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

On Mon, 21 Aug 2017 17:15:42 +0800, Yang, Yi wrote:

The issue is it is used union in

struct nsh_hdr {
    ovs_be16 ver_flags_ttl_len;
    uint8_t md_type;
    uint8_t next_proto;
    ovs_16aligned_be32 path_hdr;
    union {
        struct nsh_md1_ctx md1;
        struct nsh_md2_tlv md2;
    };
};

This should work (modulo the non-kernel type names, of course). Did you
mean to put [] after md2?

in Linux kernel build, it complained it, I changed it to

What was the error message?

struct nsh_hdr {
    ovs_be16 ver_flags_ttl_len;
    uint8_t md_type;
    uint8_t next_proto;
    ovs_16aligned_be32 path_hdr;
    union {
        struct nsh_md1_ctx md1;
        struct nsh_md2_tlv md2[0];
    };
};

I wouldn't use this. First, zero length array is a GCC extension. It
would indeed be better not to use that in uAPI. Second, I wouldn't even
use a flexible array member here, see my reply to Jan for the reasons.

Note that I commented on struct nsh_md2_tlv having __u8[] as the last
member which IMHO makes good sense. I'm not entirely sure what C99 says
about flexible array member being part of a struct inside union inside
a struct, though. GCC seems to cope with that just fine but AFAIK it
has some extension over the C standard wrt. flexible array members.

I don't know how we can support this, is it a must-have thing?

What would happen if you get a GSO packet? Ports of an ovs bridge claim
GSO support, thus they may get a GSO packet. You have to handle it one
way or the other: either software segment the packet before pushing the
header, or implement proper GSO support for NSH.

But struct nsh_hdr had different struct from struct ovs_key_nsh, we
have no way to make them completely same, do you mean we should use the
same name if they are same fields and represent the same thing?

Yes.

Thanks,

 Jiri