The FSP That Wouldn't Boot: How IOMMU Identity Mode Broke Blackwell GPUs at the Firmware Level

Introduction

In the course of a sprawling infrastructure session spanning GPU topology reconfiguration, IOMMU debugging, and speculative decoding optimization, one message stands as a turning point — a moment where a carefully constructed plan collapsed under the weight of a previously unknown hardware constraint. Message [msg 6419] records the assistant's discovery that IOMMU identity domains, which had been the target of an elaborate modprobe-based boot-time configuration strategy, are fundamentally incompatible with NVIDIA's Blackwell GPU architecture. The message captures the instant when theory met reality, and the assistant was forced to abandon weeks of effort and revert to a working baseline.

The Context: A Long Road to P2P DMA

To understand the significance of this message, one must appreciate the journey that led to it. The assistant had been working to enable peer-to-peer (P2P) DMA transfers between Blackwell GPUs on a Proxmox host with SEV-SNP IOMMU enabled. The default IOMMU configuration used DMA-FQ (DMA with Fine-grained Queuing) translation mode, which introduced overhead for GPU direct memory access. The goal was to switch the IOMMU groups for the NVIDIA-bound GPUs to identity mode, which would bypass translation and allow direct P2P transfers.

Previous attempts had failed because the systemd service responsible for setting identity domains ran after the NVIDIA driver had already initialized the GPUs. On Blackwell GPUs, the nvidia driver's initial probe locks the GPU's Firmware Security Processor (FSP/GSP) — a dedicated microcontroller that handles secure boot and firmware initialization. Once locked, no software reset mechanism (Function Level Reset, Secondary Bus Reset, or even CXL bus reset) can clear this state. The only window to set identity domains is before the nvidia driver ever touches the GPUs.

The assistant's elegant solution was a modprobe install hook — a script configured via /etc/modprobe.d/nvidia-iommu-identity.conf that would run the identity domain configuration before the nvidia module loaded. This hook was tested and verified to work correctly at runtime (the identity domains were set, the script logged its actions). The final test required a reboot to see if the hook would intercept the very first nvidia module load at boot time, before the GPUs' FSP was locked.

The Reboot and Initial Success

The user issued the reboot command in [msg 6410], and the assistant waited for the host to return. When the machine came back, the initial diagnostics in [msg 6414] and [msg 6415] were confusing: nvidia-smi reported "No devices were found," yet the IOMMU groups showed type=identity for all four NUMA0 GPUs, and the nvidia kernel modules were loaded. The assistant initially suspected a timing issue — perhaps the container wasn't ready, or nvidia-smi couldn't enumerate devices from the host when GPUs were bound to an LXC.

But then the deeper investigation in [msg 6418] revealed the truth. The kernel log contained a cascade of errors:

NVRM: GPU0 kfspProcessCommandResponse_GH100: FSP response reported error.
Task ID: 0x1 Command type: 0x14 Error code: 0x177
NVRM: GPU0 gpuHandleSanityCheckRegReadError_GH100: Possible bad register read

The error code 0x177 was new — different from the 0xffff errors seen in previous failed recovery attempts. This was not a corrupted GSP from a previous driver bind. This was a fresh failure occurring on a clean boot.

The Moment of Realization

Message [msg 6419] captures the assistant's reasoning as it connects the dots. The key insight is stated directly:

"This means the identity IOMMU domain is causing the FSP boot to fail. The nvidia driver's FSP initialization sequence requires DMA through the IOMMU, and with identity mode, something is going wrong."

This is a critical finding. The assistant hypothesizes that the Blackwell GPU's Firmware Security Processor requires specific DMA mappings that are set up by the kernel's DMA API in translation mode. In identity mode, those mappings either don't exist or are configured differently, and the FSP's boot sequence — which involves DMA transfers to load firmware into the GPU's internal memory — fails as a result.

The error code 0x177 is particularly telling. The FSP (Firmware Security Processor) on Blackwell GPUs is a sophisticated microcontroller that handles secure boot, firmware validation, and runtime management. During initialization, the nvidia driver sends a series of commands to the FSP via a mailbox interface. One of these commands (Task ID: 0x1, Command type: 0x14) apparently involves DMA operations that require the IOMMU to be in translation mode. When the IOMMU is set to identity mode, the DMA mappings that the kernel's DMA API would normally create for the FSP's firmware loading sequence are absent or malformed, causing the FSP boot to fail.

The "bad register read" at address 0x110040 returning 0xbadf4100 is a classic symptom of a GPU whose firmware initialization has failed — the register returns a recognizable error pattern (0xbadf... is a deliberate NVIDIA error signature) indicating the hardware is in an unexpected state.

The Decision to Revert

The assistant's response is immediate and decisive. There is no attempt to debug further, no experimentation with alternative configurations. The reasoning is clear: if identity IOMMU mode breaks the FSP boot at the firmware level, there is no software workaround. The approach is fundamentally incompatible with Blackwell GPUs.

The assistant executes two commands in rapid succession:

rm -f /etc/modprobe.d/nvidia-iommu-identity.conf && echo "Modprobe hook removed" && reboot

The modprobe hook that had been carefully crafted, tested, and deployed is deleted. The system is rebooted to restore the working DMA-FQ configuration. The entire IOMMU identity domain approach — spanning multiple sessions, numerous experiments, and elaborate boot-time orchestration — is abandoned in a single message.

Assumptions and Their Failure

This message reveals several assumptions that turned out to be incorrect:

Assumption 1: IOMMU mode is transparent to devices. The assistant assumed that switching IOMMU from translation to identity mode would be invisible to the GPU — that the GPU would simply see physical addresses directly instead of translated I/O virtual addresses. The discovery that the FSP's boot sequence specifically requires DMA API mappings from translation mode shattered this assumption.

Assumption 2: The modprobe hook approach would work. The hook was tested at runtime and appeared to function correctly — the identity domains were set, and the nvidia driver loaded. But the runtime test was contaminated: the GPUs' FSP had already been locked by a previous driver bind in that boot session. The true test required a clean boot, and it failed.

Assumption 3: Blackwell GPUs behave like previous generations. The assistant's experience with GPU recovery techniques (FLR, SBR, CXL bus reset) was based on previous NVIDIA architectures. Blackwell's FSP introduces a new level of firmware complexity that makes these techniques ineffective — and, as discovered here, adds new constraints on IOMMU configuration.

Input Knowledge Required

To fully understand this message, one needs:

Output Knowledge Created

This message generates several important pieces of knowledge:

  1. Identity IOMMU is incompatible with Blackwell GPUs. This is a definitive finding that can guide future deployment decisions. Any system using Blackwell GPUs with IOMMU must use translation mode (DMA-FQ or similar).
  2. The FSP boot sequence requires DMA API mappings. The kernel's DMA API, when operating in translation mode, sets up specific page table entries that the FSP relies on during its initialization. Identity mode bypasses this setup, causing the FSP boot to fail.
  3. No software-level workaround exists. The assistant's immediate reversion — without attempting FLR, SBR, or any other recovery technique — signals that this is a fundamental hardware/firmware constraint, not a configuration bug.
  4. P2P DMA via IOMMU identity domains is definitively blocked. The only remaining path for P2P DMA on this system would be if the nvidia driver's DmaRemapPeerMmio=1 parameter can be made to work correctly (it was already enabled but producing incomplete IOMMU mappings).

The Aftermath

Following this message, the assistant rebooted the host ([msg 6420]), verified that the GPUs returned with DMA-FQ IOMMU type and full functionality ([msg 6421]), started the container ([msg 6422]), and launched SGLang ([msg 6423]). The MTP (Multi-Token Prediction) speculation optimization, which had been configured in the previous session, survived the reboot and continued to provide a 12-45% throughput improvement. The system returned to a stable, production-ready state — but without the P2P DMA that had been the original goal.

Conclusion

Message [msg 6419] is a study in scientific reasoning under pressure. The assistant formulated a hypothesis (identity IOMMU breaks FSP boot), tested it with a clean reboot, observed the failure, correctly diagnosed the root cause, and took decisive corrective action — all within a single message. The willingness to abandon a complex, multi-session effort in the face of contradictory evidence is a hallmark of effective debugging. The message also serves as a cautionary tale about the assumptions we make about hardware transparency: IOMMU mode is not invisible to devices, and modern GPUs with sophisticated firmware stacks have dependencies that extend far beyond the PCIe BAR space.