netdev - Re: [PATCH net v2 1/1] net: change maximum number of UDP segments to 128

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Date: Thu, 11 Apr 2024 08:36:19 +0300
From: Yuri Benditovich <yuri.benditovich@...nix.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
	"David S . Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Shuah Khan <shuah@...nel.org>, 
	Jason Wang <jasowang@...hat.com>
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, yan@...nix.com, andrew@...nix.com
Subject: Re: [PATCH net v2 1/1] net: change maximum number of UDP segments to 128

I've just fixed the 'Fixed:' line and extended the commit message.
We can continue the discussion in the previous email thread or move it here.

On Thu, Apr 11, 2024 at 8:11 AM Yuri Benditovich
<yuri.benditovich@...nix.com> wrote:
>
> The commit fc8b2a619469
> ("net: more strict VIRTIO_NET_HDR_GSO_UDP_L4 validation")
> adds check of potential number of UDP segments 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.
> The number of segments for UDP packets is validated vs
> UDP_MAX_SEGMENTS also in udp.c (v4,v6), this does not affect
> quest-to-guest path but does affect packets sent to host, for
> example.
> It is important to mention that UDP_MAX_SEGMENTS is kernel-only
> define and not available to user mode socket applications.
> In order to request MSS smaller than MTU the applications
> just uses setsockopt with SOL_UDP and UDP_SEGMENT and there is
> no limitations on socket API level.
>
> Fixes: fc8b2a619469 ("net: more strict VIRTIO_NET_HDR_GSO_UDP_L4 validation")
> 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
> --
> 2.40.1
>