Re: IPv6 APIs rework

[Stephen Hemminger <stephen@networkplumber.org>]

On Thu, 18 Jul 2024 22:27:03 +0200
Morten Brørup <mb@smartsharesystems.com> wrote:

> > From: Robin Jarry [mailto:rjarry@redhat.com]
> > 
> > Hi folks,
> > 
> > while working on IPv6 support for grout [1], I noticed that all DPDK
> > IPv6 APIs used fixed sized arrays in the route lookup functions [2].
> > 
> >  int rte_fib6_lookup_bulk(struct rte_fib6 *fib,
> >                           uint8_t ips[][RTE_FIB6_IPV6_ADDR_SIZE],
> >                           uint64_t *next_hops,
> >                           int n);
> > 
> > If I'm not mistaken, using sized arrays in function signatures is only
> > for documentation purposes and does not result in any specific compiler
> > checks. In the above example, the ips parameter is considered as a plain
> > old `uint8_t **` pointer.
> > 
> > Also, not having a dedicated type for IPv6 addresses requires obscure
> > pointer arithmetic [3] and casting [4].
> > 
> > I'd like to introduce a real IPv6 address structure that has the same
> > alignment than a dumb `uint8_t *` pointer but has an union to ease
> > casting and most importantly presents the whole thing as an explicit
> > typed structure:
> > 
> >     #define RTE_IPV6_ADDR_SIZE 16
> > 
> >     struct rte_ipv6_addr {
> >         union {
> >             uint8_t u8[RTE_IPV6_ADDR_SIZE];
> >             uint16_t u16[RTE_IPV6_ADDR_SIZE / sizeof(uint16_t)];
> >             uint32_t u32[RTE_IPV6_ADDR_SIZE / sizeof(uint32_t)];
> >             uint64_t u64[RTE_IPV6_ADDR_SIZE / sizeof(uint64_t)];
> >         };
> >     } __rte_packed __rte_aligned(1);
> > 
> > This would require some breakage of the APIs but I think it would
> > benefit code readability and maintainability in the long term.  
> 
> In short: Although I like the idea of a unified IPv6 address type very much, I'm not sure consensus can be reached about the optimal alignment of such a type.
> 
> The long version:
> 
> Please consider this proposal in a broader perspective.
> 
> The IPv4 FIB lookup takes an uint32_t array, so the IPv4 address type here is 4 byte aligned: uint32_t *ips
> Generally, uint32_t or rte_be32_t is used for IPv4 addresses, and both these types are 4 byte aligned. In other words: IPv4 addresses are considered 4 byte aligned by DPDK.
> 
> I don't think it is similarly simple for IPv6 addresses.
> 
> The alignment of IPv6 addresses may depend on how/where they are used, e.g.:
> 1. For the FIB library, it might be good for vector implementations to have the IPv6 addresses naturally aligned (i.e. 16 byte aligned), like the uint128_t/__int128/__m128i type (or the rte_xmm_t type [XMM]). Furthermore, a simple integer type (uint128_t equivalent) might be preferable in this API.
> 2. In the IPv6 packet header, the IPv6 addresses are not 16 byte aligned, they are 8 byte aligned. So we cannot make the IPv6 address type 16 byte aligned.
> 
> I fear that broadly dumbing down the IPv6 address type to always use 1 byte alignment could potentially introduce unwanted performance penalties (now or in the future). We didn't do it for IPv4 addresses, so let's not do it for IPv6 addresses.
> 
> Perhaps we could use the lowest "non-exotic" (considering the use of IPv6 addresses) alignment, which I would guess is 8 byte - as in the IPv6 packet header.
> For reference, Ethernet addresses are defined as 2 byte aligned [ETH].
> 
> [XMM]: https://elixir.bootlin.com/dpdk/v24.03/source/lib/eal/x86/include/rte_vect.h#L42
> [ETH]: https://elixir.bootlin.com/dpdk/v24.07-rc2/source/lib/net/rte_ether.h#L74
> 
> > 
> >  int rte_fib6_lookup_bulk(struct rte_fib6 *fib,
> >                           const struct rte_ipv6_addr *ips,
> >                           uint64_t *next_hops,
> >                           int n);
> > 
> > I already have a semi-working draft and am in the process of splitting
> > the changes into small chunks to make them easier to review.
> > 
> > https://github.com/DPDK/dpdk/compare/main...rjarry:dpdk:ipv6-address-
> > rework
> > 
> > Is that something that would be of interest? If yes, I would like to
> > announce API breakage before the release of 24.07 so that the changes
> > can be integrated into 24.11.
> > 
> > Cheers!
> > 
> > [1] https://github.com/rjarry/grout
> > [2]
> > https://doc.dpdk.org/api/rte__fib6_8h.html#a924678410ccb9551cda3e75d742a
> > 11e3
> > [3] https://git.dpdk.org/dpdk/tree/lib/fib/trie_avx512.c?h=v24.07-
> > rc2#n340
> > [4] https://git.dpdk.org/dpdk/tree/lib/hash/rte_thash.h?h=v24.07-
> > rc2#n156
> > 
> > --
> > Robin  
> 

If you look at the standard netinet/in.h the storage of IPv6 addresses
is in in6_addr. DPDK has always wanted to do its own thing...

The in6_addr is a union with no explicit alignment.

struct in6_addr
  {
    union
      {
        uint8_t __u6_addr8[16];
        uint16_t __u6_addr16[8];
        uint32_t __u6_addr32[4];
      } __in6_u;

Better to not have explicit alignment and not have 64 bit value.