netdev - Re: [PATCH v4 net-next 02/12] ipv6: add IFLA_GSO_IPV6_MAX

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Message-ID: <CANn89iL3sjnRKQNwbqxh_jh5cZ-Cxo58FKeqhP+mF969u4oQkA@mail.gmail.com>
Date:   Fri, 6 May 2022 14:20:43 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Alexander H Duyck <alexander.duyck@...il.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        netdev <netdev@...r.kernel.org>, Coco Li <lixiaoyan@...gle.com>
Subject: Re: [PATCH v4 net-next 02/12] ipv6: add IFLA_GSO_IPV6_MAX_SIZE

On Fri, May 6, 2022 at 1:48 PM Alexander H Duyck
<alexander.duyck@...il.com> wrote:
>
> On Fri, 2022-05-06 at 08:30 -0700, Eric Dumazet wrote:
> > From: Coco Li <lixiaoyan@...gle.com>
> >
> > This enables ipv6/TCP stacks to build TSO packets bigger than
> > 64KB if the driver is LSOv2 compatible.
> >
> > This patch introduces new variable gso_ipv6_max_size
> > that is modifiable through ip link.
> >
> > ip link set dev eth0 gso_ipv6_max_size 185000
> >
> > User input is capped by driver limit (tso_max_size)
> >
> > Signed-off-by: Coco Li <lixiaoyan@...gle.com>
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>
> So I am still not a fan of adding all this extra tooling to make an
> attribute that is just being applied to one protocol. Why not just
> allow gso_max_size to extend beyond 64K and only limit it by
> tso_max_size?

Answer is easy, and documented in our paper. Please read it.

We do not want to enable BIG TCP for IPv4, this breaks user space badly.

I do not want to break tcpdump just because some people think TCP just works.

>
> Doing that would make this patch much simpler as most of the code below
> could be dropped.
>

Sure, but no thanks.