netdev - RE: [PATCH net-next v4] openvswitch: enable NSH support

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Date:   Mon, 21 Aug 2017 09:04:30 +0000
From:   Jan Scheurich <jan.scheurich@...csson.com>
To:     "Yang, Yi" <yi.y.yang@...el.com>, Jiri Benc <jbenc@...hat.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "dev@...nvswitch.org" <dev@...nvswitch.org>,
        "blp@....org" <blp@....org>, "e@...g.me" <e@...g.me>
Subject: RE: [PATCH net-next v4] openvswitch: enable NSH support

> struct nsh_hdr {
>     ovs_be16 ver_flags_ttl_len;
>     uint8_t mdtype;
>     uint8_t np;
>     ovs_16aligned_be32 path_hdr;
>     union {
>         struct nsh_md1_ctx md1;
>         struct nsh_md2_tlv md2;
>     };
> };
> 

The second member of the union should be a variable length array [] of struct nsh_md2_tlv

struct nsh_hdr {
    ovs_be16 ver_flags_ttl_len;
    uint8_t mdtype;
    uint8_t np;
    ovs_16aligned_be32 path_hdr;
    union {
        struct nsh_md1_ctx md1;
        struct nsh_md2_tlv md2[];
    };
};

That was the original design before Ben removed it due to missing support in Microsoft compiler. 
For the Kernel datapath we should go back to that. 

I wonder about the possible 16-bit alignment of the 32-bit fields, though. How is that handled in the kernel? Also struct nsh_md1_ctx has 32-bit members, which might not be 32-bit aligned in the packet.

BR, Jan