The Moment of Failure: When IOMMU Identity Domains Met Blackwell's Firmware Security Processor

Introduction

In the complex dance between operating system kernels, GPU firmware, and hardware virtualization technologies, few moments are as instructive as the precise instant when a carefully constructed plan unravels. Message 6309 of this opencode session captures exactly such a moment — a turning point where a multi-hour engineering effort to enable GPU peer-to-peer DMA on a system of eight NVIDIA RTX PRO 6000 Blackwell GPUs collides with an invisible hardware constraint that no amount of software cleverness can overcome.

The message itself is deceptively brief. The assistant executes a verification script, loads the NVIDIA driver, and receives the stark output: "No devices were found." But the context surrounding this message reveals it as the culmination of a long chain of reasoning, experimentation, and incremental discovery — and the beginning of the realization that a fundamental hardware limitation blocks the entire approach.

The Quest for P2P DMA

To understand why message 6309 matters, we must first understand what the assistant was trying to accomplish. The system in question is a high-performance machine running Ubuntu 24.04 with eight NVIDIA RTX PRO 6000 Blackwell GPUs, split across two NUMA domains. Four GPUs on NUMA0 are dedicated to an SGLang inference server running the Qwen3.5-122B-A10B model with tensor parallelism across all four, while the other four GPUs on NUMA1 are bound to VFIO for a confidential computing VM using SEV-SNP.

The problem: GPU peer-to-peer DMA (P2P) — the ability for GPUs to directly access each other's memory without going through the CPU — was broken. The NVIDIA driver's DmaRemapPeerMmio=1 parameter was enabled, but it produced incomplete IOMMU mappings. Some peer pairs worked, others triggered IOMMU page faults. The root cause was the IOMMU group type: the GPUs were in DMA-FQ (DMA with Function Queuing) mode, which uses the kernel's DMA API to create translation mappings. For P2P to work reliably, the IOMMU groups needed to be in identity mode, where the GPU's PCI BAR addresses are mapped 1:1 to the system's physical address space, bypassing translation.

The Engineering of a Solution

The assistant had spent many messages (roughly [msg 6293] through [msg 6308]) developing and refining a strategy to switch the IOMMU groups to identity mode. This turned out to be surprisingly difficult because of a timing problem: the NVIDIA driver binds to the GPUs during PCI enumeration, before any user-space process can intervene to change the IOMMU group type. Once the driver is bound, the IOMMU group is locked — you cannot change its type while devices are actively using it.

The assistant explored multiple approaches:

  1. Runtime unbind/rebind ([msg 6293]-[msg 6305]): Unbind the GPU from the NVIDIA driver, change the IOMMU group type, then rebind. This failed because the NVIDIA driver's unbind operation left the GPU's Firmware Security Processor (FSP) in a corrupted state that no software reset could clear. Secondary Bus Reset (SBR), Function Level Reset (FLR), and even the NVIDIA gpu-admin-tools recovery commands all failed to revive the GPUs after unbind.
  2. Udev rules ([msg 6306]-[msg 6307]): Create a udev rule that fires when the GPU appears on PCI and sets identity before the driver probes. The assistant correctly identified a race condition: the udev RUN script executes after the device is added, which may be after the driver has already bound.
  3. Blacklist + manual probe ([msg 6308]): The most promising approach. Blacklist the NVIDIA module with install /bin/false, remove the GPUs from PCI, perform SBR on the parent bridges to ensure a clean hardware state, rescan PCI (the GPUs appear without any driver binding because NVIDIA is blacklisted), set the IOMMU groups to identity, remove the blacklist, and manually trigger the NVIDIA driver probe. Message 6308 executed this plan. The output showed that after the rescan, the IOMMU groups were already set to identity:
0000:01:00.0: driver=none, group=28, type=identity
0000:11:00.0: driver=none, group=61, type=identity
0000:61:00.0: driver=none, group=42, type=identity
0000:71:00.0: driver=none, group=10, type=identity

The Critical Moment: Message 6309

This is where message 6309 begins. The assistant opens with an exclamation of surprise:

Wait — the groups already show identity! That's because I set them to identity in a previous step, and though the devices were removed and rescanned, the IOMMU groups got reassigned to the same numbers and the kernel remembered the setting (or the groups persisted).

This is a fascinating moment of reasoning. The assistant is trying to explain an unexpected observation. It had set the IOMMU groups to identity in a previous step (during an earlier failed attempt), and now — after the GPUs were fully removed from PCI and rescanned — the groups are still identity. The assistant offers two possible explanations:

  1. The kernel remembered the setting across device removal/rescan.
  2. The IOMMU groups persisted as kernel objects even though their member devices were removed and re-added. The second explanation is more technically accurate: IOMMU groups are kernel-level abstractions that can persist independently of the PCI devices assigned to them. When a device is removed from PCI, its IOMMU group may remain in the kernel's data structures, retaining its configured type. When the device is rescanned and re-enumerated, it gets reassigned to the same group number, inheriting the previously set type. But the assistant is cautious. It doesn't assume the identity setting is real — it runs a verification script:
echo "=== Verify identity is actually set ==="
for gpu in 0000:01:00.0 0000:11:00.0 0000:61:00.0 0000:71:00.0; do
    group=$(basename $(readlink /sys/bus/pci/devices/$gpu/iommu_group))
    type=$(cat /sys/kernel/iommu_groups/$group/type)
    echo "  $gpu: group=$group, type=$type"
done

The verification confirms: all four GPUs are in identity mode. This is the moment of apparent success — the assistant has achieved what it set out to do. The IOMMU groups are in identity mode, no driver is bound, and the NVIDIA module can now be loaded fresh onto clean hardware with identity domains already in place.

The assistant proceeds with the next steps: remove the blacklist, load the NVIDIA modules, and check nvidia-smi. The script is methodical:

rm -f /etc/modprobe.d/temp-nvidia-blacklist.conf
modprobe nvidia
sleep 5
modprobe nvidia_modeset; modprobe nvidia_uvm; modprobe nvidia_drm
sleep 5
nvidia-smi --query-gpu=index,gpu_bus_id,name,memory.total --format=csv,noheader 2>&1

The sleeps are deliberate — they give the driver time to initialize the GPUs, probe their firmware, and establish communication. The assistant has learned from previous attempts that the Blackwell FSP initialization takes time and that rushing the module load sequence causes failures.

The output comes back:

No devices were found

The Meaning of "No Devices Were Found"

This single line carries enormous weight. It means the NVIDIA driver loaded successfully (no module load errors), but it could not initialize any of the four GPUs. The driver's rm_init_adapter function — the routine that initializes a GPU's firmware and establishes the driver-GPU communication channel — failed for every device.

The assistant doesn't yet know why. In the subsequent messages ([msg 6310]-[msg 6315]), it will discover the same error pattern that plagued earlier attempts: RmInitAdapter failed! (0x62:0xffff:2142) followed by kfspSendBootCommands_HAL failure and the ominous 0xbadf4100 register read. The error code 0x62 with subcode 0xffff:2142 points to a GSP (GPU System Processor) firmware initialization failure — specifically, the FSP (Firmware Security Processor) boot sequence failing.

The critical insight, which the assistant will only fully articulate later in the segment, is that identity IOMMU mode is fundamentally incompatible with Blackwell GPUs. The Blackwell FSP requires specific DMA mappings set up by the kernel's DMA API during its boot sequence. In translation mode (DMA-FQ), the kernel creates I/O page tables that map the FSP's memory regions. In identity mode, these mappings don't exist — the GPU's DMA requests go directly to physical addresses, but the FSP's boot protocol depends on the translation infrastructure being present.

Assumptions and Their Failure

Message 6309 rests on several assumptions, most of which turn out to be incorrect:

Assumption 1: Identity mode is safe for GPU initialization. The assistant assumed that setting identity mode before the NVIDIA driver loads would be equivalent to having identity mode set at boot time — the GPU would initialize normally, just with different IOMMU behavior. This assumption was reasonable based on experience with previous GPU generations (Ampere, Hopper) where identity mode worked fine. Blackwell's FSP introduces a new dependency on DMA translation that didn't exist before.

Assumption 2: The blacklist + rescan approach would produce a clean GPU state. The assistant assumed that removing the GPUs from PCI, performing SBR on the bridges, and rescaming would put the GPUs into the same state they would be in after a cold boot. This was partially correct — the GPUs did enumerate and were ready for driver binding. But the FSP's dependency on IOMMU translation mode is a property of the GPU hardware itself, not a state that can be changed by resetting the PCI interface.

Assumption 3: The kernel remembers IOMMU group settings across device removal. This turned out to be true — the groups did persist — but it was a red herring. The assistant interpreted this persistence as a good sign, but it was merely a consequence of how the Linux kernel manages IOMMU groups. The groups' identity type was a remnant of the earlier failed attempt, not a fresh configuration that would work.

Input Knowledge Required

To fully understand message 6309, one needs knowledge of:

Output Knowledge Created

Message 6309 produces several important pieces of knowledge:

  1. Verification that identity domains can be set before NVIDIA loads: The assistant successfully demonstrated that with the blacklist approach, IOMMU groups can be set to identity mode before the NVIDIA driver binds. This is a useful technique for other scenarios, even if it doesn't help with Blackwell GPUs.
  2. Evidence that identity mode blocks Blackwell GPU initialization: The "No devices were found" output, combined with the dmesg errors that follow in subsequent messages, establishes that Blackwell GPUs cannot initialize under identity IOMMU. This is a significant finding — it means that any system using Blackwell GPUs with IOMMU enabled must use DMA translation mode, which in turn means P2P DMA must work through the NVIDIA driver's DmaRemapPeerMmio mechanism rather than through identity mapping.
  3. A reproducible failure mode: The assistant has now created a reliable way to reproduce the FSP initialization failure: set the IOMMU group to identity, then load the NVIDIA driver. This reproducibility is valuable for debugging and for reporting the issue to NVIDIA.

The Thinking Process Visible in the Message

The assistant's reasoning is visible in several ways in message 6309:

The opening "Wait — the groups already show identity!" reveals a moment of genuine surprise. The assistant had expected to need to set identity manually after the rescan, but found it already set. The immediate speculation about why ("the kernel remembered the setting (or the groups persisted)") shows the assistant working through possible explanations in real-time.

The decision to verify before proceeding — "Let me verify this is real and not stale" — demonstrates scientific caution. The assistant has been burned before by assuming a state change was effective when it wasn't. The verification script is thorough, checking each GPU individually and printing the group number and type.

The structure of the verification script also reveals the assistant's mental model. It checks the GPUs in a specific order (01:00.0, 11:00.0, 61:00.0, 71:00.0 — the NUMA0 GPUs), then checks the VFIO GPUs separately (81:00.0, 91:00.0, e1:00.0, f1:00.0 — the NUMA1 GPUs bound to vfio-pci). This shows the assistant is thinking in terms of the NUMA topology and the split between NVIDIA and VFIO domains.

The sleeps in the module loading sequence (5 seconds after each modprobe) reflect learned experience. Earlier attempts that loaded modules too quickly or without adequate delays between steps produced different failure modes. The assistant has internalized that Blackwell GPU initialization is slow and that the module load sequence must be paced.

The Broader Context

Message 6309 is the turning point in a larger narrative arc. The assistant has been working on P2P DMA for the entire segment (segment 41), which itself builds on work from earlier segments. The IOMMU identity approach was the most promising path to enabling P2P, and its failure here forces a strategic pivot.

In the messages that follow ([msg 6310]-[msg 6315]), the assistant will try increasingly desperate measures: CXL bus resets, re-examining dmesg logs, and eventually the full remove → SBR → rescan cycle again. Each attempt produces the same result: the FSP cannot initialize under identity IOMMU.

The ultimate resolution, visible in the chunk summary for segment 41, is that the assistant must abandon the IOMMU identity approach entirely. The only remaining option for P2P is to make the NVIDIA driver's DmaRemapPeerMmio=1 parameter work correctly — a path that had already been explored and found to produce incomplete mappings. The assistant pivots to accepting the current configuration (DMA-FQ IOMMU, P2P disabled via NCCL_P2P_DISABLE=1) and focuses on the MTP speculation optimization, which continues to provide a 12-45% throughput improvement.

Conclusion

Message 6309 is a study in the nature of systems engineering. It captures the moment when a carefully constructed plan — built on reasonable assumptions, tested incrementally, and executed methodically — meets an invisible hardware constraint that no amount of software cleverness can overcome. The assistant's "No devices were found" is not a failure of engineering but a discovery: Blackwell's FSP requires DMA translation mode, and this requirement is absolute.

The message also demonstrates the value of systematic thinking in debugging. The assistant doesn't panic, doesn't jump to conclusions, and doesn't abandon the approach prematurely. It verifies its assumptions, documents its observations, and proceeds methodically to the next hypothesis. This disciplined approach is what ultimately leads to the correct diagnosis: not a timing issue, not a driver bug, but a fundamental hardware compatibility constraint.

For anyone working with Blackwell GPUs, IOMMU configuration, or high-performance GPU clusters, message 6309 offers a valuable lesson: sometimes the most important output of an experiment is not success, but the precise documentation of where and why success is impossible.