[not-matthias]

Decode MRS Xt, RNDR and MRS Xt, RNDRRS (FEAT_RNG) on arm64. With --deterministic-rng=yes, both draw 64 bits from the calling thread's PRNG stream and always succeed (NZCV=0000), like RDRAND/RDSEED on amd64. Without the option, the host register is read and its NZCV is passed through.

Before this change, VEX did not decode these registers, so a program that executed RNDR without checking for FEAT_RNG got SIGILL. Programs that check fell back to getrandom, because Valgrind dropped AT_HWCAP2 and masked the RNDR field of ID_AA64ISAR0_EL1.

Changes:

• Decode: active when the host has FEAT_RNG, or when a deterministic RNG is installed. In deterministic mode the host register is never read, so the test also runs on hosts without FEAT_RNG.
• Advertising: FEAT_RNG is still reported only when the host has it. AT_HWCAP2 now passes through HWCAP2_RNG (all other bits are still cleared), and the emulated ID_AA64ISAR0_EL1 keeps the RNDR field. This keeps client code paths the same as a native run on the same host.
• The --help line now says "hardware RNG insns" instead of "RDRAND/RDSEED"; cmdline1/cmdline2 expectations are updated.

Verified on a ubuntu-24.04-arm GitHub runner without FEAT_RNG: none/tests/arm64/deterministic_rng, cmdline1 and cmdline2 pass. The host-passthrough path (host with FEAT_RNG, no --deterministic-rng) has not been run on hardware.

Closes COD-4061

[codspeed]

Merging this PR will not alter performance

✅ 84 untouched benchmarks
⏩ 60 skipped benchmarks¹

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_{Comparing cod-4061-deterministic-rng-support-arm64-rndrrndrrs (97727ef) with master (81d7365)}

Footnotes

1. 60 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge; no actionable issues were found.

Summary

Adds ARM64 RNDR and RNDRRS support.

• Arm64 RNDR and RNDRRS instructions return host or repeatable random values.
• Arm64 clients see RNG support only when the host has it.

No actionable issues found. Review used code inspection; no tests were run. not-matthias acknowledged that the host-passthrough path has not been run on RNG-capable hardware.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A["RNDR or RNDRRS"] --> B{"Host supports RNG or callback installed?"}
  B -->|No| C["Instruction not decoded"]
  B -->|Yes| D{"Callback installed?"}
  D -->|Yes| E["Draw two words from calling thread"]
  E --> F["Return 64-bit value and NZCV = 0"]
  D -->|No| G["Read host RNG register and NZCV"]
  F --> H["Write guest register and flags"]
  G --> H

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_{Reviews (1) · Last reviewed commit: "feat(vex): serve arm64 RNDR and RNDRRS w..." · Reviewed by Greptile}