netdev - Re: [PATCH net-next 2/4] sfc: implement encap TSO on EF100

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Date:   Fri, 30 Oct 2020 13:33:47 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Edward Cree <ecree@...arflare.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        linux-net-drivers@...arflare.com, Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 2/4] sfc: implement encap TSO on EF100

On Fri, Oct 30, 2020 at 12:43 PM Edward Cree <ecree@...arflare.com> wrote:
>
> On 30/10/2020 16:26, Willem de Bruijn wrote:
> > Then you could (as follow-up) advertise without GSO_PARTIAL and avoid
> > the whole transition through the gso layer?
>
> The thing is, non-PARTIAL offload only supports tunnels that the NIC
>  understands (single-layer UDP tunnels), but AIUI GSO_PARTIAL can
>  support all sorts of other things, such as gretaps (though we'd need
>  some more advertised features, I haven't figured out quite which
>  ones yet but when I do and can test it I'll send a follow-up) and
>  nested tunnels (as long as we don't need to edit the 'middle' IP ID,
>  e.g. if it's DF or IPv6).  So we definitely want to advertise
>  GSO_PARTIAL support.
> But possibly I don't need to have NETIF_F_GSO_UDP_TUNNEL[_CSUM] in
>  net_dev->gso_partial_features?

If the device can handle fixing the odd last segment length, indeed.

If you remove them from gso_partial_flags, then gso_features_check
won't mask them out


        /* Support for GSO partial features requires software
         * intervention before we can actually process the packets
         * so we need to strip support for any partial features now
         * and we can pull them back in after we have partially
         * segmented the frame.
         */
        if (!(skb_shinfo(skb)->gso_type & SKB_GSO_PARTIAL))
                features &= ~dev->gso_partial_features;

as called in validate_xmit_skb prior to testing whether the packet can
be passed to the nic as is in netif_needs_gso.

Until adding other tunnel types like NETIF_F_GSO_GRE_CSUM, for this
device gso_partial_features would then be 0 and thus
NETIF_F_GSO_PARTIAL is not needed at all?