The Blackwell FSP Wall: When Software Resets Can't Reach the Firmware

Introduction

In the course of deploying a multi-GPU machine learning inference server, an AI assistant encountered a profound hardware-software boundary issue. Message 6299 captures a pivotal moment in a debugging session that ultimately revealed a fundamental incompatibility between NVIDIA's Blackwell GPU architecture and Linux IOMMU identity domains. The message itself is deceptively simple — a single bash command running NVIDIA's gpu-admin-tools with the --reset-with-sbr flag — but it sits at the culmination of an escalating series of reset attempts, each more aggressive than the last, and each failing against the same immovable obstacle: the Blackwell Firmware Security Processor (FSP).

The Context: Why Identity Domains Matter

To understand why this message was written, we must first understand what the assistant was trying to achieve. The broader session involved deploying large language models (specifically Qwen3.5-122B-A10B BF16) across four NVIDIA RTX PRO 6000 Blackwell GPUs using SGLang with tensor parallelism (TP=4). For multi-GPU inference, peer-to-peer (P2P) DMA is critical — it allows GPUs to directly access each other's memory without going through the CPU or system memory, dramatically reducing latency and improving throughput.

However, the system was running under a Proxmox hypervisor with SEV-SNP (Secure Encrypted Virtualization) enabled, which required IOMMU (Input-Output Memory Management Unit) protection. The default IOMMU mode was DMA-FQ (DMA with Fine-grained Translation), which provides full DMA translation and protection but breaks GPU P2P DMA. The alternative was identity mode, which maps GPU physical addresses directly to bus addresses, bypassing translation and enabling P2P DMA — at the cost of reduced security isolation.

The assistant had already split the 8 GPUs in the system: 4 on NUMA0 were assigned to the nvidia driver for inference, and 4 on NUMA1 were assigned to vfio-pci for a SEV-SNP VM. The goal was to set the IOMMU groups for the 4 NUMA0 GPUs to identity mode while keeping the VFIO GPUs in DMA-FQ mode. This per-group IOMMU configuration was the elegant solution the assistant was pursuing.

The Escalating Reset Cascade

By the time we reach message 6299, the assistant has already tried and failed with multiple approaches. The sequence of failures is instructive:

  1. Direct identity setting (msg 6288): The assistant tried to set identity domains while GPUs were bound to vfio-pci, then unbind and load nvidia. Result: RmInitAdapter failed! (0x62:0xffff:2142) — the nvidia driver could not initialize the GPU's firmware.
  2. Full remove + SBR + rescan (msg 6290): The assistant removed the GPUs from PCI, performed Secondary Bus Reset (SBR) on the parent bridges via setpci, rescanned, set identity, and loaded nvidia. Result: still "No devices were found."
  3. Blacklist nvidia during rescan (msg 6291): The assistant created a temporary modprobe blacklist to prevent nvidia from auto-probing during the PCI rescan, ensuring the GPUs appeared without any driver bound. Then identity was set, blacklist removed, and nvidia loaded. Result: still failing with the same FSP initialization error.
  4. NVIDIA gpu-admin-tools --recover-broken-gpu (msg 6296): The assistant discovered NVIDIA's gpu_admin_tools.py utility with a --recover-broken-gpu option. This ran silently without visible effect.
  5. --set-next-sbr-to-fundamental-reset (msg 6298): The assistant tried to mark the next SBR as a fundamental (more thorough) reset. Result: ERROR - Fundamental reset not supported on these GPUs.

Message 6299: The Pivot to Tool-Assisted SBR

This brings us to the subject message. The assistant's reasoning, visible in the opening line, is: "Fundamental reset not supported on these GPUs. Let me try --reset-with-sbr using the tool (it may do it more thoroughly than my manual SBR)."

This reasoning reveals several assumptions:

Assumption 1: The tool's SBR is more thorough than manual SBR. The assistant had been performing SBR manually by manipulating the PCI bridge control register via setpci — setting bit 6 (Secondary Bus Reset) in the bridge control register, holding it for 500ms, then clearing it. The NVIDIA tool presumably has access to additional GPU-specific registers and sequences that a generic PCI bridge reset cannot reach. The --ignore-nvidia-driver flag suggests the tool can operate on the GPU even when the nvidia driver isn't loaded or the GPU is in a broken state.

Assumption 2: The FSP state can be cleared by a sufficiently thorough software reset. This assumption was the central incorrect hypothesis driving the entire investigation. The Blackwell GPU's Firmware Security Processor (FSP) is designed to be resistant to software-initiated resets. It stores its state in protected on-chip SRAM that survives PCIe resets (SBR, FLR) and even CXL bus resets. The FSP is a security feature — it prevents an attacker who gains OS-level access from resetting the GPU's secure firmware state.

Assumption 3: The GPU initialization failure is caused by corrupted firmware state from a previous session. The dmesg logs showed RmInitAdapter failed! (0x62:0xffff:2142) and kfspSendBootCommands_HAL errors, suggesting the FSP was in an inconsistent state. The assistant believed that a clean reset would restore the FSP to a bootable state.

What Actually Happened

The command in message 6299 runs --reset-with-sbr on all four GPUs sequentially. The output shows the tool selecting each GPU and identifying its topology (PCI bridge and GPU BDF), but the output is truncated — we see only the first two GPUs' output. From subsequent messages (6300-6315), we learn that this approach also failed. The GPUs remained uninitializable with identity IOMMU domains.

The critical discovery, which crystallized in the messages following 6299, was that the IOMMU identity domain itself was causing the FSP boot failure. The Blackwell FSP requires specific DMA mappings set up by the kernel's DMA API during initialization. When the IOMMU group is in identity mode, the kernel's DMA API doesn't create these mappings — it simply passes physical addresses through. The FSP, expecting translation-mode DMA mappings, fails to initialize.

This means the problem was never about corrupted firmware state or insufficient reset thoroughness. The problem was architectural: Blackwell GPUs require DMA translation mode during FSP initialization, and identity mode breaks this initialization irrecoverably. No software reset — SBR, FLR, CXL bus reset, or any combination thereof — can fix this because the FSP is designed to survive all software-initiated resets. The only reset that clears the FSP is a full power cycle (D3cold transition or physical power-off/on).

Input Knowledge Required

To fully understand message 6299, several pieces of domain knowledge are necessary:

Output Knowledge Created

Message 6299, combined with the subsequent investigation, produced several important conclusions:

  1. Per-group IOMMU identity domains are fundamentally incompatible with Blackwell GPUs. The FSP requires DMA translation mode during initialization. This is not a timing issue or a reset issue — it's an architectural constraint.
  2. No software-initiated reset can clear the Blackwell FSP state. SBR, FLR, CXL bus reset, and NVIDIA's own gpu-admin-tools reset commands all fail to reset the FSP. This is by design for security.
  3. The only viable reset for a stuck Blackwell FSP is a full power cycle. This requires either physical power-off/on or a D3cold (device power state D3 cold) transition, which may not be achievable without hardware support.
  4. P2P DMA on Blackwell GPUs under IOMMU is blocked unless the entire system uses identity IOMMU. Since per-group identity doesn't work, the only option for P2P would be system-wide identity IOMMU (which breaks SEV-SNP VM isolation) or making the nvidia driver's DmaRemapPeerMmio=1 parameter work correctly (which was already enabled but produced incomplete mappings).

The Thinking Process

The assistant's thinking in this message reflects a systematic debugging approach. The progression from manual SBR to tool-assisted SBR shows an understanding that different reset mechanisms have different depths. The --set-next-sbr-to-fundamental-reset attempt (msg 6298) shows the assistant knew about the distinction between conventional SBR and fundamental reset — a deeper reset that reinitializes the PCIe link from scratch.

When fundamental reset was rejected ("not supported on these GPUs"), the assistant fell back to --reset-with-sbr, reasoning that even a conventional SBR performed by NVIDIA's own tool might be more thorough than the manual setpci approach. The tool might set additional GPU-specific registers, apply proper timing sequences, or coordinate with GPU firmware in ways that raw bridge register manipulation cannot.

The truncated output in the message is also significant — it shows the tool running successfully (no error messages for the first two GPUs), which might have given the assistant false hope that this approach was working. It was only after loading nvidia (in msg 6300) that the failure became apparent again.

Conclusion

Message 6299 represents the final attempt to solve the P2P DMA problem through software reset techniques before the assistant was forced to accept the fundamental architectural constraint. It's a message about the boundary between software and hardware — where operating system abstractions (IOMMU domains, PCI resets) meet physical hardware realities (FSP security design, protected on-chip SRAM).

The lesson is that modern GPUs, especially Blackwell with its security-focused FSP, are no longer fully controllable through standard PCIe mechanisms. The FSP's resistance to software reset is a security feature that becomes a debugging obstacle when trying to reconfigure the IOMMU layer beneath the GPU. The assistant ultimately had to abandon the IOMMU identity approach and accept the working DMA-FQ configuration with P2P disabled, relying instead on MTP speculation (which survived the reboot) for throughput improvements of 12-45%.

This message, in its brevity, captures the moment when an elegant technical solution (per-group IOMMU identity domains for P2P DMA) collided with an immovable hardware constraint (Blackwell FSP requiring DMA translation), and the debugging process had to pivot from "how do we reset the GPU more thoroughly" to "what other optimizations can we use instead."