netdev - Re: [PATCH v3 09/19] net: psp: update the TCP MSS to reflect PSP packet overhead

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Message-ID: <686aa91268bbd_3ad0f32942b@willemb.c.googlers.com.notmuch>
Date: Sun, 06 Jul 2025 12:49:22 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Daniel Zahka <daniel.zahka@...il.com>, 
 Donald Hunter <donald.hunter@...il.com>, 
 Jakub Kicinski <kuba@...nel.org>, 
 "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, 
 Paolo Abeni <pabeni@...hat.com>, 
 Simon Horman <horms@...nel.org>, 
 Jonathan Corbet <corbet@....net>, 
 Andrew Lunn <andrew+netdev@...n.ch>
Cc: Saeed Mahameed <saeedm@...dia.com>, 
 Leon Romanovsky <leon@...nel.org>, 
 Tariq Toukan <tariqt@...dia.com>, 
 Boris Pismenny <borisp@...dia.com>, 
 Kuniyuki Iwashima <kuniyu@...gle.com>, 
 Willem de Bruijn <willemb@...gle.com>, 
 David Ahern <dsahern@...nel.org>, 
 Neal Cardwell <ncardwell@...gle.com>, 
 Patrisious Haddad <phaddad@...dia.com>, 
 Raed Salem <raeds@...dia.com>, 
 Jianbo Liu <jianbol@...dia.com>, 
 Dragos Tatulea <dtatulea@...dia.com>, 
 Rahul Rameshbabu <rrameshbabu@...dia.com>, 
 Stanislav Fomichev <sdf@...ichev.me>, 
 Toke Høiland-Jørgensen <toke@...hat.com>, 
 Alexander Lobakin <aleksander.lobakin@...el.com>, 
 Jacob Keller <jacob.e.keller@...el.com>, 
 netdev@...r.kernel.org
Subject: Re: [PATCH v3 09/19] net: psp: update the TCP MSS to reflect PSP
 packet overhead

Daniel Zahka wrote:
> From: Jakub Kicinski <kuba@...nel.org>
> 
> PSP eats 32B of header space. Adjust MSS appropriately.
> 
> We can either modify tcp_mtu_to_mss() / tcp_mss_to_mtu()
> or reuse icsk_ext_hdr_len. The former option is more TCP
> specific and has runtime overhead. The latter is a bit
> of a hack as PSP is not an ext_hdr. If one squints hard
> enough, UDP encap is just a more practical version of
> IPv6 exthdr, so go with the latter. Happy to change.
> 
> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
> Signed-off-by: Daniel Zahka <daniel.zahka@...il.com>
> ---
> 
> Notes:
>     v1:
>     - https://lore.kernel.org/netdev/20240510030435.120935-8-kuba@kernel.org/
> 
>  include/net/psp/functions.h | 12 ++++++++++++
>  include/net/psp/types.h     |  3 +++
>  net/ipv4/tcp_ipv4.c         |  4 ++--
>  net/ipv6/ipv6_sockglue.c    |  6 +++++-
>  net/ipv6/tcp_ipv6.c         |  6 +++---
>  net/psp/psp_sock.c          |  5 +++++
>  6 files changed, 30 insertions(+), 6 deletions(-)
> 

> @@ -95,6 +95,9 @@ struct psp_dev_caps {
>  #define PSP_V1_KEY	32
>  #define PSP_MAX_KEY	32
>  
> +#define PSP_HDR_SIZE	16	/* We don't support optional fields, yet */

Duplicate of PSP_HDRLEN_NOOPT?

> +#define PSP_TRL_SIZE	16	/* AES-GCM/GMAC trailer size */
> +
>  struct psp_skb_ext {
>  	__be32 spi;
>  	u16 dev_id;