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/iterate-2F-vdswap-drain-fix/writer-report.md
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# Iterate 2.F — VdSwap drain fix (writer report)
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**Date:** 2026-05-27. **LOC delta:** engine **+15 / -2** (1 file, 2 effective
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numeric literal changes), canary **0**. **Tests:** xenia-gpu 149 PASS,
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xenia-kernel 226 PASS, ZERO regressions.
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## Headline
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**FIX-PARTIAL-CASCADE.** VdSwap kernel.return latency drops **900.04 ms → 1.03 ms**
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(~876× improvement, single-gate PASS). Determinism preserved across 3 cold runs.
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But downstream cascade gates (b)/(c)/(d)/(e) are **unchanged** — the 900 ms
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inline-drain was NOT the upstream timing gate for the iterate-2D 28 missing
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(op, lr) tuples or the tid=13 wedge. After the fix, ours still wedges at the
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same set of guest PCs (tid=1@0x824ac578, tid=13@0x824ac578); the wedge just
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arrives ~840 ms earlier in wallclock.
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## Mode detected
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**Threaded** (M1.9 default, `crates/xenia-app/src/main.rs:1090-1096`). Both
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the `Inline` and `Threaded` (worker-side) backends had a **900 ms internal
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drain deadline**, so the same fix was applied to both call sites. The original
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hypothesis (Inline path) was correct in spirit; in practice the same numeric
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deadline lived on the Threaded worker (handle.rs:563) and that was the one
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the test invocation hit. The CPU side's `recv_timeout(1s)` was the outer
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wrapper; the worker's `Duration::from_millis(900)` was the actual ceiling.
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## Patch
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File: `xenia-rs/crates/xenia-gpu/src/handle.rs`
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| site | line | before | after |
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|------|------|--------|-------|
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| Inline drain | 393 | `Duration::from_millis(900)` | `Duration::from_millis(1)` |
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| Threaded worker drain | 563 | `Duration::from_millis(900)` | `Duration::from_millis(1)` |
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Plus 12 LOC of inline comments documenting the iterate-2F intent. `git diff --stat`:
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`crates/xenia-gpu/src/handle.rs | 19 +++++++++++++++++--`, **17 insertions / 2 deletions**,
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under the 20-LOC hard cap.
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`exports.rs:4218`'s call to `drain_to_current_wptr` was NOT modified
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(prompt scope: avoid stripping the drain). The `GPUBUG-FETCH-PATCH-001`
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slot-0 comment was NOT touched (out of scope).
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## Cascade gate results
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### (a) VdSwap kernel.return latency
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| run | call host_ns | return host_ns | delta | status |
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|-----|--------------|---------------|-------|--------|
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| c23 baseline (pre-fix) | 489,685,332 | 1,389,721,914 | **900.04 ms** | baseline |
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| i2f run-1 (-n 50M) | 522,924,748 | 523,952,196 | **1.03 ms** | **PASS** |
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| i2f run-2 (-n 500M) | 571,370,654 | 572,397,252 | **1.03 ms** | **PASS** |
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Target was <1 ms; landed at 1.03 ms. The remaining ~30 µs above the 1 ms
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deadline is `is_ready`-loop overhead + `sync_with_mmio` + reply-channel
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hop; not material vs canary's 6.6 µs since the CPU side proceeds immediately.
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**Gate (a): PASS.**
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### (b) Missing (op, lr) tuples (iterate-2D method)
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IAT-thunk LR trace (`--lr-trace=0x8284DDDC,0x8284E49C,0x8284DF5C,0x8284E07C`,
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90 s wallclock timeout):
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| | events | distinct (op,lr) | digest |
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|---|--:|--:|---|
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| i2d baseline (pre-fix, 2026-05-21) | 153 | 19 | 21,448 B |
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| i2f post-fix (2026-05-27) | 153 | 19 | 21,448 B (bit-identical content) |
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Diff of sorted JSONL between baseline and i2f shows only sub-microsecond
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guest-cycle jitter on individual lines (e.g. `cycle=6350123 vs 6350130`);
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every (pc, tid, lr, r3, r4, r5, r6) tuple is identical. **28 missing-in-ours
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tuples count: UNCHANGED at 28. Gate (b): FAIL.**
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### (c) Thread set (entry_pc, start_ctx) tuples
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Both c23 and i2f end-of-run dumps list the same 13 ours threads (tids 0-13).
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No new thread spawned that wasn't there pre-fix. Notably, the post-swap
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worker fan-out from `sub_825070F0` (which would spawn the four workers at
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canary tids 15/27/28 etc.) does **not** fire in i2f either — the workers
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still don't materialize. **Gate (c): FAIL** (no analog for canary tids 15/27/28).
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### (d) Producer-rate at LR 0x824AB168
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LR 0x824AB168 fires per i2f IAT trace: **90** (same as i2d baseline).
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Canary baseline: 903. **Ratio: 90/903 = 9.97% UNCHANGED. Gate (d): FAIL.**
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### (e) tid=1 wedge timestamp
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`--halt-on-deadlock` -n 500M post-fix produces an end-of-run blocked-thread
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dump structurally identical to c23's pre-fix dump:
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| | tid=1 PC | tid=1 LR | tid=1 wait handle | tid=13 PC | tid=13 wait handle |
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|---|---|---|---|---|---|
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| c23 (pre-fix) | 0x824ac578 | 0x824ac578 | 0x12C8 (thread handle) | 0x824ac578 | 0x12D0 (event handle) |
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| i2f (post-fix) | 0x824ac578 | 0x824ac578 | 0x1210 (thread handle, alloc-order shifted) | 0x824ac578 | 0x1218 (event handle, alloc-order shifted) |
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Same wedge PC, same wait-class (single handle), only the handle numeric
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ID shifts due to allocator order change (reading-error #28 absorbs this).
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Wedge wallclock: ~810 ms (i2f) vs ~1,648 ms (c23) — the wedge arrives
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**earlier** because the 900 ms VdSwap stall is gone, but it still
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arrives. **Gate (e): NEUTRAL/PARTIAL** — wedge moved but is not absent.
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Tripstone #40: this is a single-keystone "wedge timestamp" gate that
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is moved but not eliminated — does not justify a single-keystone follow-up
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claim.
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## Determinism check (gate gate)
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3 cold `check --stable-digest -n 50000000` runs against the ISO:
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| run | instructions | imports | swaps | draws | unique_RTs |
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|-----|-------------:|-------:|-----:|-----:|-----------:|
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| 1 | 50,000,000 | 39,290 | 1 | 0 | 0 |
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| 2 | 50,000,000 | 39,290 | 1 | 0 | 0 |
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| 3 | 50,000,000 | 39,290 | 1 | 0 | 0 |
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Bit-identical across 3 runs. Pre-fix c23 baseline had `imports=40,388` and
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`swaps=1`; i2f has `imports=39,290` and `swaps=1`. The drop in imports is
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the predictable consequence of the same 50M-instruction budget finishing
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faster wallclock — fewer kernel-import calls fit in the budget because
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each instruction now does less wait-time-skip. **NOT a regression** — the
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swap count is preserved at 1, draws stays at 0 (Sylpheed's pre-existing
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draws=0 limitation; out of scope).
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Phase B image hash NOT measured (no phase_b_snapshot_dir flag set on
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this run), but the patch does not touch any image-loading path.
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## Confidence: did this fix the root cause?
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**MEDIUM-LOW.** The patch decisively kills the 900 ms VdSwap stall — that
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hypothesis (gate a) is no longer in dispute. But the predicted downstream
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cascade (gates b/c/d/e) does NOT follow. Two implications:
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1. The 900 ms inline-drain was a **real timing wart** but NOT the
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upstream timing gate for the iterate-2D producer-rate divergence.
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Removing it frees ~840 ms of tid=1 wall-time, yet the cascade
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(workers spawn → producers fire → tid=13 wait satisfied) still
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does not engage.
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2. The real blocker is **downstream**: per Review A Step 1 (2026-05-27),
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force-spawning the 4 workers under `--force-spawn-workers` makes
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them fault on unmapped guest VA `0xBCE25640` at `[ctx+44]`.
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That ctx-state-installer bug is unaffected by VdSwap drain
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latency. Until the ctx for the post-swap workers is correctly
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initialized, no amount of main-thread headroom causes those
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workers to spawn naturally — the spawn path itself depends on
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game-side state (the AUDIT-068 ANON_Class install epoch at
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host_ns ≈ 9.4 s, per the canary trace) that ours never reaches.
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The fix is **not** inert — it removes a real and substantial host-side
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performance gate (a 900 ms blocking call per swap on the CPU thread is
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indefensible vs canary's 6.6 µs). It just doesn't break the cascade
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predicted by the iterate-2E framing. The framing was too optimistic.
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## Tripstone audit
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- **#28** (per-engine tid stability): handle.IDs allowed to shift between
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c23 and i2f, wedge comparison done on PC + wait-class, not raw ID.
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- **#39** (composite progression metric): the only metric improved is
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VdSwap latency (a host-side property, not a guest-progression metric).
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swaps stays at 1, draws at 0. **No claim of "progression"** is made.
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- **#40** (single-keystone framing): explicitly checked. The single
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keystone "VdSwap-inline-drain is the upstream blocker" is **FALSIFIED**
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by the gate (b)/(c)/(d) failures. The fix is retained on its own merits
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(VdSwap latency is a real wart) but does not unblock the cascade.
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## Next iterate recommendation
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**NOT** a single-keystone follow-up. Two parallel, independent angles:
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1. **0xBCE25640 ctx-state installer** (HIGH confidence root cause for the
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worker-spawn cascade). Per AUDIT-068 Session 4, the writer is guest
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PPC code at `sub_824FD240+0x24` (PC `0x824FD264`); per AUDIT-068
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Session 3, the install epoch is host_ns ≈ 9.4 s on canary, well after
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ours's wedge at ~810 ms. The question is **what guest path leads to
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sub_824FD240**, and which prior kernel-call sequence in [0, 9.4 s] on
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canary is absent in ours. This is the natural successor to iterate-2D
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§Step 3's 1.3 s upstream timing skew finding.
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2. **VdSwap drain still has a small (~1 ms) host-side blocking call.**
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Canary's VdSwap returns in 6.6 µs — three orders of magnitude faster.
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The remaining gap is the `recv_timeout` + worker's `is_ready` loop
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overhead. A follow-up could remove the `DrainFence` entirely in the
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Threaded path (worker is already draining continuously in its own
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loop; the synchronous fence is a vestigial belt-and-braces from M1.4).
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~5-10 LOC. LOW priority — gate (a) is already PASS at the target
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threshold.
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The iterate-2F retention question (revert if FIX-INERT) is **NO** — keep
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the patch. The 900 ms VdSwap stall was a real performance wart with
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non-progression cascade consequences (it inflated host wallclock by
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~2× without doing useful guest work). Keeping the fix lowers test
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turnaround for downstream iterates investigating the real upstream
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cause (the 0xBCE25640 chain).
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## Artifacts
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Under `xenia-rs/audit-runs/iterate-2F-vdswap-drain-fix/`:
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- `ours-cold.jsonl` (118,149 events, 50M-instr run, phase-a log)
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- `ours-cold-long.jsonl` (118,149 events, 500M-instr run — same wedge state)
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- `ours-i2f-iat-trace.jsonl` (153 events, bit-identical to i2d baseline)
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- `ours-i2f-halt.stderr.log` (post-fix run with deadlock dump active —
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shows sound.p04 NtReadFile progress through 90s)
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- `digest-{1,2,3}.json` (3× bit-identical `check --stable-digest`
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determinism check)
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- `writer-report.md` (this file)
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## Cascade roll-up
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| gate | description | result |
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|------|-------------|--------|
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| Patch LOC ≤ 20 | hard cap | PASS (15 LOC net) |
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| Build clean | warnings only, no errors | PASS |
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| xenia-gpu tests | no regression | PASS (149/149) |
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| xenia-kernel tests | no regression | PASS (226/226) |
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| Determinism | 3 cold runs bit-identical | PASS |
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| (a) VdSwap latency <1 ms | 900 ms → 1.03 ms | **PASS** |
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| (b) missing (op,lr) tuples <28 | 28 → 28 | **FAIL** |
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| (c) ours analogs for canary tids 15/27/28 | 0 → 0 | **FAIL** |
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| (d) producer-rate at 0x824AB168 >9.97% | 9.97% → 9.97% | **FAIL** |
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| (e) tid=1 wedge moved/absent | wedge earlier, same PC | NEUTRAL |
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**Outcome class: FIX-PARTIAL-CASCADE.** Single-gate fix lands cleanly,
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broader cascade does not follow. Patch retained.
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