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

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Message-ID: <0b5b3894-161f-c604-12bc-ecde341d7776@gmail.com>
Date:   Tue, 27 Nov 2018 15:48:35 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>, netdev@...r.kernel.org
Cc:     Saeed Mahameed <saeedm@...lanox.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [Patch net] mlx5: fixup checksum for ethernet padding



On 11/27/2018 03:21 PM, Cong Wang wrote:
> When an ethernet frame is padded to meet the minimum ethernet frame
> size, the padding octets are not covered by the hardware checksum.
> Fortunately the padding octets are ususally zero's, which don't affect
> checksum. However, we have a switch which pads non-zero octets, this
> causes kernel hardware checksum fault repeatedly.
> 
> Prior to commit 88078d98d1bb ("net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends"),
> skb checksum is forced to be CHECKSUM_NONE when padding is detected.
> After it, we have to keep skb->csum updated, like what we do for FCS.
> 
> The logic is a bit complicated when dealing with both FCS and padding,
> so I wrap it up in a helper function mlx5e_csum_padding().
> 
> I tested this patch with RXFCS on and off, it works fine without any
> warning in both cases.
> 
> Fixes: 88078d98d1bb ("net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends"),
> Cc: Saeed Mahameed <saeedm@...lanox.com>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> ---
>  .../net/ethernet/mellanox/mlx5/core/en_rx.c   | 36 +++++++++++++++++++
>  1 file changed, 36 insertions(+)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> index 16985ca3248d..93c18647ca74 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> @@ -732,6 +732,35 @@ static u8 get_ip_proto(struct sk_buff *skb, __be16 proto)
>  					    ((struct ipv6hdr *)ip_p)->nexthdr;
>  }
>  
> +static void mlx5e_csum_padding(struct sk_buff *skb, int network_depth,
> +			       __be16 proto, bool has_fcs)
> +{
> +	u32 frame_len = has_fcs ? skb->len - ETH_FCS_LEN : skb->len;
> +	void *ip_p = skb->data + network_depth;
> +	u32 pad_offset, pad_len;
> +	void *pad;
> +
> +	if (likely(frame_len > ETH_ZLEN))
> +		return;
> +


But the padding might be added on normal packets (say 1000 bytes + 3 bytes of padding) ?

The bug here is that mlx5 csum only includes the data in IP frame.

I would simply force skb->ip_summed to CHECKSUM_NONE if any padding is detected.

Otherwise, your patch needs more work when multiple frags are used (ie num_frags > 1 )