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

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Date: Thu, 11 Apr 2024 12:01:20 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Yuri Benditovich <yuri.benditovich@...nix.com>, 
 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

Yuri Benditovich 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>

Reviewed-by: Willem de Bruijn <willemb@...gle.com>

No need for a cover letter for a single patch btw.