RustCrypto aes 0.8.x and polyval 0.6.x gate their ARMv8 AES / PMULL
paths behind --cfg aes_armv8 / --cfg polyval_armv8 on aarch64 (x86_64
runtime-detects AES-NI with no flag, which is why hosts never showed
it). Without the cfgs every Apple and Android client decrypted the
media plane in SOFTWARE: 240 MiB/s/core measured on an M3 Ultra —
7 µs per 1.4 KB datagram, single-handedly capping receive throughput
at ~1.57 Gbps wire on both host pairs.
Workspace .cargo/config.toml sets both cfgs for
cfg(target_arch = "aarch64"); detection stays runtime (cpufeatures)
with a safe soft fallback. open_in_place: 240 MiB/s -> 2.42 GiB/s
(10.3x). Live sweep .173 -> M3 Ultra over 10GbE: ceiling 1572 ->
4830 Mbps wire, zero loss through a 3.5 Gbps target; the .21 pair now
saturates its physical 2.4 Gbps fabric exactly.
No in-tree build path sets RUSTFLAGS (xcframework + gradle checked),
so the config reaches all client builds; a lane that sets RUSTFLAGS
overrides config rustflags entirely and must carry the cfgs itself
(noted in the file). Shipping Apple/Android binaries stay on software
crypto until rebuilt.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
