netdev - Re: [net 1/4] net/mlx5e: Force CHECKSUM_UNNECESSARY for short ethernet frames

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Date:   Fri, 25 Jan 2019 18:58:11 +0000
From:   Saeed Mahameed <saeedm@...lanox.com>
To:     "eric.dumazet@...il.com" <eric.dumazet@...il.com>,
        "xiyou.wangcong@...il.com" <xiyou.wangcong@...il.com>
CC:     "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "nikola.ciprich@...uxbox.cz" <nikola.ciprich@...uxbox.cz>
Subject: Re: [net 1/4] net/mlx5e: Force CHECKSUM_UNNECESSARY for short
 ethernet frames

On Fri, 2019-01-25 at 10:31 -0800, Eric Dumazet wrote:
> 
> On 01/25/2019 10:22 AM, Saeed Mahameed wrote:
> > On Tue, 2019-01-22 at 13:30 -0800, Cong Wang wrote:
> > > On Fri, Jan 18, 2019 at 4:25 PM Saeed Mahameed <
> > > saeedm@...lanox.com>
> > > wrote:
> > > > From: Cong Wang <xiyou.wangcong@...il.com>
> > > 
> > > I don't know why you want to make me as the author here, but I
> > > never
> > > agree on _your_ updates on my previous patch.
> > > 
> > > Please see below.
> > > 
> > 
> > sorry, i just took your patch and worked on top of it, i thought
> > you
> > would like to get the credit for this.
> > 
> 
> I thought the issue was that the hardware csum provided by both mlx4
> and mlx5 only
>  covered the bytes included in the IP (v4 or v6) frame.
> 
> Meaning that any non zero padding bytes are not checksummed.

in case of non IP, mlx5 will provide csum complete on the whole frame.

> If this can not be fixed by a firmware change, then the fix has
> nothing to do with a frame being
> smaller than ETH_ZLEN + ETH_FCS_LEN
> 

Again, most of the switches/routers that have non-zero padding bug are
padding only small frames.

> Alternative would be for the driver to trim the frame (pretend the
> skb->len is exactly the expected one),
> but one could argue that tcpdump should be able to see padding bytes.
> 

That requires parsing the IP headers in the driver, we are trying to
avoid that, this patch is not perfect but eliminates many of the csum
warnings seen on mlx5.

>