linux-kernel - Re: [PATCH net-next] net: avoid unneeded UDP L4 and fraglist GSO resegmentation

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Date:   Sat, 31 Oct 2020 11:26:24 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Alexander Lobakin <alobakin@...me>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Antoine Tenart <atenart@...nel.org>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Network Development <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] net: avoid unneeded UDP L4 and fraglist GSO resegmentation

> >> I think it is fine to reenable this again, now that UDP sockets will
> >> segment unexpected UDP GSO packets that may have looped. We previously
> >> added general software support in commit 83aa025f535f ("udp: add gso
> >> support to virtual devices"). Then reduced its scope to egress only in
> >> 8eea1ca82be9 ("gso: limit udp gso to egress-only virtual devices") to
> >> handle that edge case.
>
> Regarding bonding and teaming: I think they should also use
> NETIF_F_GSO_SOFTWARE mask, not NETIF_F_ALL_TSO, as SCTP also has
> a software fallback. This way we could also remove a separate
> advertising of NETIF_F_GSO_UDP_L4, as it will be included in the first.
>
> So, if this one:
> 1. Add NETIF_F_GSO_UDP_L4 and NETIF_F_GSO_FRAGLIST to
>    NETIF_F_GSO_SOFTWARE;
> 2. Change bonding and teaming features mask from NETIF_F_ALL_TSO |
>    NETIF_F_GSO_UDP_L4 to NETIF_F_GSO_SOFTWARE;
> 3. Check that every virtual netdev has NETIF_F_GSO_SOFTWARE _or_
>    NETIF_F_GSO_MASK in its advertising.
>
> is fine for everyone, I'll publish more appropriate and polished v2 soon.

I think we can revert 8eea1ca82be9. Except for the part where it
defines the feature in NETIF_F_GSO_ENCAP_ALL instead of
NETIF_F_GSO_SOFTWARE. That appears to have been a peculiar choice. I
can't recall exactly why I chose that. Most likely because that was
(at the time) the only macro that covered all the devices I wanted to
capture.

As for SCTP: that has the same concern that prompted that commit for
UDP: is it safe to forward those packets to the ingress path today?