fix(encryption): probe the Armv8 Crypto Extension on aarch64 instead of NEON (LAB-4650) - #77
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…of NEON
detect_hardware_acceleration() on aarch64 without a compile-time `aes`
target feature returned cfg!(target_feature = "neon"). NEON is a default
feature on every aarch64 target, so the flag was a constant true on all
aarch64 builds, including Cortex-A72-class parts (Raspberry Pi 3/4) that
have no AES instructions and run software AES in ring.
Use std::arch::is_aarch64_feature_detected!("aes") (stable since 1.60,
within MSRV 1.85). The compile-time short-circuit stays. Add the first
unit test for the flag: it pins hardware_acceleration_enabled() to the
platform's own runtime probe on x86/x86_64 and aarch64; both probes fold
to const true under a compile-time `aes` feature, so the pin also covers
the short-circuit path. Correct the metrics and module docs that named
only AES-NI or claimed acceleration "was used (for SHA, AES, etc.)": the
flag is informational and the crypto backend dispatches on its own.
Crypto dispatch is unchanged.
Ticket: LAB-4650
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WalkthroughThe change aligns AArch64 hardware acceleration reporting with runtime AES detection. It adds platform-specific regression coverage and updates documentation for Armv8 Crypto Extension support and backend detection. ChangesAES hardware detection
Priority: ⬇️ Low Estimated code review effort: 2 (Simple) | ~10 minutes Change: Bug fix Merge Risk: 🔵 Low · up to Correct the documentation wording before merge to avoid misleading users about which component controls hardware acceleration. 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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Inline comments:
In `@src/encryption/mod.rs`:
- Line 9: Update the module documentation bullet near
detect_hardware_acceleration() to describe hardware acceleration capability
detection only, removing the claim that this module controls or uses hardware
acceleration.
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src/encryption/core.rssrc/encryption/mod.rssrc/metrics.rs
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The module header listed "Hardware acceleration detection and usage", which overstates what this module does. detect_hardware_acceleration() reports CPU capability only — ring selects its own implementation independently, as the function's own rustdoc already states.
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Problem
ZeroKnowledgeEncryptor::detect_hardware_acceleration()on aarch64 without a compile-timeaestarget feature returnedcfg!(target_feature = "neon"). NEON is a default target feature on every aarch64 target (rustc --print cfg --target aarch64-unknown-linux-gnu), sohardware_acceleration_enabled()was a compile-time constanttrueon every aarch64 build. Cortex-A72-class parts (Raspberry Pi 3/4) have NEON but no Crypto Extension: they reported "hardware accelerated" whileringran software AES. The one platform class where the flag is useful for triage is the one where it answered wrong.Change
std::arch::is_aarch64_feature_detected!("aes")(stable since Rust 1.60; MSRV is 1.85). The compile-time#[cfg(target_feature = "aes")] → trueshort-circuit is unchanged.test_hardware_acceleration_matches_platform_probepinshardware_acceleration_enabled()to the platform's own runtime probe on x86/x86_64 and aarch64. Bothstd::archprobes fold to consttruewhenaesis enabled at compile time, so the same assertion covers the short-circuit path (verified locally withRUSTFLAGS="-C target-feature=+aes").OperationMetrics::hardware_acceleratedclaimed acceleration "was used (for SHA, AES, etc.)"; it now says what the bool is (the CPU reports AES hardware; informational, the crypto backend dispatches on its own). The module doc names the Armv8 Crypto Extension alongside AES-NI.Crypto dispatch is unchanged; the flag stays informational.
Verification
cargo fmt --check,cargo clippy --all-features -- -D warnings,cargo test --all-features,cargo test --features ffi,cargo doc --all-features --no-deps: green on x86_64.aesis a default feature onaarch64-apple-darwin), not the new probe line. The probe expression was compiled foraarch64-unknown-linux-gnuwithrustc --emit=metadataand passes clippy-D warningsthere in a scratch crate; a bogus feature name fails to compile, so the feature string is checked at build time.Downstream
cachekit-rs, cachekit-ts and the protocol matrix currently document the aarch64 behaviour as "
trueon every aarch64 build (NEON check)" dated to core 0.6 (cachekit-rs#80, cachekit-ts#132, protocol#68). Those caveats can be retired once this ships in a core release.Summary by CodeRabbit
Bug Fixes
Documentation