Re: [Patch net v3] mlx5: force CHECKSUM_NONE for short ethernet frames

[Saeed Mahameed <saeedm@dev.mellanox.co.il>]

On Mon, Dec 3, 2018 at 10:14 PM Cong Wang <xiyou.wangcong@gmail.com> 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 was forced to be CHECKSUM_NONE when padding is detected.
> After it, we need to keep skb->csum updated, like what we do for RXFCS.
> However, fixing up CHECKSUM_COMPLETE requires to verify and parse IP
> headers, it is not worthy the effort as the packets are so small that
> CHECKSUM_COMPLETE can't save anything.
>
> 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@mellanox.com>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Tariq Toukan <tariqt@mellanox.com>
> Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
> ---
>  .../net/ethernet/mellanox/mlx5/core/en_rx.c   | 22 ++++++++++++++++++-
>  1 file changed, 21 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> index 624eed345b5d..1c153b8091da 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> @@ -732,6 +732,13 @@ static u8 get_ip_proto(struct sk_buff *skb, int network_depth, __be16 proto)
>                                             ((struct ipv6hdr *)ip_p)->nexthdr;
>  }
>
> +static bool is_short_frame(struct sk_buff *skb, bool has_fcs)
> +{
> +       u32 frame_len = has_fcs ? skb->len - ETH_FCS_LEN : skb->len;
> +
> +       return frame_len <= ETH_ZLEN;
> +}
> +

Do we really need to handle FCS here ?
maybe increase the small packet size to always assume FCS is there:

return skb->len <= ETH_ZLEN + ETH_FCS_LEN;
to avoid conditional statements at Data path.

>  static inline void mlx5e_handle_csum(struct net_device *netdev,
>                                      struct mlx5_cqe64 *cqe,
>                                      struct mlx5e_rq *rq,
> @@ -755,9 +762,22 @@ static inline void mlx5e_handle_csum(struct net_device *netdev,
>                 goto csum_unnecessary;
>
>         if (likely(is_last_ethertype_ip(skb, &network_depth, &proto))) {
> +               bool has_fcs = !!(netdev->features & NETIF_F_RXFCS);
> +
>                 if (unlikely(get_ip_proto(skb, network_depth, proto) == IPPROTO_SCTP))
>                         goto csum_unnecessary;
>
> +               /* CQE csum doesn't cover padding octets in short ethernet
> +                * frames. And the pad field is appended prior to calculating
> +                * and appending the FCS field.
> +                *
> +                * Detecting these padded frames requires to verify and parse
> +                * IP headers, so we simply force all those small frames to be
> +                * CHECKSUM_NONE even if they are not padded.
> +                */
> +               if (unlikely(is_short_frame(skb, has_fcs)))
> +                       goto csum_none;
> +

As Eric mentioned, goto csum_unnecessary; here, the code will handle the rest.
for l3/l4 packets the HW already verifies the checksum we must leverage that.

>                 skb->ip_summed = CHECKSUM_COMPLETE;
>                 skb->csum = csum_unfold((__force __sum16)cqe->check_sum);
>                 if (network_depth > ETH_HLEN)
> @@ -768,7 +788,7 @@ static inline void mlx5e_handle_csum(struct net_device *netdev,
>                         skb->csum = csum_partial(skb->data + ETH_HLEN,
>                                                  network_depth - ETH_HLEN,
>                                                  skb->csum);
> -               if (unlikely(netdev->features & NETIF_F_RXFCS))
> +               if (unlikely(has_fcs))
>                         skb->csum = csum_block_add(skb->csum,
>                                                    (__force __wsum)mlx5e_get_fcs(skb),
>                                                    skb->len - ETH_FCS_LEN);
> --
> 2.19.1
>