The Last Attempt: When Blackwell's Firmware Defeats IOMMU Identity Domains

Introduction

In the intricate dance between hardware and software that defines modern GPU computing, few moments are as instructive as the one captured in message 6290 of this opencode session. The message represents the final, determined attempt by an AI assistant to restore Peer-to-Peer (P2P) Direct Memory Access between NVIDIA Blackwell GPUs by setting per-IOMMU-group identity domains — an approach that, unbeknownst to the assistant at the time of writing, was already doomed by a fundamental incompatibility between the Blackwell GPU's Firmware Security Processor (FSP) and the IOMMU identity mode itself. This article examines this single message in depth: its reasoning, its assumptions, its technical context, and the knowledge it both consumes and produces.

The Message in Full

The assistant writes:

The GPUs are bound to nvidia but GSP is still corrupted from the earlier session. The SBR didn't fully clear it when the GPUs were behind vfio-pci. Let me do the full cycle again properly: unload nvidia, unbind vfio, remove GPUs, SBR, rescan (without any driver), set identity, then load nvidia.

This is followed by a large bash script executed via SSH on the remote host root@10.1.2.6, containing an eight-step procedure:

  1. Unload all nvidia kernel modules (nvidia_drm, nvidia_uvm, nvidia_modeset, nvidia)
  2. Remove GPUs from PCI by writing to the remove sysfs attribute
  3. Perform Secondary Bus Reset (SBR) on all four parent PCI bridges, holding the reset for 500ms
  4. Wait 10 seconds for the GSP firmware to fully reset
  5. Rescan the PCI bus after removing the vfio-pci new_id registration to prevent vfio-pci from grabbing the GPUs
  6. Set identity domains on each GPU's IOMMU group
  7. Load nvidia modules in the correct order
  8. Verify with nvidia-smi and check driver/group/type for both the NUMA0 GPUs and the VFIO GPUs The output shown is truncated, but reveals that Step 2 encounters errors: the unbind paths (/sys/bus/pci/devices/.../driver/unbind) don't exist because the GPUs already have no driver bound after module unloading.

Why This Message Was Written: Reasoning and Motivation

The Problem That Drove This Attempt

To understand why this message exists, we must understand the broader context. The system under configuration is a high-performance machine running Ubuntu 24.04 with eight NVIDIA RTX PRO 6000 Blackwell GPUs, split across two domains: four GPUs on NUMA0 are bound to the nvidia driver and used by SGLang to serve the Qwen3.5-122B-A10B BF16 large language model with Tensor Parallelism 4 (TP=4), while the other four GPUs are bound to vfio-pci for use by a SEV-SNP (Secure Encrypted Virtualization - Secure Nested Paging) virtual machine.

The critical problem is that GPU P2P DMA is broken. Under normal circumstances, NVIDIA GPUs can communicate directly via P2P DMA, bypassing system memory for dramatically higher bandwidth. However, when the system's IOMMU is operating in full translation mode — which is required for SEV-SNP — the P2P DMA mappings become incomplete. Some peer GPU pairs work, while others produce IOMMU faults. The nvidia driver parameter DmaRemapPeerMmio=1 is already enabled but produces only partial results.

The IOMMU Identity Domain Hypothesis

The assistant's working hypothesis throughout the preceding messages (6279–6289) has been that setting individual IOMMU groups to identity mode — where the IOMMU performs no address translation and simply passes through physical addresses — would restore full P2P DMA capability for the GPU groups while leaving other groups in translation mode for security. This is a surgical approach: rather than disabling the IOMMU globally (which would break SEV-SNP), the assistant aims to set identity only for the four GPU IOMMU groups.

The challenge is that the IOMMU group type attribute can only be set when no driver is bound to the device. And on Blackwell GPUs, the nvidia driver cannot be cleanly unbound and rebound — the GSP firmware state persists across driver unbind/bind cycles and becomes corrupted. This was discovered in earlier messages where unbinding a single GPU and rebinding it produced the dreaded RmInitAdapter failed! (0x62:0xffff:2142) error and the characteristic 0xbadf4100 register pattern indicating GSP corruption.

The Specific Motivation for Message 6290

In the immediately preceding messages (6287–6289), the assistant attempted a combined approach: unload nvidia modules, remove GPUs, SBR the bridges, rescan, set identity, and reload nvidia. However, that attempt failed because vfio-pci grabbed the GPUs during rescan (due to a lingering new_id registration from an earlier GPU split script), and the subsequent unbind from vfio-pci and identity-setting still resulted in GSP corruption. The nvidia module loaded but found no devices — the GPUs were visible on the PCI bus with driver=none, but nvidia-smi reported "No devices were found."

Message 6290 is therefore written as a corrected retry. The assistant identifies two potential causes for the previous failure:

  1. The SBR didn't fully clear the GSP state because the GPUs were behind vfio-pci at the time of the reset, and the vfio-pci driver may have interfered with the reset propagation.
  2. The vfio-pci new_id registration caused the GPUs to be claimed by vfio-pci on rescan, and the subsequent unbind from vfio-pci may not have fully cleaned up the device state. The assistant's reasoning is: if we can ensure that (a) the SBR happens with no driver bound at all, (b) we prevent vfio-pci from grabbing the GPUs by removing its new_id, and (c) we set identity domains before loading nvidia, then the fresh probe should succeed with identity domains active and GSP properly initialized.

How Decisions Were Made

The Order of Operations

The assistant makes several deliberate design decisions in constructing the eight-step procedure:

Decision 1: Unload modules before removing GPUs. By unloading the nvidia kernel modules first, the assistant ensures that when GPUs are removed from PCI, there's no lingering driver state. This is a lesson learned from earlier failures where unbinding individual devices left GSP state corrupted.

Decision 2: Remove GPUs from PCI before SBR. The SBR (Secondary Bus Reset) is a hardware reset of the PCI bridge that propagates to downstream devices. By removing the GPUs first, the assistant ensures they are in a clean state before the reset. The 500ms hold time for SBR is increased from earlier attempts (which used 200ms) to ensure the reset fully completes.

Decision 3: A 10-second wait after SBR. This is a notable choice — the assistant explicitly waits 10 seconds "for GSP to fully reset." This suggests the assistant has developed a mental model of the GSP as having a non-trivial reset latency, perhaps based on observed timing in earlier successful SBR cycles.

Decision 4: Remove vfio-pci new_id before rescan. This is the critical fix the assistant believes will solve the previous failure. By removing the 10de 2bb5 vendor/device ID registration from vfio-pci, the assistant hopes the GPUs will remain driverless after rescan, allowing identity to be set cleanly.

Decision 5: Unbind from any driver before setting identity. The script attempts to unbind from whatever driver might be bound (even though the assistant expects none), as a defensive measure.

Decision 6: Load nvidia modules in dependency order. nvidia first, then nvidia_modeset, nvidia_uvm, nvidia_drm — this respects the kernel module dependency chain.

The Tool Selection

The assistant uses a single bash tool call with an SSH command. This is a deliberate choice: all the operations (module loading, PCI sysfs manipulation, SBR via setpci, IOMMU group configuration) are Linux kernel operations that must run on the host machine. The assistant is operating remotely via SSH, suggesting the assistant itself runs on a different machine (perhaps a development workstation or another server).

Assumptions Made by the Assistant

This message rests on several critical assumptions, some of which are about to be proven wrong:

Assumption 1: The GSP corruption was caused by vfio-pci interference

The assistant states: "The SBR didn't fully clear it when the GPUs were behind vfio-pci." This assumes that the vfio-pci driver somehow prevented the SBR from properly resetting the GPU's GSP firmware. In reality, as the chunk summary reveals, the GSP corruption is not a timing or ordering issue — it is a fundamental incompatibility: the Blackwell FSP requires DMA translation mode during its initialization sequence, and identity mode breaks this initialization with error code 0x177.

Assumption 2: A clean SBR with no driver bound will fully reset GSP

The assistant assumes that SBR, when performed on a GPU with no driver bound, will completely reset the GSP firmware to a pristine state. While SBR does reset the PCI configuration space and the device's internal state, the chunk summary reveals that "no software-level reset (FLR, SBR, CXL bus reset) can clear this state." The GSP corruption from identity mode is apparently persistent across hardware resets.

Assumption 3: The identity domain approach is still viable

The entire message is predicated on the assumption that per-IOMMU-group identity domains are a workable solution for restoring P2P DMA. The assistant has not yet discovered that identity mode is fundamentally incompatible with Blackwell GPUs. The chunk summary reveals this discovery happens later in the same segment: "the nvidia Blackwell FSP boot sequence fails with error code 0x177 when IOMMU is in identity mode."

Assumption 4: The order of operations matters and can be optimized

The assistant assumes that by refining the sequence — particularly by preventing vfio-pci from grabbing the GPUs and by extending the SBR hold time and post-SBR wait — the previously failing approach can be made to work. This is a reasonable debugging assumption, but it turns out to be incorrect because the root cause is not in the procedure but in the hardware/firmware compatibility.

Assumption 5: The vfio-pci remove_id operation is effective

The assistant attempts to remove the 10de 2bb5 ID from vfio-pci's dynamic ID list. However, the output doesn't show whether this succeeded or failed (the command redirects stderr to /dev/null). This is a minor but notable assumption — the assistant assumes the remove_id interface works symmetrically with new_id, which is not always the case depending on the kernel version and driver implementation.

Mistakes and Incorrect Assumptions

The Fundamental Error: Identity Mode vs. Blackwell FSP

The most significant mistake in this message is not in the code but in the premise. The assistant is attempting to solve a problem (GSP corruption after unbind/rebind) by refining the reset and rebind procedure, when the actual problem is that identity mode itself is incompatible with Blackwell's FSP initialization. The FSP requires specific DMA mappings set up by the kernel's DMA API in translation mode. When the IOMMU group is in identity mode, these mappings are never created, and the FSP fails to boot with error 0x177.

This is a classic debugging pitfall: the assistant observed a symptom (GSP corruption after unbind/rebind) and assumed the cause was in the procedure (improper reset, driver interference), when the cause was actually in the configuration (identity mode). The earlier successful SBR cycles (which restored GPUs to working DMA-FQ state) reinforced this misattribution — the assistant had seen SBR work to fix GSP corruption, so it assumed SBR was the universal reset mechanism. What it didn't realize was that SBR only fixed corruption caused by unbind/rebind cycles under DMA-FQ; it could not fix corruption caused by identity mode itself.

The Missing Diagnostic Step

In retrospect, the assistant could have saved significant effort by checking whether identity mode was even viable before attempting the complex unbind/SBR/bind dance. A simple test — setting identity mode on a GPU group and then checking if the nvidia driver can initialize the device — would have revealed the incompatibility immediately. However, the assistant's mental model was that the GSP corruption was a timing/reset issue, not a fundamental incompatibility, so it kept trying to refine the procedure rather than questioning the premise.

The Truncated Output Blindness

The message shows truncated output — only the first few steps of the script are visible. The assistant will see the full output in the next message (the tool results are returned in the following round). This means the assistant is operating with incomplete information at the time of writing. It doesn't yet know whether this attempt succeeded or failed.

Input Knowledge Required to Understand This Message

To fully grasp what's happening in message 6290, a reader needs knowledge spanning several domains:

Linux PCI Subsystem

IOMMU Architecture

NVIDIA GPU Architecture

NVIDIA Driver Stack

SGLang and LLM Serving Context

Output Knowledge Created by This Message

Procedural Knowledge

The message documents a detailed, eight-step procedure for attempting to set IOMMU identity domains on NVIDIA Blackwell GPUs. While the procedure ultimately fails to achieve its goal (as revealed in the chunk summary), it captures valuable knowledge about:

  1. The correct sequence for GPU hot-reset: Unload modules → remove from PCI → SBR → wait → rescan → set identity → load modules
  2. The need to prevent vfio-pci from claiming devices during rescan by removing new_id registrations
  3. The dependency order for loading nvidia kernel modules
  4. The sysfs paths for PCI device manipulation, IOMMU group configuration, and driver binding

Diagnostic Knowledge

The message (and its surrounding context) produces critical diagnostic knowledge about Blackwell GPU behavior:

  1. GSP state persistence: The Blackwell GSP firmware state persists across driver unbind/bind cycles, and corruption can occur if the state is not properly reset
  2. SBR effectiveness: SBR can reset GSP state when the GPU is in DMA-FQ mode, but cannot fix corruption caused by identity mode
  3. vfio-pci interference: The vfio-pci driver can claim GPUs during rescan if a new_id registration exists, complicating the rebind process

The Ultimate Knowledge

The most important knowledge produced by this entire segment — which the assistant will discover in the subsequent messages — is that per-IOMMU-group identity domains are fundamentally incompatible with Blackwell GPUs. The FSP requires DMA translation mode during initialization, and no software-level reset can clear this requirement. This is a hardware/firmware limitation that cannot be worked around through software procedure refinement.

This knowledge has practical implications: P2P DMA restoration on Blackwell GPUs under SEV-SNP IOMMU must pursue alternative approaches, such as making the nvidia driver's DmaRemapPeerMmio=1 parameter work correctly, or accepting the performance penalty of NCCL_P2P_DISABLE=1 and relying on MTP speculation as the primary optimization.

The Thinking Process Visible in the Message

Diagnostic Reasoning

The assistant's thinking is visible in the opening sentence: "The GPUs are bound to nvidia but GSP is still corrupted from the earlier session. The SBR didn't fully clear it when the GPUs were behind vfio-pci."

This reveals a two-step inference:

  1. Observation: GPUs are bound to nvidia but GSP is corrupted (from the previous attempt)
  2. Attribution: The corruption persisted because SBR was performed while GPUs were behind vfio-pci The assistant is constructing a causal chain: vfio-pci → SBR ineffective → GSP corruption persists → nvidia fails to initialize. This is a reasonable hypothesis given the available evidence, but it turns out to be incorrect.

Experimental Design

The assistant designs the script as a controlled experiment with clear steps and verification at each stage. The structure reveals a methodical approach:

The Meta-Cognitive Gap

Perhaps the most interesting aspect of this message is what the assistant does not think about. There is no consideration of the possibility that identity mode itself might be the problem, not the procedure for achieving it. The assistant's reasoning is entirely focused on how to set identity mode, not whether identity mode is compatible with Blackwell GPUs.

This is a natural cognitive bias in debugging: once a hypothesis is formed (identity mode will fix P2P DMA), the mind focuses on overcoming obstacles to that hypothesis (GSP corruption during unbind/rebind) rather than questioning the hypothesis itself. The assistant is in what could be called "solution execution mode" — it has committed to the identity domain approach and is iterating on the execution procedure.

Conclusion

Message 6290 stands as a testament to the rigor and persistence required when debugging at the intersection of GPU firmware, IOMMU architecture, and Linux kernel subsystems. The assistant's eight-step procedure is methodically constructed, defensively programmed, and informed by lessons learned from previous failures. Yet it is built on an assumption — that identity mode is compatible with Blackwell GPUs — that is about to be proven wrong.

The message captures a pivotal moment in the debugging process: the last attempt before the paradigm shift. After this message, the assistant will discover the 0x177 error, realize that identity mode is fundamentally incompatible with Blackwell's FSP, and pivot to accepting NCCL_P2P_DISABLE=1 while relying on MTP speculation for throughput. The knowledge produced — that Blackwell's FSP requires DMA translation mode during initialization — is a hard-won piece of hardware compatibility information that will inform future deployment decisions.

In the broader narrative of this opencode session, message 6290 represents the climax of the IOMMU identity domain subplot. It is the moment of maximum effort before the inevitable reversion, the final refinement of a procedure that cannot succeed because its premise is flawed. For anyone working with NVIDIA Blackwell GPUs in virtualized environments with IOMMU requirements, the lesson is clear: P2P DMA restoration via IOMMU identity domains is not viable, and alternative optimization paths must be pursued.