docs(iommu): correct the identity-mode cost claim; pt is most of the fix, PerfOpt the last part

Reading patch v2 1/2 verbatim: amd_iommu_enable_perfopt() rehoming needs the
device ALREADY on the identity domain and switches ATS/PRI/PASID/GCR3 off via
the skip_caps path. So an iGPU in identity mode still pays a 1:1 page walk plus
those four capabilities until PerfOpt exists -- iommu=pt recovers most of the
gap, PerfOpt the last part. The first pass claimed pt alone already avoids the
cost; that was wrong and is retracted here and in the posted issue comment
(edit 23).

Also re-keys frank-npu.md, which carried the dead law 'iommu=pt is not a middle
ground' as its retirement rationale: identity mode + SVA is exactly the revival
path (amdxdna binds its own IOPT_TYPE_SVA domain), and the cost is single-digit
percent on this silicon class. Procedure step 2 now demands a local off-vs-pt
measurement instead of inheriting the vendor's 13-16%, whose slow-arm boot line
our record never identified.

serverAppConfigs-docs/log.md

@@ -2007,3 +2007,62 @@ every restart since the cutover.
   200,704-position pool behind both slots; two of ≤100k still do). Bought: 1.7
   GiB back as page cache for the 47.68 GiB n-gram table — 9.1 GiB against the
   2026-10-06 shape's 524,288 pool — and a smaller prompt-cache entry.
+
+## 2026-10-18 — issue #3 research: AMD PerfOpt retires the reason for `amd_iommu=off`
+
+- Ask: does a recent kernel change remove the IOMMU translation penalty that
+  justifies the 2026-09-11 `amd_iommu=off` boot line, i.e. can the XDNA2 NPU come
+  back? Answered and documented; **no host change** — the fix is not installable
+  from the Ubuntu 26.04 archive.
+- The change is **AMD PerfOpt** (spec §3.4.9, MMIO `0x016C` bit 13, gated by
+  extended-feature bit 45): merged via the iommu tree 2026-09-24, first release
+  **Linux 7.4**. It stops GPA→SPA translation for the *privileged iGPU* only and
+  leaves the IOMMU fully armed for every other device, NPU included. The only
+  consumer is `amdgpu_device_use_perfopt()`, which requires `AMD_IS_APU` **and the
+  identity domain** — so PerfOpt rides on top of `iommu=pt`, it does not replace
+  it. A 2026-09-28 follow-up deletes the `amdgpu.iommu_perfopt` knob and makes it
+  compulsory.
+- Local census (read-only): kernel `7.0.0-38-generic`; **zero** `perfopt` symbols
+  in its System.map; `linux-image-generic` and `linux-generic-hwe-26.04-edge` both
+  pin `7.0.0-38.38` → 7.4 needs a mainline PPA or a local build. `CONFIG_IOMMU_
+  DEFAULT_DMA_LAZY=y` (STRICT unset). `/dev/accel/accel0` already exists as
+  `crw-rw---- root:render 261,0` with `dank` in `render`, so the quadlet law is
+  `AddDevice=/dev/accel/accel0` + `GroupAdd=keep-groups` — the ComfyUI `/dev/kfd`
+  pattern, no new udev rule.
+- **Three premises of the issue corrected.** (1) The 13–16 % is the *vendor's* A/B
+  against *their* previous boot line, which our record never identified — we
+  copied the figure on 2026-09-11 without ever measuring here. Strict flush and
+  bounce buffering are not the mechanism either (Ubuntu ships lazy DMA, and a
+  coherent iGPU on unified memory takes no SWIOTLB bounce). What identity mode
+  actually costs the iGPU is the 1:1 page walk plus **ATS/PRI/PASID/GCR3** being
+  switched on at attach — the same four the PerfOpt enable path switches back off
+  (the `skip_caps` label in `attach_device`), and why it arms only "when an
+  integrated GPU is already in identity mode". (2) The NPU does **not** have to
+  wait for 7.4: `iommu=pt` enables the IOMMU, which is all `err=-19` asks for.
+  Making passthrough the AMD default was rejected on the list (Robin Murphy,
+  2026-03-26), so `pt` stays the sanctioned route. (3) The 460 tok/s baseline is
+  stale — the 2026-10-17 native-v2 payload runs **1,513–1,604 t/s cold prefill** —
+  so the window measures `off` vs `pt` locally and inherits no number.
+- Silicon check: our IOMMU is `1022:1508` Strix/Strix Halo rev 02, and Phoronix's
+  **Strix Halo desktop** arm measured only **2–4 %** from PerfOpt (18–23 % on
+  Strix Point / Krackan laptops) — the modest arm is the one that describes this
+  box. Kernel pick: **skip 6.18–6.18.7**, where an IOMMU regression disables SVA
+  outright.
+- NPU legality under identity mode: `amdxdna` binds via `iommu_sva_bind_device()`
+  + PASID + GCR3 into its own `IOPT_TYPE_SVA` domain, independent of the default
+  domain type (`drivers/accel/amdxdna/aie2_pci.c`); `err=-19` needs only an
+  *enabled* IOMMU, which `pt` gives.
+- Self-correction inside the same session: the first pass — and the first version
+  of the posted comment — claimed `iommu=pt` alone already avoids the cost. It
+  does not. Patch v2 1/2 shows an identity attach keeps the page walk and enables
+  ATS/PRI/PASID/GCR3, so `pt` recovers **most** of the gap and PerfOpt is the last
+  part of it. The comment was edited to the corrected text.
+- Artifacts: new section in
+  `serverAppConfigs-docs/LLM/dankclaw-iommu-translation-off.md` (+6 citations,
+  front-matter re-keyed to "vendor-claimed, never measured locally"),
+  `frank-npu.md` § *Why it is retired* corrected — the "`pt` is not a middle
+  ground" law is dead — the AGENTS.md key-lesson bullet re-keyed, a 4-step
+  maintenance-window procedure that measures locally instead of inheriting the
+  vendor number, and the plan posted to Gitea issue #3 (comment 23, then edited).
+  bats 371/371; commits `7643381` plus this correction. Nothing deployed — no
+  service was touched.