handoff: VSync/event-wedge fixes + iterate 2.A–2.BC research notes
Source changes (dormant parity infra, retained from iterate 2.AI/2.AO): - xenia-kernel/exports.rs: nt_create_event manual_reset polarity + related event wiring - xenia-gpu/mmio_region.rs: D1MODE_VBLANK_VLINE_STATUS hardcode parity Also lands the audit-runs/ analysis notes (.md/.txt/.json digests) for the iterate 2.x VSync/0x10e8/0x1004 wedge investigation. Raw trace dumps (.jsonl/.gz/.csv/.stdout) and agent worktrees (.claude/) are gitignored as regenerable local artifacts — see memory + HANDOFF for the running findings. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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MechaCat02
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audit-runs/phase-c21-wait-begin-floating-absorb/investigation.md
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audit-runs/phase-c21-wait-begin-floating-absorb/investigation.md
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# Phase C+21 investigation — wait.begin floating-absorb (2026-05-14)
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## Framing (extends C+20 reading-error #32)
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C+20 escalated the divergence at canary tid=6 idx=104,606 to "scheduler
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determinism" because the cross-3-cold-run canary jitter survey showed
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the wait.begin was **host-scheduler-driven** in canary itself:
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| jitter | idx 104,606 event |
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|--------|----------------------------------------------------------------|
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| 1 | `wait.begin sid=75ae880ec432eb36` |
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| 2 | `kernel.return RtlEnterCriticalSection` (fast path — matches ours) |
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| 3 | offset-shifted; wait.begin at idx 104,603 with different SID |
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The diff harness's per-tid_event_idx matching anchors to whatever
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canary cold sample is chosen. The bug is observational — ours's
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behavior is structurally equivalent to canary's fast-path; the
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divergence is induced by canary's slow-path entry on contention
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which doesn't reproduce.
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C+20 deferred a fix because the scope said "no scheduler
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determinism". C+21 takes the lighter-weight option per the prompt:
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**extend the C+18 floating-absorb pattern to wait.begin events
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referencing shared-global SIDs**. This works because:
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1. The SIDs at issue are shared-global dispatcher SIDs (same
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pointer-derived recipe as the `KEVENT`/`KSEMAPHORE` cases C+18
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addressed).
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2. The wait.begin events themselves are observation-side artifacts of
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thread-scheduling contention — they belong to the same harness-
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observation-error class as the floating handle.create events.
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## Verified observational
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3 archived canary cold jitter jsonls plus a fresh C+21 canary cold
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captured under wiped-cache conditions:
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```
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SID `75ae880ec432eb36`:
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- canary-jitter-1: handle.create on tid=9 idx=295; wait.begin on
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tid=6/9/10/18 (15× total)
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- canary-jitter-2: similar multi-tid usage (NOT at idx 104,606)
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- canary-jitter-3: similar; wait.begin shifted to idx 104,603
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- canary-cold-c21 (fresh): same SID pattern; idx 104,606 fast-paths
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- ours-cold-c19: SID never appears (contention never reproduces)
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```
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Multi-tid SID usage on tids 6/9/10/18 (or 6/9/10/16/17/18/26 for the
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related SID `a25a16a4f6f547aa`) is a robust shared-global signature.
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The canary `EmitHandleCreateSharedGlobal` (`event_log.cc:435`)
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asymmetry — it hashes the dispatcher VA but stashes
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`object->handle()` as raw_handle_id — means canary's shared-global
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handle.create events are NOT self-recognizable by the C+18 recipe
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check alone. The C+21 fix adds a complementary cross-tid usage
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heuristic that detects them through their multi-tid presence in
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either handle.create OR wait.begin events.
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## The fix (diff tool only — no engine changes)
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`xenia-rs/tools/diff-events/diff_events.py`:
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1. `collect_shared_global_sids(canary_by_tid, ours_by_tid)`: new
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pre-pass union of (a) recipe-matching handle.create SIDs (C+18)
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AND (b) any SID referenced by handle.create OR wait.begin on 2+
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distinct tids in either engine (C+21 cross-tid heuristic).
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2. `is_shared_global_wait_begin(ev, shared_sids)`: classifies a
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wait.begin event as floating if ANY of its
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`handles_semantic_ids` is in `shared_sids` (covers `wait_type=any`
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and `wait_type=all`).
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3. `diff_one_tid`: extends the two-pointer walk to absorb floating
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wait.begin events on kind mismatch, mirroring the C+18
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handle.create absorption logic. Per-thread waits remain strict —
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only shared-global waits float.
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Engine source UNCHANGED. Wire format UNCHANGED (`schema_version=1`
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holds; payload structure is identical).
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Total LOC: ~140 lines additive across `diff_events.py`,
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`test_diff_events.py`, and `schema-v1.md`. 16 new diff-tool test
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assertions on top of the existing 14 — 30 total, all PASS.
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## 3-jitter verification (per RE class #32 discipline)
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Pre-C+21 jitter-1 result (from C+19/C+20 baseline): tid=6→1 main
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matched 102,553 (C+18) or 104,606 (C+20 — different SID).
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Post-C+21:
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| run | tid=6→1 matched | floating_wait (c/o) |
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|-------------------|-----------------|---------------------|
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| jitter-1 | **104,607** | 1 / 0 |
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| jitter-2 | **104,607** | 0 / 0 |
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| jitter-3 | **104,607** | 3 / 0 |
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| fresh cold-c21 | **104,607** | 0 / 0 |
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All four canary cold samples converge on the SAME matched-prefix
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(104,607). The C+21 absorb is doing exactly what it should:
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- jitter-1 contended → 1 wait.begin absorbed → advance past 104,606.
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- jitter-2 fast-pathed → 0 absorbed; matches strictly.
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- jitter-3 had 3 absorbable contended waits scattered → 3 absorbed.
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- fresh c21 fast-pathed → 0 absorbed; matches strictly.
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Sister chains UNCHANGED:
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| chain | C+19/C+20 | C+21 | delta |
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|---------------|-----------|---------|-------|
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| 4 → 11 | 11 | 11 | 0 |
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| 7 → 2 | 32 | 32 | 0 |
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| 12 → 7 | 3 | 3 | 0 |
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| 14 → 9 | 41 | 41 | 0 |
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| 15 → 10 | 16 | 16 | 0 |
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The `floating_create` column shows `0/1` on tid=15→10 (C+18's fix
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still operating) and `1/0` on tid=6→1 of jitter-3 (jitter-3 had an
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extra canary-side handle.create that C+21's recipe match detected).
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No spurious absorption.
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## The new divergence beyond the jitter cloud
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At canary tid=6 idx 104,607 (ours tid=1 idx 104,607 post-absorb):
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```
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[104604] ours+canary import.call RtlEnterCriticalSection
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[104605] ours+canary kernel.call RtlEnterCriticalSection
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[104606] ours+canary kernel.return RtlEnterCriticalSection (both fast-path)
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[104607] canary import.call RtlEnterCriticalSection (ANOTHER CS)
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[104607] ours import.call RtlLeaveCriticalSection (leaves CS)
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```
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This is a **REAL structural divergence** — canary entered a different
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CS while ours moved on to leave one. Not in scope for C+21. Will be
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addressed in C+22 framing.
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## Reading-error class #32 — locked in
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The C+20 documentation introduced #32. C+21 confirms its taxonomy
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applies broadly:
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> **#32 Canary itself is non-deterministic across cold runs in
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> contention-dependent regions. Single-canary-cold-run sampling is
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> unreliable for matched-prefix in those regions.**
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The C+21 fix is the diff-tool counter-measure: SIDs that are
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referenced by multi-tid usage are floating; their wait.begin events
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get the same observation-side treatment as their handle.create
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events. The matched-prefix metric becomes **deterministic across
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canary cold samples** within shared-global contention windows.
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## Cascade outcome
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- A=design floating-absorb extension: PASS.
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- B=implement + test in diff tool: PASS (~140 LOC, 16 new tests).
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- C=verifies across all 3 jitter jsonls: PASS — all yield 104,607.
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- D=fresh canary measurement: matched-prefix > 104,606: PASS (104,607).
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## Scope adherence
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- Engine sources: UNCHANGED.
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- Diff tool: `diff_events.py` + `test_diff_events.py` only.
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- Docs: `schema-v1.md` v1.3 + this audit-run dir.
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- GPU/audio/HID: untouched.
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- D-NEW-2 (`KeWaitForSingleObject` timeout_ns mismatch on tid=12→7
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idx=3): NOT fixed in C+21 — still the next downstream divergence
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on the tid=12→7 chain (matched=3).
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