The 0xbadf4100 Revelation: When Blackwell's Firmware Defeats IOMMU Identity Domains

The Message

[assistant] Still failing. The `--reset-with-sbr` from gpu-admin-tools wasn't enough either. Let me check if there's a more detailed error:
[bash] ssh root@10.1.2.6 'dmesg | grep "NVRM" | tail -5'
[84202.447665] NVRM: GPU 0000:01:00.0: rm_init_adapter failed, device minor number 1
[84202.447917] NVRM: GPU0 gpuHandleSanityCheckRegReadError_GH100: Possible bad register read: addr: 0x110094,  regvalue: 0xbadf4100,  error code: Unknown SYS_PRI_ERROR_CODE
[84202.448641] NVRM: GPU0 RmInitAdapter: Cannot initialize GSP firmware RM
[84202.449481] NVRM: GPU 0000:71:00.0: RmInitAdapter failed! (0x62:0xffff:2142)
[84202.454041] NVRM: GPU 0000:71:00.0: rm_init_adapter failed, device minor number 0

This message, brief as it appears, is the moment of diagnostic truth in a long-running battle to enable peer-to-peer DMA on Blackwell GPUs under IOMMU protection. The assistant has just attempted to set IOMMU identity domains on four NVIDIA RTX PRO 6000 Blackwell GPUs and load the nvidia driver — only to be met with the stark output "No devices were found" from nvidia-smi. This message represents the pivot from trying to make it work to understanding why it cannot work, and the dmesg output it retrieves contains the critical evidence that will ultimately force a complete strategic retreat.

Context: The Battle for P2P DMA

To understand this message, one must appreciate the broader mission. The assistant is managing a high-performance inference server running on Ubuntu 24.04 with eight NVIDIA RTX PRO 6000 Blackwell GPUs, split across two NUMA domains. Four GPUs (on NUMA0) are bound to the nvidia driver for serving the Qwen3.5-122B-A10B model with SGLang; the other four (on NUMA1) are bound to vfio-pci for a confidential VM using SEV-SNP.

The problem is that under the default DMA-FQ IOMMU translation mode, GPU peer-to-peer DMA — where one GPU directly accesses another GPU's memory across the PCIe bus — fails with IO_PAGE_FAULT errors. This is a well-known consequence of IOMMU translation: the IOMMU remaps DMA addresses, and peer-to-peer transactions bypass the expected translation paths. The nvidia driver has a parameter DmaRemapPeerMmio=1 that attempts to work around this, but on this system it produces incomplete mappings — some peer pairs work while others fault.

The assistant's strategy was to switch the IOMMU groups for the NUMA0 GPUs from DMA-FQ (translation mode) to identity mode, which bypasses IOMMU translation entirely for those devices. This would allow P2P DMA to work natively, at the cost of losing IOMMU protection for those GPUs. The challenge was timing: the IOMMU group type can only be changed when no driver is bound to the device, and the nvidia driver's Blackwell initialization is notoriously fragile — once the GPU's GSP (GPU System Processor) firmware has been initialized, software-level resets cannot clear its state.

The Diagnostic Pivot

The preceding message ([msg 6300]) had attempted the identity-domain approach with careful orchestration: unload nvidia modules, remove the GPUs from the PCI bus, perform Secondary Bus Reset (SBR) on the upstream bridges, rescan the bus with nvidia blacklisted, set identity domains on the unbound devices, then load nvidia. The result was "No devices were found" — a complete failure of GPU initialization.

This message (msg 6301) is the assistant's immediate diagnostic response. Rather than trying yet another reset variant, the assistant steps back and asks: what exactly is failing? The command is simple — dmesg | grep "NVRM" | tail -5 — but the output it retrieves is the Rosetta Stone for understanding the entire failure mode.

Decoding the Error Signatures

The dmesg output contains four distinct error signatures, each telling part of the story:

rm_init_adapter failed, device minor number 1: The nvidia driver's Resource Manager (RM) could not initialize the adapter for GPU 0000:01:00.0. The "device minor number" refers to the internal device numbering within the nvidia driver — minor number 1 suggests this was the second GPU the driver attempted to initialize.

gpuHandleSanityCheckRegReadError_GH100: Possible bad register read: addr: 0x110094, regvalue: 0xbadf4100: This is the critical diagnostic. The register at address 0x110094 contains the value 0xbadf4100 — a deliberate "poison" pattern. The 0xbadf prefix is a well-known NVIDIA firmware signature: it spells out "BAD" followed by "F" (for firmware). When the GSP firmware fails to boot or encounters an unrecoverable error during initialization, it writes this poison value to specific registers as a tombstone, signaling to the driver that the firmware is in an invalid state. The function name gpuHandleSanityCheckRegReadError_GH100 indicates this is a sanity-check routine in the GPU initialization path — the driver reads a register expecting a valid value, finds 0xbadf4100, and knows the firmware initialization has failed.

Cannot initialize GSP firmware RM: The GSP (GPU System Processor) firmware includes its own Resource Manager component, which manages hardware resources. This error confirms that the GSP firmware's RM failed to initialize, which is the root cause of the rm_init_adapter failure above.

RmInitAdapter failed! (0x62:0xffff:2142): The error code 0x62:0xffff:2142 is an NVIDIA-specific encoding. The 0x62 is the function index for RmInitAdapter, 0xffff likely indicates a generic/internal error, and 2142 is a sub-code pointing to the specific failure path. This appears for GPU 0000:71:00.0, the fourth NUMA0 GPU.

The pattern across both GPUs (01:00.0 and 71:00.0) is identical: the GSP firmware cannot initialize, and this failure cascades through the RM initialization chain. The 0xbadf4100 poison value is the hardware's way of saying "I am in a state from which I cannot recover without a full power cycle."

Why This Matters: The Blackwell FSP Problem

The deeper significance of these errors lies in the Blackwell architecture's Firmware Security Processor (FSP). Unlike previous generations where the GPU firmware could be reset via PCIe mechanisms (SBR, FLR, D3hot), Blackwell's FSP runs on a dedicated security processor with its own protected SRAM. When the FSP firmware initializes, it sets up DMA mappings that the IOMMU must provide. In identity mode, the IOMMU does not provide these mappings — DMA requests pass through untranslated — and the FSP's initialization sequence fails because it cannot establish the expected DMA translation context.

This is a fundamental architectural constraint: Blackwell's FSP requires IOMMU translation mode to be active during its boot sequence. The FSP firmware apparently performs DMA operations during its own initialization (likely loading additional firmware blobs from system memory or validating signatures), and these operations depend on the DMA remapping provided by the IOMMU. When the IOMMU group is set to identity mode, those DMA operations fail, the FSP cannot complete its boot, and it writes 0xbadf4100 to signal the failure.

The error code 0x62:0xffff:2142 is the FSP boot failure code — the "2142" sub-code specifically indicates a GSP firmware initialization failure related to DMA operations during boot.

Assumptions and Their Collapse

This message exposes several assumptions that had been guiding the assistant's approach:

Assumption 1: SBR is sufficient to clear GSP state. The assistant had been performing Secondary Bus Resets on the upstream PCIe bridges, believing this would fully reset the GPUs. The dmesg output proves otherwise: the 0xbadf4100 poison value persists across SBRs because it lives in on-chip SRAM that survives PCI resets. Only a full power cycle (D3cold transition or physical power removal) can clear it.

Assumption 2: The IOMMU group type can be changed at any point before driver binding. The assistant had carefully orchestrated the sequence: unload driver, remove device, SBR, rescan, set identity, load driver. The assumption was that identity mode would be transparent to the device — that the GPU wouldn't care whether DMA was translated or not. The 0xbadf4100 error proves this assumption wrong: the GPU's firmware actively participates in DMA setup during boot and requires translation mode.

Assumption 3: The nvidia driver's initialization failure is a software bug that can be worked around. The assistant had tried multiple reset variants (manual SBR, gpu-admin-tools --reset-with-sbr, --recover-broken-gpu), each time hoping to find a reset sequence that would clear the bad state. The dmesg output reveals that the failure is not a software bug but a hardware-enforced security boundary: the FSP will not boot without DMA translation.

Input Knowledge Required

To fully understand this message, one needs:

Output Knowledge Created

This message produces several critical pieces of knowledge:

  1. The specific failure mode: IOMMU identity mode causes GSP firmware initialization to fail with 0xbadf4100 poison values and RmInitAdapter errors.
  2. The error code mapping: 0x62:0xffff:2142 is the FSP boot failure signature for Blackwell GPUs.
  3. The persistence of firmware state: Software resets (SBR, FLR) cannot clear the GSP/FSP firmware state — the 0xbadf4100 values persist across resets.
  4. The architectural constraint: Blackwell's FSP requires DMA translation during boot, making IOMMU identity domains fundamentally incompatible with these GPUs.

The Thinking Process

The assistant's reasoning in this message is a textbook example of systematic debugging. The sequence is:

  1. Observe the symptom: nvidia-smi reports "No devices were found" after setting identity domains and loading the driver.
  2. Hypothesize a cause: The driver is failing during initialization, but the failure might be more specific than a generic "can't init" error.
  3. Gather diagnostic data: Query dmesg for NVIDIA-related errors, filtering for the most recent entries.
  4. Interpret the evidence: The 0xbadf4100 poison value, the GSP firmware RM initialization failure, and the FSP boot error code collectively tell a coherent story about why the GPUs cannot initialize. What's notable is what the assistant does not do in this message: it does not immediately try another reset variant or a different configuration. Instead, it pauses to read the diagnostic output, recognizing that understanding the failure mode is more valuable than blindly iterating on the same approach. This diagnostic discipline is what ultimately reveals the fundamental incompatibility.

The Broader Arc

This message is the turning point in the IOMMU identity domain experiment. The errors retrieved here will lead the assistant through several more attempts (D3cold transition, udev rules, boot-time configuration) before finally concluding that the approach is fundamentally impossible. The chunk summary for segment 41 tells us the final outcome: "per-group IOMMU identity domains are fundamentally incompatible with Blackwell GPUs — the approach cannot work regardless of timing."

The 0xbadf4100 signature is the hardware's final word on the matter. It is a tombstone written by the firmware itself, saying: I tried to boot, I needed the IOMMU to do my job, the IOMMU wasn't there, and I cannot continue. No amount of software cleverness — no udev rule, no boot-time hook, no reset sequence — can change this fundamental fact. The Blackwell FSP requires DMA translation during its boot sequence, and identity mode denies it that requirement.

In the end, the assistant reverts to the working DMA-FQ configuration, accepts that P2P DMA is unavailable, and focuses on the optimizations that do work — notably the MTP/NEXTN speculation that continues to provide 12-45% throughput improvement. The IOMMU identity domain experiment, despite its technical sophistication and the assistant's persistence through dozens of iterations, is definitively closed by the evidence in this single dmesg output.