xenia-rs/audit-runs/phase-a-diff-harness/ours-changes.md

Phase A — ours (xenia-rs) changes

All changes are additive and cvar-gated default-off. With the cvar unset, xenia-rs check --stable-digest -n 50000000 produces a byte-identical digest to the pre-patch baseline (verified in validation.md).

File	Change	Why
crates/xenia-kernel/src/event_log.rs	NEW (≈340 LOC)	Phase A emitter. Lazy file open, per-tid monotonic tid_event_idx counter, FNV-1a 64-bit semantic_id, JSONL writer with Mutex<BufWriter<File>>. is_enabled() is a relaxed atomic-bool load → zero overhead when disabled. Includes unit tests for FNV-1a vs the standard "foobar" test vector and for semantic_id stability.
crates/xenia-kernel/src/lib.rs	+1 line (pub mod event_log;)	Register the new module.
crates/xenia-kernel/src/state.rs	+33 LOC inside call_export	Single hook site for import.call / kernel.call / kernel.return. All three emits are inside if phase_a_on guards. Reads tid and cycle_count from the running thread when enabled.
crates/xenia-app/src/main.rs	+1 CLI flag, +6 LOC dispatch	Adds --phase-a-event-log <PATH> to the Exec subcommand. Env-var fallback XENIA_PHASE_A_EVENT_LOG. Calls xenia_kernel::event_log::init(path) once at startup; None keeps the emitter disabled.

Total surface area

≈ 380 LOC additive across 4 files; no existing logic modified.

Hook coverage (v1)

• import.call — emitted at the syscall dispatcher (call_export in state.rs) once per kernel/import invocation, before kernel.call.
• kernel.call — same site, after import.call, before the export body runs.
• kernel.return — same site, after the export body returns, with r3 as the integer return value.
• schema_version header — emitted once on file open (synthetic tid=0).

The following event kinds are part of schema v1 but not yet wired in either engine in this phase. Some have a Rust emitter function ready (function exists in event_log.rs, no call site); others have no Rust function yet:

• thread.create, thread.exit — emitter ready (emit_thread_create, emit_thread_exit), no call sites.
• thread.suspend, thread.resume — declared in schema, no Rust function yet.
• handle.create, handle.destroy — emitter ready (emit_handle_create, emit_handle_destroy), no call sites.
• wait.begin, wait.end — emitter ready (emit_wait_begin, emit_wait_end), no call sites.
• mem.write — declared in schema, no Rust function yet; gated behind a separate cvar (phase_a_event_log_mem_writes) declared in canary defaulted false.
• vfs.open, vfs.read, vfs.close — declared in schema, no Rust function yet.

Wiring any of these is additive and can land in a follow-up without touching the schema or the diff tool.

Verification

cd "/home/fabi/RE - Project Sylpheed/xenia-rs"

# Build
cargo build --release -p xenia-app

# Unit tests (FNV-1a + semantic_id stability)
cargo test -p xenia-kernel event_log

# Cvar-OFF determinism
cp target/release/xenia-rs target/release/xenia-rs-phaseA-post
target/release/xenia-rs-phaseA-post check --stable-digest -n 50000000 \
  --out /tmp/digest-after.json \
  "/home/fabi/RE - Project Sylpheed/Project Sylpheed - Arc of Deception (USA, Europe) (En,Ja).iso"
diff audit-runs/phase-a-diff-harness/digest-pre-patch.json /tmp/digest-after.json
# expect empty diff (byte-identical)

# Cvar-ON sanity run
rm -f audit-runs/phase-a-diff-harness/ours-sanity.jsonl
target/release/xenia-rs-phaseA-post exec -n 50000000 \
  --phase-a-event-log audit-runs/phase-a-diff-harness/ours-sanity.jsonl --quiet \
  "/home/fabi/RE - Project Sylpheed/Project Sylpheed - Arc of Deception (USA, Europe) (En,Ja).iso"
head -1 audit-runs/phase-a-diff-harness/ours-sanity.jsonl  # must start with schema_version