linux-kernel - [PATCH] net: change maximum number of UDP segments to 128

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Message-Id: <20240406082513.78692-1-yuri.benditovich@daynix.com>
Date: Sat,  6 Apr 2024 11:25:13 +0300
From: Yuri Benditovich <yuri.benditovich@...nix.com>
To: willemdebruijn.kernel@...il.com,
	davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	shuah@...nel.org,
	linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org,
	linux-kselftest@...r.kernel.org
Cc: yan@...nix.com,
	andrew@...nix.com
Subject: [PATCH] net: change maximum number of UDP segments to 128

Earlier commit fc8b2a619469378 ("net: more strict VIRTIO_NET_HDR_GSO_UDP_L4 validation")
added check of potential number of UDP segment vs UDP_MAX_SEGMENTS
in linux/virtio_net.h.
After this change certification test of USO guest-to-guest
transmit on Windows driver for virtio-net device fails,
for example with packet size of ~64K and mss of 536 bytes.
In general the USO should not be more restrictive than TSO.
Indeed, in case of unreasonably small mss a lot of segments
can cause queue overflow and packet loss on the destination.
Limit of 128 segments is good for any practical purpose,
with minimal meaningful mss of 536 the maximal UDP packet will
be divided to ~120 segments.

Signed-off-by: Yuri Benditovich <yuri.benditovich@...nix.com>
---
 include/linux/udp.h                  | 2 +-
 tools/testing/selftests/net/udpgso.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/udp.h b/include/linux/udp.h
index 3748e82b627b..7e75ccdf25fe 100644
--- a/include/linux/udp.h
+++ b/include/linux/udp.h
@@ -108,7 +108,7 @@ struct udp_sock {
 #define udp_assign_bit(nr, sk, val)		\
 	assign_bit(UDP_FLAGS_##nr, &udp_sk(sk)->udp_flags, val)

-#define UDP_MAX_SEGMENTS	(1 << 6UL)
+#define UDP_MAX_SEGMENTS	(1 << 7UL)

 #define udp_sk(ptr) container_of_const(ptr, struct udp_sock, inet.sk)

diff --git a/tools/testing/selftests/net/udpgso.c b/tools/testing/selftests/net/udpgso.c
index 1d975bf52af3..85b3baa3f7f3 100644
--- a/tools/testing/selftests/net/udpgso.c
+++ b/tools/testing/selftests/net/udpgso.c
@@ -34,7 +34,7 @@
 #endif

 #ifndef UDP_MAX_SEGMENTS
-#define UDP_MAX_SEGMENTS	(1 << 6UL)
+#define UDP_MAX_SEGMENTS	(1 << 7UL)
 #endif

 #define CONST_MTU_TEST	1500
-- 
2.34.3