netdev - Re: [Patch net v2] mlx5: fixup checksum for short ethernet frame padding

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Message-ID: <CAM_iQpVPy2sSz53wiF=fFVm=PPRudsnc58bAn2YX156D0-TRyg@mail.gmail.com>
Date:   Wed, 28 Nov 2018 19:53:11 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Saeed Mahameed <saeedm@...lanox.com>
Subject: Re: [Patch net v2] mlx5: fixup checksum for short ethernet frame padding

On Wed, Nov 28, 2018 at 7:50 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Wed, Nov 28, 2018 at 7:40 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
> >
> > On Wed, Nov 28, 2018 at 4:07 PM Eric Dumazet <edumazet@...gle.com> wrote:
> > >
> > > A NIC is supposed to deliver frames, even the ones that 'seem' bad.
> >
> > A quick test shows this is not the case for mlx5.
> >
> > With the trafgen script you gave to me, with tot_len==40, the dest host
> > could receive all the packets. Changing tot_len to 80, tcpdump could no
> > longer see any packet. (Both sender and receiver are mlx5.)
> >
> > So, packets with tot_len > skb->len are clearly dropped before tcpdump
> > could see it, that is likely by mlx5 hardware.
>
> Or a router, or a switch.
>
> Are your two hosts connected back to back ?

Both should be plugged into a same switch. I fail to see why a
switch could parse IP header as the packet is nothing of interest,
like a IGMP snooping.