On Fri, 11 Oct 2024 14:37:35 +0200 Morten Brørup <m...@smartsharesystems.com> wrote:
> > From: Robin Jarry [mailto:rja...@redhat.com] > > Sent: Thursday, 10 October 2024 22.08 > > > > Morten Brørup, Oct 06, 2024 at 10:18: > > > This has been discussed before, but I want to double check... > > > > > > If - sometime in the future - we want to add a union to offer a 2- > > byte > > > access variant and make the structure to 2-byte aligned (which is the > > > common case in Ethernet packets), it will break both API and ABI. > > This > > > seems unlikely to get accepted at a later time, so I think we are > > > - right now - in a situation where it's now or never: > > > > > > struct rte_ipv6_addr { > > > __extension__ > > > union { > > > unsigned char a[RTE_IPV6_ADDR_SIZE]; > > > uint16_t w[RTE_IPV6_ADDR_SIZE / 2]; > > > }; > > > }; > > > > > > Unless some of the CPU folks want the 2-byte aligned variant, stick > > > with what you offered. > > > > I was also thinking the same. I could have added an unnamed union which > > only contains one field. > > > > However, it does not make much sense if we never want to change the > > default alignment. > > > > Important note: DPDK is compiled with the following C flags: > > > > -Wno-address-of-packed-member > > > > Added in 2017 https://git.dpdk.org/dpdk/commit/?id=95cd37070af44 > > > > If we had struct rte_ipv6_addr aligned on 2 bytes (or more), > > applications that do not silence that warning would have a hard time. > > Example in grout: > > > > ../modules/ip6/datapath/ip6_input.c:99:54: error: taking address of > > packed member of ‘struct rte_ipv6_hdr’ may result in an unaligned > > pointer value [-Werror=address-of-packed-member] > > 99 | nh = ip6_route_lookup(iface->vrf_id, &ip- > > >dst_addr); > > | > > ^~~~~~~~~~~~~ > > I don't understand the choice of packing for these types... > > Why are the IPv4 [ipv4h] and IPv6 header [ipv6h] types packed? Is it so they > can be stored on unaligned addresses? > > E.g. the IPv4 header's natural alignment is 4 byte (due to the rte_be32_t > src_addr, dst_addr fields). The IPv4 header is most often 2 byte aligned, > being located after an Ethernet header. > > Maybe they should be 2-byte aligned, like the Ethernet address type [etha] > and the Ethernet header type [ethh]. > > The VLAN tag type [vlanh] is packed. Why? > > I wonder if the compiler would spit out the same warnings if the above types > were __rte_aligned(2) instead of __rte_packed. I am in the never use packed camp for network headers. The Linux, BSD and glibc headers avoid use of packed. Windows seems to love using it.
