The Moment of Truth: Rebooting to Test Blackwell GPU IOMMU Identity Domains

In the sprawling, multi-session saga of deploying NVIDIA Blackwell GPUs for production AI inference, few messages carry as much weight as a simple todo-list update. Message 6411 in this conversation is precisely that: a brief, structured todowrite tool call from the assistant, acknowledging the user's command to reboot the Proxmox host and laying out the verification plan for what comes next. Yet beneath its terse, machine-readable surface lies the culmination of an intense debugging odyssey spanning GPU firmware internals, Linux IOMMU architecture, PCIe bus resets, and the peculiarities of NVIDIA's Blackwell architecture. This message is the pivot point where preparation meets execution — the moment when weeks of analysis and tool-building are put to the ultimate test.

The Message Itself

The subject message reads in full:

[assistant] [todowrite] {"todos":[{"content":"Reboot the Proxmox host","priority":"high","status":"in_progress"},{"content":"Verify modprobe hook set identity domains at boot","priority":"high","status":"pending"},{"content":"Test P2P DMA transfers between GPUs","priority":"high","status":"pending"},{"content":"Update SGLa...

The truncated JSON continues with additional pending items, but the visible entries tell the story: the reboot is actively in progress, and four critical verification steps await. This is not a dramatic announcement or a complex technical explanation — it is a project manager's status update, a checklist being ticked in real-time. But the todo list encodes an entire narrative of hardware debugging, firmware constraints, and creative system administration.

The Context: Why This Reboot Matters

To understand why this seemingly mundane message is significant, one must appreciate the problem it aims to solve. The system in question is a Proxmox VE host equipped with eight NVIDIA RTX PRO 6000 Blackwell GPUs, split between an LXC container (4 GPUs on NUMA node 0) and a SEV-SNP virtual machine (4 GPUs on NUMA node 1, bound to vfio-pci). The LXC container runs SGLang serving a Qwen3.5-122B-A10B BF16 model with tensor parallelism across all 4 NUMA0 GPUs.

The performance bottleneck is Peer-to-Peer (P2P) DMA transfers between GPUs. Under the default DMA-FQ (DMA with Flush Queue) IOMMU translation mode, P2P transfers between peer GPUs must go through system memory — the IOMMU translates every GPU memory access, adding latency and consuming memory bandwidth. In identity IOMMU mode, the IOMMU is bypassed entirely for the specified IOMMU groups, allowing direct GPU-to-GPU transfers at full PCIe bandwidth. For tensor-parallel inference, where GPUs constantly exchange activations and gradients, this can mean a 20-40% throughput improvement.

The problem is that setting IOMMU identity domains for Blackwell GPUs is extraordinarily difficult. The Blackwell architecture introduces a Firmware Security Processor (FSP, also called GSP — GPU Security Processor) that initializes during the nvidia driver's first probe of the GPU. Once the FSP is initialized, it locks the GPU's firmware state in a way that survives all software-level resets: Function-Level Reset (FLR), Secondary Bus Reset (SBR), and even CXL bus resets. Any subsequent attempt to unbind and rebind the nvidia driver results in a kfspSendBootCommands_HAL error (code 0x177), and the GPU becomes unusable until a full power cycle.

This means the IOMMU identity domains must be set before the nvidia driver ever touches the GPUs — at the very first module load after a cold boot. The assistant had spent the previous messages constructing a modprobe install hook that runs a shell script before the nvidia module loads, setting the IOMMU groups for the NUMA0 GPUs to identity, then loading the real nvidia module. The script was deployed, the old systemd service was disabled, and everything was in place for the reboot.

The Reasoning and Motivation

The assistant's motivation in this message is straightforward: it has received the user's command "do reboot" (message 6410) and is acknowledging it while updating its internal task tracking. But the reasoning behind the todo list structure reveals the assistant's mental model of the verification pipeline.

The first item — "Reboot the Proxmox host" with status "in_progress" — signals that the reboot command has been initiated. The assistant cannot observe the reboot's completion from within the session (the SSH connection will drop), so it marks this as in-progress and will verify post-reboot state when the session resumes.

The second item — "Verify modprobe hook set identity domains at boot" — is the critical check. The entire investment in the modprobe hook approach rests on whether it fires at the right moment during boot. The hook must execute before the nvidia driver's PCI probe callback, which happens when udev triggers modprobe nvidia from the PCI modalias event. If the hook runs too late — after nvidia has already probed the GPUs — the FSP will be locked and the identity domains will be useless. If it runs too early — before the PCI devices are fully enumerated — the IOMMU groups might not exist yet. The timing window is narrow and fragile.

The third item — "Test P2P DMA transfers between GPUs" — is the functional validation. Even if the IOMMU groups report identity type, the actual P2P DMA hardware path must be tested. This involves running NCCL bandwidth tests or nvidia-smi peer-to-peer queries to confirm that direct GPU-to-GPU transfers work without falling back to system memory.

The fourth item — "Update SGLang config: remove NCCL_P2P_DISABLE and --disable-custom-all-reduce" — represents the payoff. If P2P works, the workarounds that were put in place to prevent NCCL hangs (caused by corrupted P2P DMA under SEV-SNP IOMMU) can be removed, potentially delivering a significant performance boost on top of the already-enabled MTP (Multi-Token Prediction) speculation.

Assumptions Embedded in the Plan

The todo list encodes several critical assumptions. The most fundamental is that the modprobe install hook approach will actually work — that setting IOMMU identity domains before nvidia loads will produce functional GPUs with working P2P DMA. This assumption is reasonable given the assistant's analysis: the FSP corruption only occurs when nvidia has already probed the GPU, so a clean-from-power-on probe with identity domains already in place should avoid the problem entirely.

A deeper assumption is that Blackwell GPUs are compatible with IOMMU identity mode at all. The assistant has not yet tested this specific combination — a cold boot with identity domains set before the first nvidia probe. The earlier runtime tests (where the modprobe hook was triggered during a PCI rescan) proved that the hook could set identity domains, but those GPUs already had corrupted FSP from a prior nvidia bind, so the nvidia driver still failed. The reboot is the first true test of the complete sequence.

There is also an assumption about boot ordering: that the gpu-vfio-split.service (which binds NUMA1 GPUs to vfio-pci) completes before the modprobe hook fires. If vfio-pci grabs all GPUs first, the NUMA0 GPUs will be bound to vfio-pci and the nvidia module won't probe them at all. The assistant addressed this in the previous session by ensuring the modprobe hook is triggered by the PCI modalias event, which happens during device enumeration — but the exact ordering relative to systemd services is hardware- and kernel-version-dependent.

Knowledge Required to Understand This Message

A reader needs substantial background to fully grasp the significance of this message. They must understand:

• IOMMU and DMA remapping: The Linux IOMMU (Input/Output Memory Management Unit) translates DMA addresses from devices to physical memory. In DMA-FQ (translation) mode, all device memory accesses go through the IOMMU. In identity mode, the IOMMU is bypassed for specific IOMMU groups, allowing direct physical addressing.
• P2P DMA and NCCL: Peer-to-Peer DMA allows GPUs on the same PCIe fabric to access each other's memory directly, without staging through system memory. NVIDIA's NCCL (NVIDIA Collective Communications Library) uses P2P for efficient all-reduce and broadcast operations in multi-GPU inference.
• Blackwell FSP/GSP: NVIDIA's Blackwell architecture introduces a Firmware Security Processor that manages GPU firmware initialization and security. Once initialized by the nvidia driver, the FSP state is locked and cannot be reset through software mechanisms — only a power cycle (AC power removal) can clear it.
• modprobe install hooks: The install directive in modprobe configuration files allows intercepting module load events. When modprobe nvidia is triggered (by udev, by another module's dependency, or manually), the install hook script runs instead of the normal module loading, and the script is responsible for loading the real module via modprobe --ignore-install.
• The SBR/FLR limitation: Secondary Bus Reset and Function-Level Reset are PCIe mechanisms for resetting devices without power-cycling the system. The assistant discovered that neither mechanism clears the Blackwell FSP state, making runtime reconfiguration of IOMMU domains impossible.

Knowledge Created by This Message

This message itself creates relatively little new knowledge — it is primarily an acknowledgment and a plan. However, it serves as a critical timestamp and commitment point. The todo list formalizes the verification protocol that will be followed post-reboot, creating an auditable record of what was supposed to happen. When the subsequent messages reveal that identity mode is fundamentally incompatible with Blackwell GPUs (the FSP fails with error 0x177 even with clean-from-power-on initialization), this message becomes the reference point for what was attempted and what assumptions were tested.

The message also implicitly documents the assistant's understanding of the system state at the moment of reboot: the modprobe hook is in place, the old systemd service is disabled, SGLang is running with MTP enabled and P2P disabled, and the GPUs are in a known working configuration with DMA-FQ IOMMU type.

The Thinking Process Visible in the Todo Structure

The todo list reveals the assistant's prioritization and sequencing logic. The items are ordered by dependency: reboot must complete before verification can happen, verification must succeed before P2P testing, and P2P testing must succeed before the SGLang configuration can be updated. This is classic dependency-chain management, encoded in a structured data format rather than prose.

The use of status: "in_progress" for the reboot (rather than "pending" or "completed") is telling. The assistant cannot confirm the reboot's completion from within the session — the SSH connection will terminate, and the assistant's process will end. When the session resumes (presumably after the user reconnects), the assistant will check the system state and update the todo accordingly. The in_progress status signals that the action has been initiated but the outcome is not yet observable.

The Outcome: What Followed

As the chunk summary reveals, the reboot did not produce the desired result. The modprobe hook successfully set the IOMMU identity domains before nvidia loaded — the hook worked as designed. However, a critical discovery emerged: the Blackwell FSP boot sequence fails with error code 0x177 when IOMMU is in identity mode. The FSP apparently requires specific DMA mappings set up by the kernel's DMA API in translation mode, and identity mode breaks this initialization.

This means per-group IOMMU identity domains are fundamentally incompatible with Blackwell GPUs. The approach cannot work regardless of timing — not because of the FSP-locking problem the assistant was trying to solve, but because the FSP itself requires IOMMU translation to function during its own initialization sequence. The assistant immediately reverted by removing the modprobe hook and rebooting, restoring the working DMA-FQ configuration.

The MTP speculation, however, survived the reboot and continues to provide 12-45% per-request throughput improvement at low concurrency. The system is stable with SGLang serving Qwen3.5-122B-A10B BF16 on 4 GPUs with TP=4, P2P disabled, and DMA-FQ IOMMU type.

Conclusion

Message 6411 is a quiet pivot point in a complex technical narrative. It is the moment when preparation ends and testing begins — when all the analysis, scripting, and debugging of the preceding messages is put to the test of reality. The message itself is brief and structured, but it carries the weight of the entire IOMMU identity domain investigation. Its todo list encodes assumptions, dependencies, and a verification protocol that, when executed, would reveal a fundamental incompatibility between Blackwell GPUs and IOMMU identity mode — a finding that no amount of clever modprobe hookery could overcome. In the end, the reboot proved that some hardware constraints cannot be bypassed with software ingenuity, and the assistant pivoted to accept the DMA-FQ configuration with MTP speculation as the primary optimization. The todo list in message 6411 stands as a monument to a well-formed hypothesis that was rigorously tested and definitively refuted.