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.
This commit is contained in:
Frank
@@ -1107,15 +1107,16 @@ linked runbooks.
|
||||
times (`RuntimeError: No HIP GPUs are available`).
|
||||
- SearXNG outbound traffic must stay on the Gluetun tunnel; do not move its
|
||||
ports off the gluetun service.
|
||||
- `amd_iommu=off` on Dankclaw (bought 13–16 % prefill for frank-dragoon,
|
||||
2026-09-11) retires the XDNA2 NPU: the driver's SVA/PASID bind needs an
|
||||
enabled IOMMU (`/dev/accel/accel0` fails `err=-19`). That trade is on an
|
||||
expiring path: **AMD PerfOpt** (iommu tree 2026-09-24, Linux 7.4) stops
|
||||
iGPU GPA→SPA translation while the IOMMU stays enabled for every other
|
||||
device, so the next boot-line window is `iommu=pt` with the NPU usable —
|
||||
measured only 2–4 % residual on a Strix Halo desktop (18–23 % on Strix
|
||||
Point laptops). Not installable here yet: Ubuntu 26.04 caps at
|
||||
`7.0.0-38`. Full research in
|
||||
- `amd_iommu=off` on Dankclaw (adopted 2026-09-11 on the vendor's 13–16 %
|
||||
prefill figure, never measured here) retires the XDNA2 NPU: the driver's
|
||||
SVA/PASID bind needs an enabled IOMMU (`/dev/accel/accel0` fails `err=-19`).
|
||||
That trade is on an expiring path: **AMD PerfOpt** (iommu tree 2026-09-24,
|
||||
Linux 7.4) makes the IOMMU skip GPA→SPA translation for the *privileged iGPU*
|
||||
while staying fully armed for every other device — and it arms only when the
|
||||
iGPU is already in the identity domain, so the next boot-line window is
|
||||
`iommu=pt` with the NPU usable, not `off`. PerfOpt buys 2–4 % back on a Strix
|
||||
Halo desktop (18–23 % on Strix Point laptops); it is not installable here
|
||||
yet (Ubuntu 26.04 caps at `7.0.0-38`). Research in
|
||||
`serverAppConfigs-docs/LLM/dankclaw-iommu-translation-off.md`.
|
||||
- Rootless Quadlet containers run with `--replace --rm`: `podman logs` covers
|
||||
only the CURRENT instance; history across recreations lives in
|
||||
|
||||
@@ -1,7 +1,7 @@
|
||||
---
|
||||
type: research
|
||||
title: Dankclaw IOMMU translation off (amd_iommu=off) — decision record
|
||||
description: Why Dankclaw boots with amd_iommu=off since 2026-09-11, what it buys (13-16% prefill for frank-dragoon), what it costs (the XDNA2 NPU endpoint, machine-wide DMA translation), what was deliberately not copied from the reference machine, and how to revert.
|
||||
description: Why Dankclaw boots with amd_iommu=off since 2026-09-11, what it buys (a vendor-claimed 13-16% prefill for frank-dragoon, never measured locally), what it costs (the XDNA2 NPU endpoint, machine-wide DMA translation), the 2026-10-18 AMD PerfOpt research that puts the trade on an expiring path, and how to revert.
|
||||
tags:
|
||||
- dankclaw
|
||||
- iommu
|
||||
@@ -92,15 +92,15 @@ deployment — reverting one does not require reverting the other.
|
||||
## 2026-10-18 research: the penalty now has a hardware fix (AMD PerfOpt)
|
||||
|
||||
Asked and answered — **the decision changes, the host does not.** Nothing was
|
||||
changed on Dankclaw today; this section records why the 13–16 % figure is on an
|
||||
expiring path.
|
||||
changed on Dankclaw today; this section records why the figure behind the
|
||||
decision is on an expiring path — and that it was never measured on this host.
|
||||
|
||||
**Mechanism.** AMD IOMMU **PerfOpt**, spec-defined in the AMD I/O Virtualization
|
||||
Technology (IOMMU) spec §3.4.9: MMIO `0x016C` bit 13 `PERF_OPT_EN`, gated by
|
||||
extended-feature bit 45 `FEATURE_PERF_OPT`. Armed, it stops GPA→SPA translation
|
||||
for the *privileged integrated GPU* — only the IR/IW permission bits are
|
||||
enforced — while the IOMMU stays fully enabled for every other device, the NPU
|
||||
included. Driver: `drivers/iommu/amd/{iommu.c,amd_iommu_types.h}`. The only
|
||||
included. Driver: `drivers/iommu/amd/{iommu.c,init.c,amd_iommu_types.h}`. The only
|
||||
consumer is **amdgpu**: `amdgpu_device_use_perfopt()` requires `AMD_IS_APU` **and
|
||||
the identity domain**, and the shared per-IOMMU bit is refcounted so one
|
||||
device's teardown cannot clear it under a peer.
|
||||
@@ -119,13 +119,15 @@ PerfOpt compulsory.
|
||||
laptops, **2–4 % on a Strix Halo desktop** — which is also the evidence that our
|
||||
silicon implements the bit (our IOMMU is `1022:1508` Strix/Strix Halo rev 02).
|
||||
|
||||
**The September number was never strict-flush cost.** Ubuntu ships
|
||||
`CONFIG_IOMMU_DEFAULT_DMA_LAZY=y` (STRICT not set), so IOTLB invalidation was
|
||||
already batched, and a coherent iGPU on unified memory takes no SWIOTLB bounce.
|
||||
What we measured was **translated-domain** cost — and `iommu=pt` avoids that
|
||||
today, with or without PerfOpt. The March attempt to make passthrough the AMD
|
||||
default was rejected outright, so `iommu=pt` / `iommu.passthrough=1` stays the
|
||||
sanctioned route.
|
||||
**Identity mode is not "no IOMMU".** Ubuntu ships `CONFIG_IOMMU_DEFAULT_DMA_LAZY=y`
|
||||
(STRICT not set), so IOTLB invalidation was already batched, and a coherent iGPU
|
||||
on unified memory takes no SWIOTLB bounce — strict flush and bounce buffering do
|
||||
not explain the September figure. But a device attached to the identity domain
|
||||
still has its DMA walked through 1:1 page tables, and the AMD attach path enables
|
||||
**ATS/PRI/PASID/GCR3** for it; PerfOpt's own enable path has to switch those four
|
||||
off again (the `skip_caps` label in `attach_device`) to get the speedup, and the
|
||||
cover letter says it arms only "when an integrated GPU is **already in identity
|
||||
mode**". So `pt` is most of the recovery and PerfOpt is the last part of it.
|
||||
|
||||
**The NPU is compatible with identity mode.** `amdxdna` binds through
|
||||
`iommu_sva_bind_device()` + PASID + GCR3 into its own `IOPT_TYPE_SVA` domain,
|
||||
@@ -143,11 +145,15 @@ pattern ComfyUI uses for `/dev/kfd`.
|
||||
**Revised procedure for the next boot-line window** (still a maintenance window,
|
||||
still an explicit OK per run):
|
||||
|
||||
1. `iommu=pt` in place of `amd_iommu=off` — NPU usable, translated cost gone,
|
||||
residual 2–4 % on this silicon class.
|
||||
2. Re-measure prefill against the **current** payload. The 13–16 % belongs to the
|
||||
2026-09-11 shape; the 2026-10-17 native-v2 boot runs 1,513–1,604 t/s cold
|
||||
prefill, so the old baseline is not comparable.
|
||||
1. `iommu=pt` in place of `amd_iommu=off`. The NPU becomes usable at that reboot;
|
||||
the iGPU pays the identity-walk cost until PerfOpt exists on this kernel, which
|
||||
the desktop-class measurement prices in single-digit percent.
|
||||
2. **Measure `off` vs `pt` on this host, on the current payload** (bench law: stop
|
||||
+ mask `frank-dragoon` before any GPU A/B). The 13–16 % was the vendor's A/B
|
||||
against *their* previous boot line, which our record never identified — the
|
||||
2026-09-11 adoption copied their figure without a local run. The current
|
||||
native-v2 payload does 1,513–1,604 t/s cold prefill, so that baseline is not
|
||||
comparable regardless.
|
||||
3. Keep the fabric-clock pin (`pp_dpm_fclk` level 7) for GPU+NPU co-run.
|
||||
4. On 7.4: nothing to do — PerfOpt arms itself when the iGPU is in the identity
|
||||
domain. Confirm with `dmesg | grep -i perfopt`; force off with
|
||||
|
||||
@@ -42,9 +42,21 @@ upstream project on this exact silicon. That single flag is the whole decision:
|
||||
out `/dev/accel/accel0` — every open fails with `err=-19` ("No such device").
|
||||
That is precisely the failure the endpoint was rescued from on 2026-08-30,
|
||||
recorded in the next section.
|
||||
- `iommu=pt` is not a middle ground for the NPU *and* the prefill number: the
|
||||
passthrough domain still costs the prefill throughput the vendor measured.
|
||||
- The operator chose prefill, so the NPU endpoint goes.
|
||||
- `iommu=pt` was recorded here as "not a middle ground", on the theory that the
|
||||
passthrough domain still costs the vendor's prefill number. **That claim did
|
||||
not survive the 2026-10-18 research.** Identity mode is not free for the iGPU
|
||||
— its DMA still walks 1:1 page tables and the AMD attach path enables
|
||||
ATS/PRI/PASID/GCR3 for the device — but on this silicon class the cost is
|
||||
single-digit percent, and **AMD PerfOpt** (Linux 7.4, merged 2026-09-24)
|
||||
removes it: the IOMMU then enforces only the IR/IW permission bits for the
|
||||
privileged iGPU and stays fully armed for every other device, the NPU
|
||||
included. So `pt` IS the revival path for this endpoint. See
|
||||
[[dankclaw-iommu-translation-off]] § *2026-10-18 research* for the kernel
|
||||
census, the versions to avoid (6.18–6.18.7 disable SVA), the device-access
|
||||
quadlet law (`AddDevice=/dev/accel/accel0` + `GroupAdd=keep-groups`), and the
|
||||
measurement step that must precede any decision.
|
||||
- The operator took the vendor's prefill figure on the September evidence, so the
|
||||
NPU endpoint went.
|
||||
|
||||
Consequences accepted at retirement: **audio input left the fleet with no
|
||||
replacement** (halogen reads images only, no audio or video), and the surviving
|
||||
|
||||
@@ -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.
|
||||
|
||||
Reference in New Issue
Block a user