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

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Message-ID: <CANn89i+oyQRa3f=3V=hbs4xzGttXLuHVS7wG3qEFqaHJyLtpSQ@mail.gmail.com>
Date:   Wed, 28 Nov 2018 20:09:03 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     netdev <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 7:53 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> 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.

Well, _something_ is dropping the frames.
It can be mlx5, or something else.

Does ethtool -S show any increasing counter ?