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

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Date:   Fri, 6 May 2022 14:37:41 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Eric Dumazet <edumazet@...gle.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 2:20 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> 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.

I have 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.

The changes I suggested don't enable it for IPv4. What your current
code is doing now is using dev->gso_max_size and if it is the correct
IPv6 type you are using dev->gso_ipv6_max_size. What I am saying is
that instead of adding yet another netdev control you should just make
it so that we retain existing behavior when gso_max_size is less than
GSO_MAX_SIZE, and when it exceeds it all non-ipv6 types max out at
GSO_MAX_SIZE and only the ipv6 type packets use gso_max_size when you
exceed GSO_MAX_SIZE.

The big thing I am not a fan of is adding protocol level controls down
in the link level interface. It makes things confusing. For example,
say somebody has existing scripts to limit the gso_max_size and they
were using IPv6 and your new control is added. Suddenly the
gso_max_size limitations they were setting won't be applied because
you split things up at the protocol level.