The Moment Blackwell Broke: When IOMMU Identity Domains Met the GSP Firmware Wall

In the sprawling infrastructure saga of deploying large language models across eight NVIDIA RTX PRO 6000 Blackwell GPUs, there are moments of quiet triumph — and moments of stark, silent failure. Message [msg 6376] belongs squarely to the latter category. It is the verification step that reveals everything is technically correct yet fundamentally broken. The assistant runs a bash command to confirm that four GPUs are bound to the nvidia driver with IOMMU identity domains, and the response comes back with a devastating contradiction: the GPUs are on the nvidia driver, but nvidia-smi reports "No devices were found." This single message captures the exact instant when a carefully engineered approach to enabling GPU peer-to-peer DMA crashes into an immovable hardware limitation.

The Context: A Multi-Session Quest for P2P DMA

To understand why this message exists, one must trace back through the preceding sessions. The system in question is a Proxmox host running Ubuntu 24.04 with eight NVIDIA RTX PRO 6000 Blackwell GPUs, split across two NUMA domains. Four GPUs (NUMA 0) are assigned to an LXC container running SGLang for LLM inference, while the other four (NUMA 1) are bound to vfio-pci for a SEV-SNP confidential VM. The challenge is that the AMD IOMMU, which must remain in full translation mode (amd_iommu=on) to support SEV-SNP, breaks GPU peer-to-peer DMA. Without P2P, the SGLang server cannot use NCCL's custom all-reduce, costing significant throughput in tensor-parallel inference across the four NUMA 0 GPUs.

The previous session (segment 40) had already diagnosed this problem extensively. The assistant discovered that per-IOMMU-group identity domains — a feature of the Linux 6.14 kernel — could theoretically allow P2P DMA for specific PCI devices while keeping the rest of the IOMMU in translation mode. This is the ideal solution: enable P2P for the inference GPUs without compromising the VM's security isolation. The assistant created a systemd service (gpu-iommu-identity.service) to set these identity domains at boot, before the nvidia driver loads. The user rebooted, and message [msg 6376] is the first check after that reboot.

What the Message Actually Shows

The assistant issues a single, comprehensive bash command that queries four things simultaneously:

1. IOMMU group types for the NUMA 0 GPUs — checking that the identity domains were correctly applied at boot
2. nvidia-smi output — the definitive test of whether the GPUs are functional
3. IOMMU group types for the VFIO GPUs — verifying that the SEV-SNP VM's GPUs remain in DMA-FQ translation mode
4. IO_PAGE_FAULT count — a diagnostic for IOMMU-related errors The output is a study in contradiction. The first section shows exactly what the assistant wanted: all four GPUs (0000:01:00.0, 0000:11:00.0, 0000:61:00.0, 0000:71:00.0) are bound to the nvidia driver, each in an IOMMU group set to identity. The VFIO GPUs are correctly in DMA-FQ mode. But then comes the killer line: nvidia-smi returns "No devices were found." This is the message's entire payload — a single verification command that simultaneously confirms success (the identity domains are set) and announces catastrophic failure (the GPUs are dead to the nvidia driver).

The Reasoning Behind the Verification

The assistant's decision to run this particular verification command reveals its mental model of the problem. The command is structured as a hypothesis test: "If the identity domains were set at boot before the nvidia driver loaded, then the GPUs should be functional on the nvidia driver with identity IOMMU type, and P2P should work." The assistant checks each link in this causal chain:

• Identity domains applied? Check /sys/kernel/iommu_groups/$group/type — yes, all four show identity.
• GPUs bound to nvidia? Check the driver symlink — yes, all four show driver=nvidia.
• GPUs actually working? Check nvidia-smi — no, "No devices were found."
• IOMMU faults? Check dmesg | grep IO_PAGE_FAULT — the count matters because identity domains can cause DMA faults if the GPU tries to use translation-mode mappings. The contradiction between the driver symlink and nvidia-smi is the key insight. The PCI device is bound to the nvidia driver (the kernel thinks the driver owns it), but the driver itself cannot communicate with the GPU (nvidia-smi fails). This points to a firmware-level initialization failure — the nvidia driver's probe succeeded at the PCI level but failed at the GSP (GPU System Processor) firmware level.

The Hidden Assumption That Broke Everything

The assistant made a critical assumption that seemed reasonable but turned out to be false: that unbinding a GPU from one driver and rebinding it to another is a clean operation on Blackwell hardware. This assumption is visible in the script written in message [msg 6373], which unbinds GPUs from vfio-pci, sets identity domains, and then rebinds to nvidia via drivers_probe. The script even includes a sleep 1 between operations, suggesting the assistant expected a straightforward handoff.

The reality, which the assistant discovers in the following messages ([msg 6377] onward), is far more insidious. The nvidia Blackwell GPU contains a Firmware Security Processor (FSP) that initializes during the driver's first probe at boot. This firmware establishes a secure session that survives all software-level resets — Function Level Reset (FLR), Secondary Bus Reset (SBR), CXL bus reset, and module unload. Once the nvidia driver has touched a Blackwell GPU, any subsequent unbind/rebind cycle leaves the FSP in a locked state. The GPU's PCI configuration space may show it as "bound to nvidia," but the firmware refuses to re-initialize, producing the 0xbadf4100 error code seen in message [msg 6377].

This is a hardware design constraint that no amount of software cleverness can overcome. The Blackwell FSP is designed to prevent unauthorized firmware tampering — a security feature that, in this context, becomes an obstacle to legitimate driver management.

Input Knowledge Required

To fully understand this message, the reader needs knowledge spanning several domains:

• IOMMU architecture: Understanding that the AMD IOMMU can operate in translation mode (DMA-FQ, which remaps device addresses for security/isolation) or identity mode (pass-through, which allows direct physical memory access for P2P). The kernel 6.14 feature of per-group domain type switching is relatively new and essential context.
• Blackwell GPU firmware: The GSP/FSP architecture, where a dedicated security processor on the GPU manages initialization and requires specific DMA mappings set up by the kernel's DMA API. This is not a well-documented detail — the assistant discovered it through trial and error.
• PCI device driver model: How Linux binds PCI devices to drivers, the role of drivers_probe, unbind, and the modalias autoloading mechanism. The assistant's earlier fix in [msg 6373] relies on writing PCI IDs to vfio-pci/new_id and triggering drivers_probe, which requires understanding this subsystem.
• SEV-SNP and confidential computing: Why the IOMMU must remain in translation mode for the VM, and why identity domains for specific groups are the only viable path to P2P without breaking VM security.
• The system topology: Four NUMA 0 GPUs for inference, four NUMA 1 GPUs for the VM, with specific PCI addresses and IOMMU group numbers that the assistant has memorized through repeated debugging.

Output Knowledge Created

This message produces several important pieces of knowledge:

1. The identity domain mechanism works at the kernel level — the IOMMU groups were successfully switched to identity mode at boot, confirming that the systemd service and script are functionally correct.
2. The boot ordering is wrong — the identity service ran before the vfio-split service, causing vfio-pci to grab all GPUs. This is a systemd dependency ordering bug, not a fundamental flaw.
3. The nvidia driver rebind fails silently — the driver claims the device but cannot initialize it. The nvidia-smi failure is the only symptom; there is no kernel panic or driver crash.
4. The GSP firmware is the root cause — subsequent messages reveal that the FSP boot sequence fails with error 0x177 when IOMMU is in identity mode during the initial probe, and that any unbind/rebind after the first probe corrupts the FSP state permanently.
5. The approach must be fundamentally rethought — setting identity domains after the nvidia driver has already probed the GPUs is impossible. The only viable path is to set identity domains before the nvidia driver ever touches the GPUs, which requires preventing nvidia from autoloading during boot.

The Thinking Process Visible in the Message

The message itself is terse — a single command and its output — but it reveals a sophisticated diagnostic strategy. The assistant is running a "smoke test" that checks multiple independent indicators of GPU health. The structure of the command shows the assistant's mental checklist:

1. Verify the configuration (IOMMU types)
2. Verify the binding (driver assignments)
3. Verify the functionality (nvidia-smi)
4. Verify the side effects (IO_PAGE_FAULTs) This is the same pattern a systems engineer would use: check that what you think you configured is actually configured, then check that the system is actually working, then check for hidden damage. The absence of any follow-up action in this message is also telling. The assistant does not immediately try to fix the problem — it just observes and reports. This restraint shows discipline: the assistant recognizes that the output is unexpected and needs analysis before action. The real debugging begins in the next message ([msg 6377]), where the assistant checks dmesg and discovers the 0xbadf4100 GSP error.

The Broader Significance

Message [msg 6376] is a classic "it compiles but doesn't work" moment, elevated to the hardware level. Everything the assistant did was technically correct: the IOMMU groups are in identity mode, the GPUs are bound to the nvidia driver, the VFIO GPUs are undisturbed. Yet the system is non-functional. This is the kind of failure that no amount of code review or configuration checking can catch — it requires understanding the undocumented behavior of the Blackwell firmware's security processor.

The message also represents a turning point in the session. Before this message, the assistant believed that runtime driver rebinding with identity domains was a viable approach. After this message, the assistant must confront the possibility that P2P DMA on Blackwell with IOMMU identity domains is simply impossible — a conclusion that the chunk summary confirms: "per-group IOMMU identity domains are fundamentally incompatible with Blackwell GPUs."

For the reader following this infrastructure saga, message [msg 6376] is the moment when a promising solution dies, and the search for alternatives begins.