IPv6 APIs rework

[Robin Jarry]

Morten Brørup, Jul 19, 2024 at 12:46:
> When passing an IPv4 address as a value, alignment does matter; it 
> must be 4 byte aligned.

I was expecting the compiler to do what is necessary to copy the data to 
an aligned register before jumping to the function.

> On a CPU with 128 bit registers, I would probably also pass an IPv6 
> address as a value. With such a CPU, the parameter type should be 
> uint128_t or rte_be128_t, depending on byte order.

I don't think there is a portable/standard uint128_t yet. Everything 
I could find is either GCC or linux specific.

> There's a 3rd option:
> Have an IPv6 type that is simply an array of 16 bytes with no explicitly specified alignment:
>
> struct rte_ipv6_addr {
> 	unsigned char addr_bytes[RTE_IPV6_ADDR_LEN];
> };
>
> Or:
>
> typedef struct rte_ipv6_addr {
> 	unsigned char addr_bytes[RTE_IPV6_ADDR_LEN];
> } rte_ipv6_addr_t;
>
> If used as is, it will be unaligned.
>
> And if alignment offers improved performance for some use cases, 
> explicit alignment attributes can be added to the type in those use 
> cases.
>
> Not using an uint128_t type (or a union of other types than unsigned 
> char) will also avoid byte order issues.
>
> I guess Stephen was right to begin with. :-)

Having the type as a union (as is the POSIX type) makes casting to 
integers a lot less tedious and makes the structure overall more 
flexible.

We could completely add an unaligned be128 member to the union by the 
way. I don't see what is wrong with having sub union members.

About your concern with byte order, since the union members have 
explicit rte_be*_t types, I don't think confusion can happen. I have 
also renamed the members, replacing the "u" prefix with "a" so that it 
does not indicate that it should be used as a host integer.

        struct __rte_aligned(1) rte_ipv6_addr {
                union {
                        unsigned char a[16];
                        unaligned_be16_t a16[8];
                        unaligned_be32_t a32[4];
                        unaligned_be64_t a64[2];
                        unaligned_be128_t a128[1];
                };
        } __rte_packed;