The Moment the Blackwell GSP Broke: A Pivot Point in IOMMU Identity Domain Debugging

Introduction

In the course of a marathon infrastructure debugging session spanning dozens of messages, message [msg 6289] stands as a quiet but devastating turning point. It is the moment when a carefully constructed sequence of operations — module unloading, PCI removal, secondary bus reset (SBR), PCI rescan, IOMMU identity domain configuration, and module reloading — collapses into failure, revealing a fundamental hardware incompatibility between NVIDIA's Blackwell GPU architecture and Linux's IOMMU identity domains. The message is brief, almost laconic: "Still 'No devices were found' — the nvidia module loaded but didn't auto-probe the GPUs." But the dmesg output it carries tells a deeper story: the Blackwell GPU's Firmware Security Processor (FSP) cannot initialize when IOMMU is in identity mode, and this is not a recoverable error.

The Message Itself

The assistant writes:

Still "No devices were found" — the nvidia module loaded but didn't auto-probe the GPUs. Let me check if the devices need to be triggered:

``bash ssh root@10.1.2.6 'dmesg | grep -E "NVRM|nvidia" | tail -10; echo "---"; for gpu in 0000:01:00.0 0000:11:00.0 0000:61:00.0 0000:71:00.0; do driver=$(basename $(readlink /sys/bus/pci/devices/$gpu/driver) 2>/dev/null || echo "none"); echo " $gpu: driver=$driver"; done' ``

`` [83924.268961] NVRM: GPU 0000:71:00.0: RmInitAdapter failed! (0x62:0xffff:2142) [83924.269534] NVRM: GPU 0000:71:00.0: rm_init_adapter failed, device minor number 0 [83924.269864] NVRM: GPU2 gpuHandleSanityCheckRegReadError_GH100: Possible bad register read: addr: 0x110094, regvalue: 0xbadf4100, error code: Unknown SYS_PRI_ERROR_CODE [83924.270637] NVRM: GPU2 RmInitAdapter: Cannot initialize GSP firmware RM [83924.271650] NVRM: GPU 0000:01:00.0: RmInitAdapter failed! (0x62:0xffff:2142) [83924.... ``

The output is truncated, but the pattern is unmistakable: every GPU on the NUMA0 bus (0000:01:00.0, 0000:11:00.0, 0000:61:00.0, 0000:71:00.0) fails with RmInitAdapter failed! and the critical message Cannot initialize GSP firmware RM. The error code 0x62:0xffff:2142 is a proprietary NVIDIA error, but the context makes its meaning clear: the GSP firmware cannot complete its boot sequence because the DMA mappings it expects from the kernel's DMA API are not present under identity mode.

The Reasoning and Motivation: Why This Message Was Written

To understand why this message exists, one must trace the thread of reasoning that led to it. The assistant had been engaged in a multi-session effort to deploy large language models (specifically Qwen3.5-122B-A10B BF16) across 8 NVIDIA RTX PRO 6000 Blackwell GPUs, split between an LXC container (4 GPUs on NUMA0) and a SEV-SNP VM (4 GPUs on NUMA1). A critical performance bottleneck had been identified: NCCL P2P (peer-to-peer) DMA between GPUs was failing under the SEV-SNP IOMMU configuration, causing NCCL operations to hang. The workaround was NCCL_P2P_DISABLE=1, which eliminated the hangs but also removed the fastest path for inter-GPU communication.

The assistant hypothesized that if the IOMMU groups for the NUMA0 GPUs could be switched from the default DMA-FQ (DMA with Fine-Grained Queuing) translation mode to identity mode — where the GPU sees the physical address space directly without translation — then P2P DMA might work correctly. This is a well-known technique in virtualization and GPU computing: identity domains bypass IOMMU translation for specific devices, allowing direct memory access.

The preceding messages in segment 41 document a series of increasingly sophisticated attempts to achieve this. The assistant tried:

Unbind → identity → rebind ([msg 6281]): Unbinding a single GPU from the nvidia driver, setting its IOMMU group to identity, and rebinding. This failed because the Blackwell GPU's GSP firmware state gets corrupted across unbind/rebind cycles.
Module unload → identity → module reload ([msg 6285]): Unloading all nvidia kernel modules, setting identity domains, and reloading. This also failed — the GSP firmware state persisted across module unload.
Module unload → PCI remove → SBR → PCI rescan → identity → module reload ([msg 6287]): The most elaborate attempt. By removing the GPUs from the PCI bus, issuing a secondary bus reset to the parent bridges (which fully resets the GSP firmware), rescaming the bus, setting identity domains while no driver was bound, and then loading the nvidia module, the assistant hoped to present the GPUs with identity mode from the very first moment the nvidia driver touched them. Message [msg 6289] is the result of this third attempt. The assistant runs a diagnostic command to understand why nvidia-smi reports "No devices were found" despite the nvidia module loading successfully. The dmesg output reveals the truth: even with a clean GSP state (fresh from SBR), the Blackwell GPU's firmware initialization fails under identity IOMMU mode.

Input Knowledge Required

To fully grasp this message, the reader needs a substantial foundation in several technical domains:

PCIe and IOMMU architecture: Understanding that PCI devices have IOMMU groups, that these groups can be in translation (DMA-FQ, DMA-API, etc.) or identity mode, and that identity mode bypasses the IOMMU's address translation. The Linux kernel exposes /sys/kernel/iommu_groups/<group>/type for this purpose.

NVIDIA Blackwell GPU architecture: Knowledge that Blackwell GPUs include a Firmware Security Processor (FSP) — previously called GSP on Hopper and earlier architectures — which handles secure boot, firmware management, and certain runtime operations. The FSP requires specific DMA mappings set up by the kernel's DMA API during initialization. The error RmInitAdapter: Cannot initialize GSP firmware RM indicates that the FSP's runtime manager (RM) cannot boot.

Secondary Bus Reset (SBR): Understanding that SBR is a PCIe mechanism that resets all devices behind a bridge, clearing their internal state including firmware. This is more thorough than Function Level Reset (FLR) and is necessary for Blackwell GPUs to recover from certain error states.

The modprobe and kernel module system: Knowing that nvidia, nvidia_modeset, nvidia_uvm, and nvidia_drm are separate kernel modules that must be loaded in a specific order, and that unloading them requires no users of the devices.

The specific hardware topology: The system has 8 Blackwell GPUs split across two NUMA domains. The NUMA0 GPUs (BDFs 01:00.0, 11:00.0, 61:00.0, 71:00.0) are managed by the nvidia driver for the LXC container, while the NUMA1 GPUs (81:00.0, 91:00.0, e1:00.0, f1:00.0) are bound to vfio-pci for the SEV-SNP VM. The parent bridges for the NUMA0 GPUs are at 00:01.1, 10:01.1, 60:01.1, and 70:01.1.

The Thinking Process Visible in the Message

The assistant's reasoning in this message is diagnostic and confirmatory. The phrase "Still 'No devices were found'" indicates that this outcome was unexpected — the assistant had reason to believe the third attempt would work. The preceding messages show the assistant progressively refining the approach, each time addressing a specific failure mode:

First attempt failed because GSP state persisted across unbind/rebind.
Second attempt failed because module unload doesn't reset GSP.
Third attempt added SBR to fully reset GSP before identity mode was applied. The assistant's working hypothesis was that if the GSP could be brought up fresh in identity mode — never having seen translation mode — it would initialize correctly. The message reveals this hypothesis to be false. The GSP apparently requires the kernel's DMA API to establish specific mappings during its boot sequence, and identity mode short-circuits this process. The diagnostic command itself is revealing: the assistant checks both dmesg for NVIDIA errors and the per-device driver status. The dmesg output is the primary diagnostic tool here, as it captures the kernel-level failure messages from the nvidia driver. The driver status check (driver=none for all GPUs) confirms that the nvidia driver failed to bind to any of the devices — they remain driverless despite the module being loaded.

Mistakes and Incorrect Assumptions

The central incorrect assumption in this message — and the sequence leading to it — is that the Blackwell FSP could initialize under identity IOMMU mode if presented with a clean state. This assumption was reasonable given the available evidence: identity domains are a standard Linux feature, and the nvidia driver supports them on other architectures (e.g., on AMD GPUs or older NVIDIA generations). However, Blackwell's FSP has a specific dependency on DMA translation that earlier architectures may not share.

A secondary assumption was that SBR fully resets all Blackwell GPU state. While SBR does reset the PCIe link and the GPU's internal logic, the error RmInitAdapter: Cannot initialize GSP firmware RM suggests that the FSP's boot sequence depends on DMA mappings that the kernel sets up during driver probing — mappings that never materialize under identity mode. This is not a state persistence issue but a fundamental initialization dependency.

There is also a subtle methodological mistake: the assistant did not test identity mode on a single GPU before attempting the full four-GPU sequence. Earlier messages show testing with single-GPU unbind/rebind ([msg 6281]), but the full identity domain test was only performed in the batch. Had a single-GPU test been done with the SBR + identity sequence first, the failure would have been discovered with less disruption.

Output Knowledge Created

This message produces several critical pieces of knowledge:

Definitive proof that Blackwell GPUs cannot initialize under IOMMU identity mode. The error RmInitAdapter: Cannot initialize GSP firmware RM with code 0x62:0xffff:2142 is a hard failure, not a transient condition. The GSP firmware requires DMA translation mappings that identity mode does not provide.
Confirmation that SBR does not resolve the identity mode incompatibility. Even with a completely fresh GSP state (post-SBR), the initialization fails. This means the issue is architectural, not state-dependent.
A diagnostic pattern for Blackwell GPU initialization failures. The combination of RmInitAdapter failed!, Cannot initialize GSP firmware RM, and the 0x62:0xffff:2142 error code is a signature for IOMMU identity mode incompatibility. Future engineers encountering this pattern will know the root cause.
The boundary of software-based P2P DMA restoration. With identity domains ruled out, the only remaining option for P2P DMA is the nvidia driver's DmaRemapPeerMmio=1 parameter, which was already enabled but producing incomplete mappings. This message effectively closes the identity domain approach as a dead end.

The Broader Context and Aftermath

The chunk summary for segment 41 reveals what happens next: the assistant immediately reverts the change by removing the modprobe hook and rebooting, restoring the working DMA-FQ configuration. The MTP/NEXTN speculation enabled in the previous session survives the reboot and continues providing 12-45% per-request throughput improvement. The system stabilizes with SGLang serving Qwen3.5-122B-A10B BF16 on 4 GPUs with TP=4, P2P disabled, and DMA-FQ IOMMU type.

The key outcome, as summarized in the chunk analysis, is that "P2P DMA restoration via IOMMU identity domains is definitively blocked — Blackwell's FSP requires DMA translation mode during initialization, and no software-level reset (FLR, SBR, CXL bus reset) can clear this state." This message is the moment that conclusion becomes apparent.

Conclusion

Message [msg 6289] is a classic example of a negative result that is as valuable as a positive one. It definitively closes one line of investigation — IOMMU identity domains for Blackwell GPUs — and forces the project to accept the NCCL_P2P_DISABLE=1 workaround while focusing optimization efforts elsewhere (MTP speculation, model configuration, etc.). The message's brevity belies its significance: in a few lines of dmesg output, it reveals a fundamental architectural constraint of the Blackwell platform that no amount of software cleverness can circumvent. For the engineer reading the session log, this message is the pivot point where the P2P DMA restoration effort ends and the system stabilizes into its final, working configuration.