[Intel-wired-lan] [RFC PATCH 2/5] mlx5: Add support for UDP tunnel segmentation with outer checksum offload

[Alexander Duyck]

On Wed, Apr 20, 2016 at 10:40 AM, Saeed Mahameed
<saeedm at dev.mellanox.co.il> wrote:
> On Tue, Apr 19, 2016 at 10:06 PM, Alexander Duyck <aduyck at mirantis.com> wrote:
>> This patch assumes that the mlx5 hardware will ignore existing IPv4/v6
>> header fields for length and checksum as well as the length and checksum
>> fields for outer UDP headers.
>>
>> I have no means of testing this as I do not have any mlx5 hardware but
>> thought I would submit it as an RFC to see if anyone out there wants to
>> test this and see if this does in fact enable this functionality allowing
>> us to to segment UDP tunneled frames that have an outer checksum.
>>
>> Signed-off-by: Alexander Duyck <aduyck at mirantis.com>
>> ---
>>  drivers/net/ethernet/mellanox/mlx5/core/en_main.c |    7 ++++++-
>>  1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
>> index e0adb604f461..57d8da796d50 100644
>> --- a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
>> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
>> @@ -2390,13 +2390,18 @@ static void mlx5e_build_netdev(struct net_device *netdev)
>>         netdev->hw_features      |= NETIF_F_HW_VLAN_CTAG_FILTER;
>>
>>         if (mlx5e_vxlan_allowed(mdev)) {
>> -               netdev->hw_features     |= NETIF_F_GSO_UDP_TUNNEL;
>> +               netdev->hw_features     |= NETIF_F_GSO_UDP_TUNNEL |
>> +                                          NETIF_F_GSO_UDP_TUNNEL_CSUM |
>> +                                          NETIF_F_GSO_PARTIAL;
>>                 netdev->hw_enc_features |= NETIF_F_IP_CSUM;
>>                 netdev->hw_enc_features |= NETIF_F_RXCSUM;
>>                 netdev->hw_enc_features |= NETIF_F_TSO;
>>                 netdev->hw_enc_features |= NETIF_F_TSO6;
>>                 netdev->hw_enc_features |= NETIF_F_RXHASH;
>>                 netdev->hw_enc_features |= NETIF_F_GSO_UDP_TUNNEL;
>> +               netdev->hw_enc_features |= NETIF_F_GSO_UDP_TUNNEL_CSUM |
>> +                                          NETIF_F_GSO_PARTIAL;
>> +               netdev->gso_partial_features = NETIF_F_GSO_UDP_TUNNEL_CSUM;
>>         }
>>
>>         netdev->features          = netdev->hw_features;
>>
>
> Hi Alex,
>
> Adding Matt, VxLAN feature owner from Mellanox,
> Matt please correct me if am wrong, but We already tested GSO VxLAN
> and we saw the TCP/IP checksum offloads for both inner and outer
> headers handled by the hardware.
>
> And looking at mlx5e_sq_xmit:
>
> if (likely(skb->ip_summed == CHECKSUM_PARTIAL)) {
>         eseg->cs_flags = MLX5_ETH_WQE_L3_CSUM;
> if (skb->encapsulation) {
>         eseg->cs_flags |= MLX5_ETH_WQE_L3_INNER_CSUM |
> MLX5_ETH_WQE_L4_INNER_CSUM;
>         sq->stats.csum_offload_inner++;
> } else {
>         eseg->cs_flags |= MLX5_ETH_WQE_L4_CSUM;
> }
>
> We enable inner/outer hardware checksumming unconditionally without
> looking at the features Alex is suggesting in this patch,
> Alex, can you elaborate more on the meaning of those features ? and
> why would it work for us without declaring them ?

Well right now the feature list exposed by the device indicates that
TSO is not used if a VxLAN tunnel has a checksum in an outer header.
Since that is not exposed currently that is completely offloaded in
software via GSO.

What the GSO partial does is allow us to treat GSO for tunnels with
checksum like it is GSO for tunnels without checksum by precomputing
the UDP checksum as though the frame had already been segmented and
restricts us to an even multiple of MSS bytes that are to be segmented
between all the frames.  One side effect though is that all of the IP
and UDP header fields are also precomputed, but from what I can tell
it looks like the values that would be changed by a change in length
are ignored or overwritten by the hardware and driver anyway.

- Alex