnus Karlsson <magnus.karlsson@...el.com>
---
 Documentation/networking/af_xdp.rst | 33 +++++++++++++++++------------
 1 file changed, 19 insertions(+), 14 deletions(-)

diff --git a/Documentation/networking/af_xdp.rst b/Documentation/networking/af_xdp.rst
index dceeb0d763aa..72da7057e4cf 100644
--- a/Documentation/networking/af_xdp.rst
+++ b/Documentation/networking/af_xdp.rst
@@ -329,23 +329,24 @@ XDP_SHARED_UMEM option and provide the initial socket's fd in the
 sxdp_shared_umem_fd field as you registered the UMEM on that
 socket. These two sockets will now share one and the same UMEM.
 
-There is no need to supply an XDP program like the one in the previous
-case where sockets were bound to the same queue id and
-device. Instead, use the NIC's packet steering capabilities to steer
-the packets to the right queue. In the previous example, there is only
-one queue shared among sockets, so the NIC cannot do this steering. It
-can only steer between queues.
-
-In libbpf, you need to use the xsk_socket__create_shared() API as it
-takes a reference to a FILL ring and a COMPLETION ring that will be
-created for you and bound to the shared UMEM. You can use this
-function for all the sockets you create, or you can use it for the
-second and following ones and use xsk_socket__create() for the first
-one. Both methods yield the same result.
+In this case, it is possible to use the NIC's packet steering
+capabilities to steer the packets to the right queue. This is not
+possible in the previous example as there is only one queue shared
+among sockets, so the NIC cannot do this steering as it can only steer
+between queues.
+
+In libxdp (or libbpf prior to version 1.0), you need to use the
+xsk_socket__create_shared() API as it takes a reference to a FILL ring
+and a COMPLETION ring that will be created for you and bound to the
+shared UMEM. You can use this function for all the sockets you create,
+or you can use it for the second and following ones and use
+xsk_socket__create() for the first one. Both methods yield the same
+result.
 
 Note that a UMEM can be shared between sockets on the same queue id
 and device, as well as between queues on the same device and between
-devices at the same time.
+devices at the same time. It is also possible to redirect to any
+socket as long as it is bound to the same umem with XDP_SHARED_UMEM.
 
 XDP_USE_NEED_WAKEUP bind flag
 -----------------------------
@@ -822,6 +823,10 @@ A: The short answer is no, that is not supported at the moment. The
    switch, or other distribution mechanism, in your NIC to direct
    traffic to the correct queue id and socket.
 
+   Note that if you are using the XDP_SHARED_UMEM option, it is
+   possible to switch traffic between any socket bound to the same
+   umem.
+
 Q: My packets are sometimes corrupted. What is wrong?
 
 A: Care has to be taken not to feed the same buffer in the UMEM into
-- 
2.42.0