The Blackwell FSP Barrier: When IOMMU Identity Domains Meet NVIDIA's Firmware Security Processor

In the intricate dance between GPU hardware and system software, some boundaries cannot be crossed. Message [msg 6276] captures one such boundary — a moment of discovery where a carefully orchestrated sequence of PCI bus resets, driver unbinding, and IOMMU reconfiguration collides with an immovable constraint deep inside NVIDIA's Blackwell GPU architecture. This single message, containing a bash command and its output, represents the culmination of an escalating series of attempts to restore GPU peer-to-peer (P2P) DMA capability, and it delivers the definitive answer: IOMMU identity domains are fundamentally incompatible with Blackwell GPUs.

The Mission: Restoring P2P DMA

To understand why this message matters, we must first understand what the assistant was trying to accomplish. Earlier in the session (segment 40, [msg 6255] onward), the system had been configured with 8 NVIDIA RTX PRO 6000 Blackwell GPUs split across two domains: 4 GPUs bound to the nvidia driver for an SGLang inference server, and 4 GPUs assigned to a SEV-SNP confidential VM via vfio-pci. The nvidia-bound GPUs were running with NCCL_P2P_DISABLE=1 because P2P DMA transfers were causing corruption under the SEV-SNP IOMMU configuration.

The root cause of the P2P corruption was that the IOMMU was operating in "DMA-FQ" (DMA with Fine-Grained Queue) translation mode, which required the kernel to set up DMA mappings for every peer access. When these mappings were incomplete or incorrect (as was happening with the DmaRemapPeerMmio=1 nvidia driver parameter), peer GPU accesses would fault or corrupt data. The solution seemed straightforward: switch the IOMMU groups for the nvidia-bound GPUs from DMA-FQ to identity mode, which bypasses DMA translation entirely and lets GPUs address each other's memory directly using raw PCIe bus addresses.

The Escalating Reset Saga

The assistant's journey to this message had already been long and arduous. Starting at [msg 6262], the assistant had attempted:

1. Simple unbind/rebind (<msg id=6263-6265>): Unbind the nvidia driver, set identity, rebind. Result: GPUs failed to initialize with RmInitAdapter failed! (0x62:0xffff:2142).
2. PCI remove/rescan ([msg 6268]): Remove the GPU devices from the PCI bus and rescan. Result: GPUs reappeared but still failed — the GSP firmware state persisted.
3. Function Level Reset (FLR) (<msg id=6271-6272>): Use the PCIe FLR capability to reset each GPU. Result: FLR succeeded but GSP remained corrupted, evidenced by badf4100 register reads.
4. Secondary Bus Reset (SBR) (<msg id=6273-6275>): Attempt bus reset via the parent PCIe bridge. Result: Bus reset method was not supported on these devices.
5. SBR via parent bridge registers ([msg 6275]): Directly manipulate the PCI bridge control register to assert Secondary Bus Reset. This was the most aggressive reset attempted — it toggled bit 6 (SBR) in the BRIDGE_CONTROL register of each upstream PCIe switch port. After that SBR, the GPUs had "fallen off the bus" and required a full PCI remove + rescan cycle to recover. The assistant then recovered them successfully (see <msg id=6277-6282>), but each time the nvidia driver was rebound, the GSP firmware failed with the same WPR2 already up error.

The Subject Message: A Pivotal Discovery

Message [msg 6276] is the moment where the assistant, having just executed the most aggressive reset available (SBR via parent bridge registers), attempts the identity domain configuration one more time. The message opens with the assistant's reasoning:

"GPUs exist but no driver bound (which is expected — the SBR + remove/rescan wiped the driver binding). The nvidia driver didn't auto-probe because it was already unbound. Now let me set identity domains and bind nvidia."

This reasoning reveals a critical assumption: that the SBR had fully reset the GPU state, and that a clean unbind → set identity → rebind sequence would now work. The assistant believed the earlier failures were due to insufficient reset depth, and that SBR — the most thorough software-triggerable PCIe reset — would clear the GSP firmware state.

The bash command executed in this message performs three steps:

1. Check current IOMMU group types: The output shows all 4 GPUs already at type=identity — remnants from a previous attempt that had set the type but failed to bind the driver.
2. Set identity domains: All 4 succeed with "identity OK". This is the easy part — writing to a sysfs file.
3. Bind to nvidia driver: ALL 4 GPUs FAIL. The output is truncated but shows 0000:01:00.0 -> nvidia FAILED, 0000:11:00.0 -> nvidia FAILED, and so on. The subsequent nvidia-smi check returns "No devices were found", confirming the GPUs are non-functional. The dmesg tail would reveal the critical error that explains everything.

The Critical Discovery: WPR2 and the FSP Boot Failure

While the dmesg output is truncated in the message, the assistant's analysis in subsequent messages (particularly <msg id=6293-6294>) and the chunk summary reveal the crucial finding. The nvidia driver's GSP (GPU System Processor) firmware boot sequence fails with error code 0x177 when IOMMU is in identity mode. The specific error is:

_kgspBootGspRm: unexpected WPR2 already up, cannot proceed with booting GSP

WPR2 stands for "Write Protected Region 2" — a protected memory region within the GPU's on-chip SRAM that the FSP (Firmware Security Processor) uses for its secure boot sequence. When the IOMMU is in identity mode, the kernel's DMA API does not set up the specific DMA mappings that the FSP requires during its initialization. The FSP attempts to access certain memory regions through DMA translations that simply don't exist in identity mode, causing the boot to fail.

This is fundamentally different from a transient state issue that could be cleared with a reset. The FSP boot sequence requires DMA translation mappings to be set up by the kernel — it cannot operate in a pass-through addressing mode. Identity mode, by definition, bypasses all DMA translation, which means those required mappings never get created.

Why No Software Reset Can Fix This

The assistant attempted every software-triggerable reset mechanism available:

| Reset Method | Mechanism | Result | |---|---|---| | Driver unbind/rebind | Remove PCI device from driver, reattach | GSP corrupted | | PCI remove/rescan | Remove device from bus, rescan | GSP persisted | | Function Level Reset (FLR) | PCIe FLR via sysfs | GSP persisted | | Bus reset (sysfs) | PCIe bus reset via reset_method | Not supported | | SBR via bridge registers | Toggle bit 6 of BRIDGE_CONTROL | GPUs fell off bus |

None of these could clear the FSP state because the FSP firmware lives in protected on-chip SRAM that survives software-initiated resets. The only reset that works is a power cycle (D3cold → D0 transition), which requires physically removing and restoring power to the GPU — something that cannot be done at runtime on a production server without a full reboot.

Assumptions and Their Consequences

The assistant made several assumptions that proved incorrect:

1. "SBR fully resets the GPU state": While SBR resets the PCIe link and the GPU's PCIe configuration space, it does not reset the FSP firmware running inside the GPU's security processor. The FSP's WPR2 region persists across SBR.
2. "A clean unbind/rebind will work after SBR": The assistant assumed the earlier failures were due to accumulated state from multiple unbind/rebind cycles, and that a single clean cycle after a full reset would work. In reality, the failure is not about state accumulation — it's a fundamental incompatibility between identity IOMMU mode and the FSP boot sequence.
3. "The IOMMU type can be changed at runtime": While the sysfs interface allows changing the IOMMU group type at runtime, the practical constraint is that the nvidia driver must be loaded after the identity type is set. But loading nvidia triggers the FSP boot, which fails under identity mode. This creates a chicken-and-egg problem: you can't set identity after nvidia loads (because the driver is already using DMA mappings), and you can't load nvidia after setting identity (because the FSP boot fails).

Input Knowledge Required

To fully understand this message, one needs:

• PCIe architecture: Understanding of PCIe bus topology, bridge devices, Secondary Bus Reset, Function Level Reset, and how PCI device enumeration works via sysfs.
• IOMMU concepts: Knowledge of DMA remapping, translation vs. identity modes, and how the Linux kernel exposes IOMMU groups via /sys/kernel/iommu_groups/.
• NVIDIA driver internals: Familiarity with the GSP (GPU System Processor) firmware, the FSP (Firmware Security Processor), and the nvidia driver's initialization sequence.
• Blackwell architecture specifics: Understanding that Blackwell GPUs have a more sophisticated security processor than previous generations, with protected memory regions (WPR) that survive software resets.
• Linux kernel driver model: How PCI drivers bind/unbind via sysfs, how driver_override works, and how module loading interacts with device enumeration.

Output Knowledge Created

This message produces several important pieces of knowledge:

1. Definitive proof that IOMMU identity domains are incompatible with Blackwell GPUs: The FSP boot requires DMA translation mappings that don't exist in identity mode.
2. A complete taxonomy of reset mechanisms and their effectiveness: SBR via bridge registers works to reset the PCIe link but not the FSP. FLR is supported but insufficient. No software-triggered reset can clear the FSP state.
3. The error signature for this failure mode: RmInitAdapter failed! (0x62:0xffff:2142) followed by _kgspBootGspRm: unexpected WPR2 already up and badf4100 register reads.
4. A dead end for the P2P restoration approach: The identity domain strategy is definitively blocked. The only remaining option for P2P is the nvidia driver's DmaRemapPeerMmio=1 parameter, which was already enabled but producing incomplete IOMMU mappings.

The Aftermath

The assistant's response to this discovery was swift and pragmatic. In subsequent messages (<msg id=6282-6295>), it attempted several more variations — module unload/reload, vfio-pci unbinding, temporary blacklisting — but each attempt confirmed the same fundamental limitation. By [msg 6295], the assistant had pivoted to exploring nvidia_gpu_tools.py for GPU recovery, but the chunk summary tells us the final outcome: the assistant reverted the modprobe hook, rebooted, and accepted the DMA-FQ configuration with NCCL_P2P_DISABLE=1 as the stable production state.

The MTP (Multi-Token Prediction) speculation enabled in the previous session survived the reboot and continues to provide 12-45% throughput improvement, serving as the primary active optimization. The system is stable with SGLang serving Qwen3.5-122B-A10B BF16 on 4 GPUs with TP=4, P2P disabled, and DMA-FQ IOMMU type.

Conclusion

Message [msg 6276] is a testament to the reality that some hardware boundaries cannot be crossed through software ingenuity alone. The Blackwell GPU's FSP represents a new security architecture that fundamentally changes the relationship between the GPU and the system IOMMU. What worked on previous GPU generations — switching IOMMU to identity mode for P2P DMA — is no longer viable. This discovery, while disappointing for the P2P restoration goal, is valuable knowledge that saves countless future hours of debugging. It also highlights an important principle in systems engineering: sometimes the most important output of a debugging session is not a solution, but a definitive proof that a particular approach cannot work.