The Moment the Blackwell FSP Refused: When IOMMU Identity Domains Met an Immovable Object

Introduction

In the high-stakes world of large-scale GPU deployment, few things are more frustrating than a hardware security feature that refuses to cooperate. Message 6292 of this opencode session captures a pivotal moment in a weeks-long investigation into enabling peer-to-peer (P2P) DMA across NVIDIA Blackwell RTX PRO 6000 GPUs. The message represents the culmination of an elaborate, multi-hour attempt to set IOMMU identity domains on four Blackwell GPUs — and the moment that attempt definitively failed. What makes this message so significant is not just the failure itself, but what the failure revealed about the Blackwell architecture's Firmware Security Processor (FSP) and its ironclad resistance to software-level manipulation.

The Scene: A System Under Siege

To understand message 6292, one must first understand the battlefield. The assistant was working on a Proxmox host with 8 NVIDIA RTX PRO 6000 Blackwell GPUs, split between a VM (4 GPUs on NUMA0 for SGLang inference serving) and an LXC container (4 GPUs on NUMA1 for a SEV-SNP confidential computing VM). The system had been running stably with SGLang serving Qwen3.5-122B-A10B BF16 at TP=4, delivering impressive throughput. But there was a persistent performance bottleneck: P2P DMA between GPUs was broken.

The root cause traced back to the IOMMU configuration. Under the default DMA-FQ (DMA with Flush Queue) translation mode, the IOMMU remaps all DMA addresses through page tables. While this provides protection, it introduces latency for GPU peer-to-peer transfers. The NVIDIA driver's DmaRemapPeerMmio=1 parameter was supposed to create the necessary IOMMU mappings for P2P, but it was producing incomplete mappings — some peer pairs worked, others triggered IO_PAGE_FAULT errors. The alternative was to switch the IOMMU groups to identity mode, which bypasses translation entirely and allows direct physical addressing — the ideal configuration for GPU P2P performance.

But there was a catch. The Blackwell GPUs' FSP (Firmware Security Processor) — a hardened security co-processor that manages GPU initialization, firmware authentication, and secure boot — apparently requires specific DMA mappings set up by the kernel's DMA API during initialization. Identity mode, which bypasses those mappings, breaks the FSP boot sequence.

The Long Road to Message 6292

The assistant had been chasing this problem for hours before message 6292. The journey included:

Attempting runtime module reload: Unloading the nvidia kernel modules, setting identity domains, and reloading. Result: "No devices were found" — the GPUs' GSP (GPU System Processor, the FSP's predecessor/sibling) was corrupted.
Secondary Bus Reset (SBR): Removing GPUs from PCI, triggering SBR on parent bridges, then rescanning. This worked to restore GPUs to a clean state, but the nvidia driver would auto-probe during rescan, binding before identity could be set.
VFIO interference: The vfio-pci driver kept grabbing GPUs because of a pre-existing new_id registration. Each attempt required unbinding from vfio-pci, removing the ID, then proceeding.
CXL bus reset: Discovering that the GPUs supported cxl_bus reset and trying it. Result: the FSP state persisted — CXL reset also couldn't clear it.
NVIDIA GPU admin tools: Using nvidia_gpu_tools.py with --reset-with-sbr and --recover-broken-gpu flags. None of these could reset the FSP.
D3cold investigation: Checking if the GPUs could be transitioned to D3cold (a deep power state). They supported it, but the power state was "unknown" — the GPUs were in a bad state that prevented software control.
Udev rule approach: Creating a udev rule to set identity domains the instant a GPU appears on PCI, before the nvidia driver binds. This had race condition problems.
The blacklist breakthrough: Finally, the assistant realized the key insight — prevent nvidia from auto-probing by temporarily blacklisting the module with install nvidia /bin/false, then perform the remove → SBR → rescan cycle, set identity domains while the GPUs are driverless, remove the blacklist, and manually load nvidia. Message 6291 (the immediate predecessor) executed this blacklist approach and succeeded in getting all 4 GPUs detected with NO driver bound and the nvidia module NOT loaded. This was the first time the assistant had achieved this state — a clean slate with all GPUs present, driverless, and ready for identity domain configuration.

The Subject Message: Message 6292

Here is the message in full:

[assistant] All 4 GPUs detected with NO driver bound, and nvidia module is NOT loaded. Now I can set identity domains and then load nvidia:

[bash] ssh root@10.1.2.6 'set -e GPUS="0000:01:00.0 0000:11:00.0 0000:61:00.0 0000:71:00.0"

echo "=== Set identity domains on all 4 GPU groups ===" for gpu in $GPUS; do group=$(basename $(readlink /sys/bus/pci/devices/$gpu/iommu_group)) echo "identity" > /sys/kernel/iommu_groups/$group/type type=$(cat /sys/kernel/iommu_groups/$group/type) echo " $gpu: group=$group, type=$type" done

echo "" echo "=== Remove blacklist ===" rm -f /etc/modprobe.d/temp-nvidia-blacklist.conf

echo "" echo "=== Load nvidia modules ===" modprobe nvidia && echo " nvidia loaded" sleep 5 modprobe nvidia_modeset && echo " nvidia_modeset loaded" modprobe nvidia_uvm && echo " nvidia_uvm loaded" modprobe nvidia_drm && echo " nvidia_drm loaded" sleep 5

echo "" echo "=== FINAL VERIFICATION ===" echo "nvidia-smi:" nvidia-smi --query-gpu=index,gpu_bus_id,name,memory.total --format=csv,noheader 2>&1

echo "" echo "IOMMU group types:" for gpu in $GPUS; do group=$(basename $(readlink /sys/bus/pci/devices/$gpu/iommu_group)) driver=$(basename $(readlink /sys/bus/pci/devices/$gpu/driver) 2>/dev/null || echo "none") type=$(cat /sys/kernel/iommu_groups/$group/type) echo " $gpu: driver=$driver, group=$group, type=$type" done

echo "" echo "VFIO GPUs (should be DMA-FQ):" for gpu in 0000:81:00.0 0000:91:00.0 0000:e1:00.0 0000:f1:00.0; do group=$(basename $(readlink /sys/bus/pci/devices/$gpu/iommu_group)) driver=$(basename $(readlink /sys/bus/pci/devices/$gpu/driver) 2>/dev/null || echo "none") type=$(cat /sys/kernel/iommu_groups/$group/type) echo " $gpu: driver=$driver, group=$group, type=$type" done

echo "" echo "IO_PAGE_FAULT check (should be empty after this):" dmesg | grep IO_PAGE_FAULT | tail -3'

=== Set identity domains on all 4 GPU groups === 0000:01:00.0: group=28, type=identity 0000:11:00.0: group=61, type=identity 0000:61:00.0: group=42, type=identity 0000:71:00.0: group=10, type=identity

=== Remove blacklist ===

=== Load nvidia modules === nvidia loaded nvidia_modeset loaded nvidia_uvm loaded nvidia_drm loaded

=== FINAL VERIFICATION === nvidia-smi: No devices were found

The message is structured as a single bash command executed over SSH on the remote host (10.1.2.6). The assistant's reasoning text at the top — "All 4 GPUs detected with NO driver bound, and nvidia module is NOT loaded. Now I can set identity domains and then load nvidia" — reveals the triumphant yet premature confidence that this attempt would finally succeed.

The Reasoning and Assumptions Behind the Approach

The assistant's reasoning in this message reflects a careful, methodical approach built on lessons from previous failures. Several key assumptions and decisions are embedded in the code:

Assumption 1: Timing is everything. The assistant had learned that the critical window for setting identity domains is between PCI rescan (when GPUs appear as driverless devices) and nvidia module loading (when the driver probes and initializes the GPUs). By using a temporary blacklist (install nvidia /bin/false in /etc/modprobe.d/temp-nvidia-blacklist.conf), the assistant prevented the nvidia module from auto-loading during PCI rescan, creating a clean window to set identity domains.

Assumption 2: Identity domains persist across module load. The assistant assumed that once an IOMMU group type is set to identity, it would remain in that mode when the nvidia driver later binds and initializes the GPU. This assumption turned out to be correct in terms of the sysfs interface — the identity setting did persist — but the underlying hardware interaction was more complex.

Assumption 3: The FSP/GSP initialization problem was a race condition. The assistant had been chasing the theory that the FSP boot failure was caused by the IOMMU being in translation mode when the nvidia driver probed, and that switching to identity mode before probe would fix it. The repeated "Cannot initialize GSP firmware RM" and "RmInitAdapter failed! (0x62:0xffff:2142)" errors were interpreted as the FSP failing because the DMA mappings it needed weren't available. The assumption was that identity mode would provide those mappings (or bypass the need for them).

Assumption 4: The SBR + remove/rescan cycle fully resets the GPU. The assistant had observed that this sequence worked to restore GPUs to a working state (when nvidia auto-probed with default DMA-FQ). The assumption was that this same reset would work regardless of what IOMMU mode was subsequently configured.

Decision: Set identity before loading nvidia. This was the critical tactical decision. Instead of loading nvidia first and then trying to switch IOMMU mode (which had failed repeatedly because the driver wouldn't release the GPUs), the assistant chose to configure the IOMMU while the devices were driverless. This is the correct approach in principle — the IOMMU group type should be set before the driver binds, as changing it afterward requires unbinding and rebinding the driver.

Decision: Remove the blacklist before loading nvidia. The assistant removed the temporary blacklist file before calling modprobe nvidia, ensuring that the module would load normally and all its dependencies (nvidia_modeset, nvidia_uvm, nvidia_drm) would be satisfied.

Decision: Sequential module loading with sleeps. The assistant loaded modules one at a time with 5-second sleeps between nvidia and the dependent modules, and another 5-second sleep after all modules were loaded before verification. This was a reasonable precaution to allow the driver to fully initialize all GPUs.

The Failure and What It Revealed

The output tells a devastating story:

=== FINAL VERIFICATION ===
nvidia-smi:
No devices were found

All four IOMMU groups were successfully set to identity. All nvidia modules loaded without errors. But nvidia-smi reports "No devices were found" — the GPUs failed to initialize.

The assistant's immediate next message (msg 6293) checked dmesg and found the familiar errors:

NVRM: GPU0 _kfspPrintCms2Log_GB100: CMS2 Log:
NVRM: GPU0 nvDbgDumpBufferBytes: ... 00  00  14  00  9f  00  00  00 ...
NVRM: GPU0 nvDbgDumpBufferBytes: ... 00  00  14  00  77  01  00  00 ...

The 77 01 error code in the CMS2 log is the FSP boot failure — the same error that had appeared in every previous attempt. The identity domain approach hadn't fixed it.

But this failure was different from the earlier ones. In previous attempts, the assistant could always restore the GPUs by doing a remove + SBR + rescan cycle (without identity), which would let nvidia auto-probe with default DMA-FQ and succeed. After message 6292, the assistant tried the same restoration procedure — and it still worked. The GPUs could be restored to working state under DMA-FQ. This confirmed that the FSP issue was specifically tied to the IOMMU identity mode, not a permanent hardware corruption.

The Deeper Insight: Blackwell's FSP Architecture

The persistent failure revealed something fundamental about the Blackwell GPU architecture. The FSP (Firmware Security Processor) on Blackwell GPUs is a hardened security module that manages:

Secure boot: Authenticating and loading GPU firmware
Key management: Handling encryption keys for protected memory regions
DMA policy: Enforcing DMA access controls based on IOMMU configuration
Firmware lifecycle: Managing firmware updates and recovery The FSP apparently reads the IOMMU configuration during its initialization sequence. When the IOMMU is in identity mode, the FSP cannot establish the DMA mappings it needs for its own operation (e.g., accessing system memory for firmware loading, logging, or secure channel establishment). This causes the FSP boot to fail with error 0x177 (encoded as 77 01 in the CMS2 log), which cascades into RmInitAdapter failed! (0x62:0xffff:2142). The critical discovery was that no software-level reset can clear this FSP state. The assistant tried: - FLR (Function Level Reset): Standard PCIe reset — didn't work - SBR (Secondary Bus Reset): Bridge-level reset — didn't work - CXL bus reset: The most aggressive software reset available — didn't work - NVIDIA GPU admin tools: Vendor-provided reset utilities — didn't work The FSP state is stored in protected on-chip SRAM that survives all software-initiated resets. The only way to clear it is a full power cycle (D3cold transition or physical power removal). This is a deliberate security design — the FSP is meant to be resistant to tampering, including reset-based attacks.

Output Knowledge Created by This Message

Despite being a failure, message 6292 produced several valuable pieces of knowledge:

Identity domains can be set on driverless Blackwell GPUs: The sysfs interface works correctly — the IOMMU group type can be changed to identity when no driver is bound. This was confirmed by the type=identity output for all four groups.
The nvidia driver loads cleanly with identity domains: modprobe nvidia and its dependent modules all reported success. There were no module loading errors.
GPU initialization fails silently under identity mode: The driver loads but nvidia-smi reports "No devices were found." The GPUs are present on PCI (the sysfs entries exist) but the driver's RmInitAdapter fails during FSP initialization.
The blacklist approach works for preventing auto-probe: The temporary install /bin/false blacklist successfully prevented nvidia from auto-loading during PCI rescan, giving the assistant a clean window to configure IOMMU settings.
The failure is deterministic and reproducible: Every attempt to load nvidia with identity domains produced the same result — FSP boot failure with error 0x177. This is not a race condition or timing issue; it's a fundamental incompatibility.
VFIO GPUs are unaffected: The 4 VFIO-bound GPUs (on NUMA1 for the SEV-SNP VM) remained on DMA-FQ mode as expected, confirming that the identity domain changes were correctly scoped to the NUMA0 GPU groups.

The Broader Implications

Message 6292 represents a turning point in the investigation. Before this message, the assistant still believed that the right timing and sequence could make IOMMU identity domains work with Blackwell GPUs. After this message, it became clear that the approach was fundamentally impossible — the Blackwell FSP requires DMA translation mode during initialization, and no amount of software gymnastics can circumvent this requirement.

The assistant's subsequent messages show the pivot: from trying to make identity domains work, to accepting that P2P DMA restoration via IOMMU identity is definitively blocked, and focusing on alternative optimizations like MTP speculation (which was already delivering 12-45% throughput improvements).

The message also demonstrates a crucial lesson in systems engineering: sometimes the most valuable outcome of an experiment is not success, but the definitive proof that a particular approach cannot work. The clean, reproducible failure in message 6292 saved countless future hours that might have been spent chasing increasingly exotic solutions. The assistant could now confidently rule out IOMMU identity domains and focus energy elsewhere.

Conclusion

Message 6292 is a masterclass in systematic debugging under difficult constraints. The assistant's approach — carefully isolating variables, creating clean test conditions with the blacklist, setting identity domains on driverless devices, and methodically verifying each step — represents the gold standard for hardware-software boundary investigation. The failure was clean, informative, and final. It revealed not just a bug or a configuration error, but a fundamental architectural constraint of the Blackwell GPU platform.

For anyone deploying Blackwell GPUs in environments requiring IOMMU identity domains (such as high-performance computing clusters needing optimal P2P DMA), this message carries a sobering message: the Blackwell FSP will not cooperate. The security architecture that protects these GPUs also prevents certain low-level performance optimizations. The only remaining path for P2P DMA on Blackwell is through the NVIDIA driver's DmaRemapPeerMmio=1 parameter — which, as the assistant discovered, produces incomplete IOMMU mappings with some peer pairs working and others faulting.

In the end, the system was deployed stably with MTP speculation as the primary optimization, P2P disabled via NCCL_P2P_DISABLE=1, and all 4 NUMA0 GPUs working correctly on the nvidia driver with DMA-FQ IOMMU type. Message 6292 was the moment the team learned that sometimes, the hardware decides which battles can be won.