vendor: libs3rdma 0.3.0 — resolve devices by IPv6 address - #260
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The RDMA data path resolved its device from an IPv4 address only, so a client on an IPv6-only fabric failed at init with "no RDMA device found for IP". 0.3.0 takes an address of either family: a RoCEv2 GID *is* a 128-bit IPv6 address, IPv4 merely occupies the mapped `::ffff:a.b.c.d` form, so there is no separate v6 transport path -- only the parsing changed. Accepted forms are now `2001:db8::1`, `[2001:db8::1]:9000` (the shape `net.JoinHostPort` and most SDK config produce), and `fe80::1%eth0`, whose zone is honoured rather than dropped: a link-local address is unique only per link, so on a multi-rail host the same `fe80::` address can sit on two rails and the zone is the only thing that says which was meant. Two behaviour changes worth knowing when you pass a device selector: - `0.0.0.0` / `::` used to fail and now mean what an empty selector means -- every device with an ACTIVE port. - An invalid pin (`mlx5_0:1:256`) no longer falls back to the default rail; it fails naming what was written. Measured on 400 Gb/sec NDR hardware, single rail: 390.01 Gbps over IPv4 vs 388.11 (link-local) and 388.08 (global) over IPv6. The 0.49% gap is the IPv6 header, not a regression. No ABI change -- the header diff is comments only, so this needs no recompile beyond picking up the new .so. Both arches (x86_64 + aarch64). s3rdma v0.3.0, built from the tag (s3rdma@c623d98).
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Estimated code review effort: 1 (Trivial) | ~3 minutes Merge Risk: ⚪ Minimal · up to The IPv6 device-resolution behavior presents no supplied merge-blocking risk; only a non-blocking cleanup of conversational wording in the public API documentation remains. Poem
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Inline comments:
In `@vendor/s3rdma/include/s3rdma.h`:
- Around line 58-60: Update the documentation comment describing the ip selector
to retain its supported values and empty-selector behavior, but remove the
conversational dual-rail rationale; change only this comment and preserve the
API contract wording.
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Review on this PR noted that the header documents every address form in detail and never the device forms beside them, nor what happens to an out-of-range port/GID pin. Fixed upstream in the s3rdma library rather than here, since this file is synced verbatim by that project's vendoring script and an edit made downstream would be reverted on the next drop. The synced header now covers the "mlx5_0" / "mlx5_0:3" / "mlx5_0:1:3" forms and states that an invalid pin fails naming the input instead of silently selecting the default GID index. It also corrects the client selector documentation, which still described device names only although IP addresses have been accepted since the IPv6 change, and records the server precedence: an empty selector defers to the configured device before falling back to every ACTIVE port. Comments only. The libraries are unchanged and byte-identical to the previous commit -- these headers are hand-maintained C and nothing in the Rust build reads them.
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Picks up libs3rdma 0.3.0, which resolves the RDMA device from an IP address of either family. Before this, the address was parsed as IPv4 only and matched against the GID table as an IPv4-mapped GID, so an IPv6-only fabric failed at init with
no RDMA device found for IP.There is no separate IPv6 transport path, because RoCEv2 does not need one: a GID is a 128-bit IPv6 address, and IPv4 merely occupies the mapped
::ffff:a.b.c.dform. Only the address parsing changed.Accepted device selectors
A scoped address keeps its zone deliberately. A link-local address is unique only per link, so on a multi-rail host the same
fe80::address can appear in two GID tables, and the zone is the only thing that says which rail was meant; resolution matches only GID entries that interface backs.Two behaviour changes worth knowing
0.0.0.0/::used to fail and now mean what an empty selector means: open every device with an ACTIVE port. Neither names an interface, and no GID carries them.mlx5_0:1:256used to open GID index 0 while the caller believed they had pinned 256; it now fails naming exactly what was written.Performance
Measured — with the
p2p_allmeshbenchmark from the p2p-rdma library, not through this SDK's data path. Two hosts, 400 Gb/sec RoCE, single rail, 1 MiB transfers, 8 threads, 10 s:IPv6 lands 0.49% behind IPv4, and that gap is the IPv6 header rather than a regression: 4096 B of payload rides in 4158 wire bytes with IPv4 and 4178 with IPv6, which predicts 0.479%. PFC engaged on the lossless priority with zero discards, so IPv6 traffic is classified into the lossless class the same as IPv4.
Not measured: this SDK's own PUT/GET path over IPv6. It shares the address-resolution code that was measured above, but the token/descriptor path over an IPv6 GID has only been exercised against SoftRoCE loopback. No benchmark of this crate's RDMA path was run.
If you deploy this over IPv6, check one thing first
mlx5 synthesizes a MAC-derived link-local GID whether or not IPv6 is configured on the host. So a v6 address resolves, a queue pair comes up, descriptors exchange — and then zero bytes move, silently. RoCEv2 needs the destination MAC and the kernel resolves it from the neighbour table, so with
net.ipv6.conf.all.disable_ipv6=1there is no NDP to resolve with.A GID in the table is not evidence the path works. Verify
disable_ipv6is 0 and thatip -6 neighshows the peerREACHABLE.Scope
Vendored binaries and header only, both arches (
x86_64+aarch64), built from thev0.3.0tag. No ABI change — the header diff is comments — so nothing needs recompiling beyond picking up the new.so.s3rdma_version()now reports0.3.0, which is how a caller can tell whether the library beside it understands IPv6.Generated with Claude Code
https://claude.ai/code/session_01FgJAkB2sSfpnakZtjruTrt
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