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

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Date:   Thu, 29 Nov 2018 16:30:05 -0800
From:   Saeed Mahameed <saeedm@....mellanox.co.il>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        Linux Netdev List <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 8:09 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> 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.
>

mlx5 HW should deliver such packets.
just tested with scapy :
sender:

#scapy
p = Ether(dst="24:8a:07:b4:24:6e")/IP(dst="1.2.3.4", len = 1000)
sendp(p, iface = "p5p1")

receiver:
tcpdump: listening on p5p1, link-type EN10MB (Ethernet), capture size
262144 bytes
16:24:26.427563 80:18:44:e5:2f:c4 > 24:8a:07:b4:24:6e, ethertype IPv4
(0x0800), length 60: truncated-ip - 954 bytes missing! (tos 0x0, ttl
64, id 1, offset 0, flags [none], proto Options (0), length 1000)
    10.20.2.212 > 1.2.3.4:  ip-proto-0 980


> Does ethtool -S show any increasing counter ?