Re: IPv6 APIs rework

[Vladimir Medvedkin <medvedkinv@gmail.com>]

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Hi Robin,

Thanks, that is a good idea.




чт, 18 июл. 2024 г. в 21:27, Morten Brørup <mb@smartsharesystems.com>:

> > 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.
>

I think alignment should be 1 since in FIB6 users usually don't copy IPv6
address and just provide a pointer to the memory inside the packet. Current
vector implementation loads IPv6 addresses using unaligned access (
_mm512_loadu_si512) so it doesn't rely on alignment.


> 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.
>
Not necessary, if Ethernet frame in mbuf starts on 8b aligned address, then
IPv6 is aligned only by 2 bytes.


> 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
>
>

-- 
Regards,
Vladimir

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