udp.7: add UDP_GRO

UDP_GRO was added in commit e20cf8d3f1f7
("udp: implement GRO for plain UDP sockets.")

    $ git describe --contains e20cf8d3f1f7
    linux/v5.0-rc1~129^2~379^2~8

Kernel source has example code in tools/testing/selftests/net/udpgro*

Per https://www.kernel.org/doc/man-pages/patches.html,
"Describe how you obtained the information in your patch":
I reviewed the relevant UDP_GRO patches.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Cc: <pabeni@redhat.com>
Cc: <netdev@vger.kernel.org>
[ alx: srcfix ]
Signed-off-by: Alejandro Colomar <alx@kernel.org>

diff --git a/man7/udp.7 b/man7/udp.7
index 79fe130fbdefad384f7b85e2b81f6de48e838260..a9fdfbac9c151dfca5c0596d05d7dafd7f9fc44b 100644 (file)
--- a/man7/udp.7
+++ b/man7/udp.7
@@ -233,6 +233,20 @@ calls by passing it as a
 .BR cmsg (7).
 A value of zero disables the feature.
 This option should not be used in code intended to be portable.
+.TP
+.BR UDP_GRO " (since Linux 5.0)"
+Enables UDP receive offload.
+If enabled,
+the socket may receive multiple datagrams worth of data
+as a single large buffer,
+together with a
+.BR cmsg (7)
+that holds the segment size.
+This option is the inverse of segmentation offload.
+It reduces receive cost by handling multiple datagrams worth of data
+as a single large packet in the kernel receive path,
+even when that exceeds MTU.
+This option should not be used in code intended to be portable.
 .SS Ioctls
 These ioctls can be accessed using
 .BR ioctl (2).