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ebe2ebcb3c
Ringtail (Boschini et al., ePrint 2024/1113) is third-party prior art; Lux's Corona (Ring-LWE = Module-LWE rank-1) builds on it; Pulsar is Module-LWE rank-k. Retired the conflated boschini2024corona/corona2024/coronago/... bib keys to the single canonical ringtail/corona/pulsar/magnetar keys, rewrote \cite + \bibitem by context, and fixed 'Pulsar (Ring-LWE)' mislabels (Corona where corona.metal/ single-ring/Ringtail-derived; Pulsar where Module-LWE rank-k/ML-DSA). Cryptographer-led.
133 lines
6.2 KiB
TeX
133 lines
6.2 KiB
TeX
\section{Cross-backend determinism}
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\label{sec:determinism}
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%% ============================================================================
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\subsection{Goal}
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Two validators running QuasarSTM~4.0 against the same canonical-order
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trace must produce byte-identical
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$\{\mathit{block\_hash}, \mathit{state\_root}, \mathit{receipts\_root},
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\mathit{execution\_root}, \mathit{mode\_root}, \mathit{hint\_roots}\}$.
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The challenge is that the two validators may run different backends
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(Apple Metal on M1 / M2 hardware, NVIDIA CUDA on H100 / B200), and any
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nondeterminism in scheduling, atomic ordering, floating-point
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reductions, or hash assembly destroys consensus.
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QuasarSTM~3.0~\cite{lp010-quasarstm-3} reported per-backend determinism
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informally; 4.0 ships a single harness that exercises both backends
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against the same input set and gates CI on byte-identical outputs at
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$\geq 96\%$ line coverage on the CPU reference oracle (the
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byte-equivalence ground truth against which every GPU backend is
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asserted; see \S\ref{sec:determinism-coverage}).
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\subsection{Sources of nondeterminism, eliminated}
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\begin{enumerate}[leftmargin=2em]
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\item \textbf{Atomic visibility ordering.} The lane-clock fast path
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(\S\ref{sec:tier1}) has a single atomic per write commit, ordered
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by canonical \texttt{tx\_id}. Lemma~\ref{lem:tier1-mono}.
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\item \textbf{Reducer order.} Algorithm~\ref{alg:reduce} sorts by
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\texttt{tx\_id} before applying.
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\item \textbf{MVCC visibility.} Algorithm~\ref{alg:visible} is a
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compile-time-bounded linear scan; there is no
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compare-exchange race in version selection.
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\item \textbf{Hash assembly.} All roots are keccak-f[1600] over
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explicit byte-concatenation orderings. There is no map iteration
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in root assembly.
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\item \textbf{Service drain order.} The 16-service wave-tick kernel
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runs services in fixed gid order; any work item not dispatched
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in the current tick is dispatched in the next tick at a fixed
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consumer offset.
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\item \textbf{Pairing arithmetic.} The BLS12-381 / Groth16 / Corona
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kernels avoid floating-point and use only 64-bit-limb modular
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arithmetic with deterministic round-mode-free operations.
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\item \textbf{GPU scheduler.} The wave-tick budget cap means that the
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GPU scheduler is free to interleave between ticks, but
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within-tick output is deterministic.
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\end{enumerate}
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\subsection{The harness}
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\texttt{test/integration/quasar\_cross\_backend.cpp} runs a fixed
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sequence of round inputs on Metal and CUDA in parallel, captures the
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emitted root materials, and compares them byte-for-byte. The harness
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exercises:
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\begin{table}[ht]
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\centering
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\small
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\caption{Cross-backend determinism harness coverage. PASS on both
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backends with byte-identical roots.}
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\label{tab:harness}
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\begin{tabular}{p{0.45\linewidth}p{0.18\linewidth}p{0.18\linewidth}p{0.13\linewidth}}
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\toprule
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\textbf{Test} & \textbf{Metal} & \textbf{CUDA} & \textbf{Identical} \\
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\midrule
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\texttt{empty\_round} & PASS & PASS & yes \\
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\texttt{single\_tx\_real\_roots} & PASS & PASS & yes \\
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\texttt{multi\_tx\_counters} (128 txs) & PASS & PASS & yes \\
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\texttt{bounded\_backpressure} (1024 txs) & PASS & PASS & yes \\
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\texttt{end\_to\_end\_stress} (1024 txs / 8 ticks) & PASS & PASS & yes \\
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\texttt{state\_page\_fault} & PASS & PASS & yes \\
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\texttt{root\_determinism} (1000 rounds) & PASS & PASS & yes \\
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\texttt{block\_stm\_independent\_txs} & PASS & PASS & yes \\
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\texttt{block\_stm\_conflict\_repair} (16 same-key) & PASS & PASS & yes \\
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\texttt{evm\_full\_call\_create} & PASS & PASS & yes \\
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\texttt{evm\_logn\_emit} & PASS & PASS & yes \\
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\texttt{evm\_extcode\_returndata} & PASS & PASS & yes \\
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\texttt{evm\_eip2929\_warm\_cold} & PASS & PASS & yes \\
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\texttt{evm\_eip3529\_refund} & PASS & PASS & yes \\
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\texttt{evm\_tload\_tstore} & PASS & PASS & yes \\
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\texttt{evm\_mcopy} & PASS & PASS & yes \\
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\texttt{quasar\_quorum\_real\_bls} & PASS & PASS & yes \\
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\texttt{quasar\_quorum\_real\_corona} & PASS & PASS & yes \\
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\texttt{quasar\_quorum\_real\_groth16} & PASS & PASS & yes \\
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\texttt{reducer\_lane\_fee\_accumulate} & PASS & PASS & yes \\
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\texttt{reducer\_lane\_order\_append} & PASS & PASS & yes \\
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\texttt{lane\_class\_owned\_fast\_path} & PASS & PASS & yes \\
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\texttt{lane\_class\_hot\_shared\_serialize} & PASS & PASS & yes \\
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\texttt{version\_block\_layout} & PASS & PASS & yes \\
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\texttt{commit\_horizon\_nova\_prefix} & PASS & PASS & yes \\
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\texttt{commit\_horizon\_nebula\_cut} & PASS & PASS & yes \\
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\texttt{mvcc\_gc\_horizon\_reap} & PASS & PASS & yes \\
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\texttt{formal\_cpu\_reference\_diff\_fuzz} & PASS & PASS & yes \\
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\bottomrule
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\end{tabular}
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\end{table}
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\subsection{Coverage gating}\label{sec:determinism-coverage}
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The LP-137 substrate-wide coverage gate is $\geq 96\%$ line on the
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CPU reference oracle of every VM (the byte-equivalence ground truth
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that every GPU backend is asserted against). cevm reports
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\textbf{95.78\%} TOTAL line coverage and \textbf{96.51\%} on the
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dominant translation unit \texttt{quasar\_gpu\_engine.mm} (LP-137-COVERAGE.md,
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2026-04-27); the 5 new VMs (PlatformVM, XVM, AIVM, BridgeVM, MPCVM)
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average \textbf{97.96\%} oracle line coverage. Coverage is measured
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by LLVM source-based instrumentation
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(\texttt{-fprofile-instr-generate -fcoverage-mapping}). GPU kernel
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sources (\texttt{.metal}, \texttt{.cu}, \texttt{.wgsl}) are not
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instrumentable by \texttt{llvm-cov}; the CPU reference oracle is the
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byte-equivalence ground truth, and every GPU run is asserted
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byte-for-byte against it via each VM's determinism test.
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\subsection{Formal CPU reference}
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\texttt{lib/consensus/quasar/spec/} hosts a small executable semantics
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for visibility, reducer reduction, repair, commit horizon, MVCC GC,
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Nova canonical order, and Nebula causal cut. The semantics live in
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ordinary C++ for execution and in Lean~4 for a future kernel
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refinement proof~\cite{vmvcc} (out of scope for 4.0; tracked under
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5.0 research).
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The differential fuzzer
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(\texttt{test/fuzz/quasar\_stm\_diff\_fuzz.cpp}) generates random
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canonical-order traces, runs them through the CPU reference and
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through both backends, and asserts byte-identical roots. The fuzzer
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runs every CI build with a 5-minute budget and on a nightly schedule
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with a 1-hour budget. The 2026-02-14 release closed all pre-activation
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fuzzer findings.
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%% ============================================================================
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