The Blackwell FSP Barrier: When Software Resets Fail and IOMMU Identity Domains Collide with GPU Firmware

Introduction

In the high-stakes world of large-scale ML inference deployment, few things are more frustrating than a hardware-level roadblock that no amount of software ingenuity can overcome. Message 6315 of this coding session captures a pivotal moment in a multi-hour debugging odyssey: the assistant has just discovered that none of the standard PCIe software reset mechanisms — FLR, SBR, or even CXL bus reset — can clear the Blackwell GPU's Firmware Security Processor (FSP) state. This message represents the turning point where the assistant pivots from trying to reset the GPUs into a clean state to instead trying to time the IOMMU domain configuration perfectly, preventing the nvidia driver from binding until the identity domain is set. It is a message of renewed hope, a carefully orchestrated plan, and ultimately — as the subsequent messages reveal — a plan that fails against the immutable physics of Blackwell's security architecture.

The Problem: P2P DMA and the IOMMU Barrier

To understand why this message matters, we must step back and appreciate the larger context. The system in question is a high-end ML inference server running Ubuntu 24.04 with eight NVIDIA RTX PRO 6000 Blackwell GPUs. Four of these GPUs (on NUMA node 0) are bound to the nvidia driver for running SGLang serving the Qwen3.5-122B-A10B BF16 model with tensor parallelism across 4 GPUs. The other four are bound to vfio-pci for a SEV-SNP (Secure Encrypted Virtualization - Secure Nested Paging) virtual machine.

The critical problem is that P2P (peer-to-peer) DMA between GPUs is broken. Under normal circumstances, NVIDIA GPUs can communicate directly via P2P DMA over the PCIe bus, bypassing system memory for dramatically faster data transfers. However, when the IOMMU is in translation mode (specifically DMA-FQ — DMA with Fine-Grained translation), the kernel's DMA API creates mappings that can cause P2P transactions to fault. The SEV-SNP security feature on AMD EPYC processors requires the IOMMU to be active, creating an inherent tension: security demands IOMMU translation, but GPU performance demands identity mapping that bypasses it.

The nvidia driver has a parameter DmaRemapPeerMmio=1 that attempts to work around this by creating special IOMMU mappings for P2P traffic, but on this system it produces incomplete results — some peer pairs work while others fault. The only definitive fix would be to set the IOMMU groups for the NUMA0 GPUs to identity mode, which bypasses translation entirely for those devices. But this must be done before the nvidia driver initializes the GPU's firmware, because — as the assistant is about to discover — the Blackwell FSP has specific requirements about the DMA environment it boots into.

The Failed Software Resets: A Catalog of Ineffectiveness

The messages immediately preceding msg 6315 (msg 6301 through msg 6314) document a systematic attempt to reset the Blackwell GPUs using every available PCIe software reset mechanism. The assistant tried:

FLR (Function Level Reset): The standard PCIe reset that resets the device's PCI configuration space and internal state. On Blackwell, FLR completed without error but did not clear the FSP — the firmware remained in its corrupted state.

SBR (Secondary Bus Reset): A more aggressive reset that asserts the PCIe reset signal on a downstream port, resetting all devices behind a bridge. The assistant performed SBR on the parent bridges (0000:00:01.1, 0000:10:01.1, 0000:60:01.1, 0000:70:01.1) with varying hold times. SBR also failed to clear the FSP.

CXL Bus Reset: A new reset type available on CXL (Compute Express Link) capable devices. The assistant discovered that the GPUs supported cxl_bus reset and tried it with apparent success (the sysfs write returned "OK"), but the subsequent nvidia driver load still failed with the same FSP initialization error.

The error signature was consistent across all attempts: RmInitAdapter failed! (0x62:0xffff:2142) followed by kfspSendBootCommands_HAL errors and the cryptic gpuHandleSanityCheckRegReadError_GH100: Possible bad register read: addr: 0x110094, regvalue: 0xbadf4100. The error code 0x62 indicates a GSP (GPU System Processor) firmware initialization failure, and the 0xbadf4100 register value suggests the FSP is reading back garbage — a classic sign of a device that has been put into an inconsistent state by earlier operations and cannot be recovered without a full power cycle.

The assistant's conclusion in msg 6315 is stark: "None of the software resets (FLR, SBR, CXL bus) are clearing the Blackwell GPU's Firmware Security Processor." This is a significant finding. On previous GPU architectures (like GH100/Ampere or earlier), SBR was typically sufficient to reset the GPU's internal state. Blackwell's FSP represents a new security architecture designed to be resistant to software-initiated resets — a feature intended to prevent physical attacks but which becomes a liability when debugging system integration issues.

The Pivot: Timing the Identity Domain Configuration

Despite the bleak assessment of software resets, the assistant identifies a glimmer of hope. Earlier in the session, a specific sequence did produce working GPUs: "remove from PCI → SBR on parent bridge → rescan." When this sequence was executed, the GPUs came back clean and the nvidia driver initialized successfully. The problem was that during this sequence, the nvidia driver auto-probed the fresh GPUs before the assistant could set the IOMMU groups to identity mode.

This observation leads to a refined strategy: prevent nvidia from auto-probing during the rescan, set the identity domains while the GPUs are driverless, then manually trigger nvidia probe. The assistant articulates this clearly: "So the solution is clear: I need to prevent nvidia from auto-probing during the rescan, then set identity, then manually trigger nvidia probe."

The mechanism chosen is a modprobe blacklist using the install=/bin/false directive. This is a clever trick: instead of the usual blacklist nvidia directive (which only prevents loading by name but doesn't stop auto-probing via module alias matching), the install directive replaces the module loading command with /bin/false, which always fails. When the kernel's PCI subsystem tries to auto-probe a newly discovered device by loading the nvidia driver, the modprobe invocation will fail silently, leaving the device driverless.

The Orchestrated Sequence: A Step-by-Step Analysis

The bash script executed in msg 6315 is a carefully orchestrated seven-step sequence. Let us examine each step and the reasoning behind it:

Step 1: Ensure nvidia is unloaded. The assistant unloads all four nvidia kernel modules (nvidia_drm, nvidia_uvm, nvidia_modeset, nvidia) in reverse dependency order. This is necessary because the GPU removal in step 3 requires no driver to be bound to the devices.

Step 2: Create nvidia blacklist with install=/bin/false. The blacklist file is written to /etc/modprobe.d/temp-nvidia-blacklist.conf with install directives for all four modules. The use of a "temp" prefix in the filename signals that this is a temporary measure — the file will be removed after the identity domains are configured.

Step 3: Remove GPUs from PCI. Each GPU is removed from the PCI bus by writing 1 to its /sys/bus/pci/devices/*/remove sysfs attribute. This triggers the kernel's PCI hotplug removal path, which tears down the device's software state including its IOMMU group membership.

Step 4: SBR on all bridges. Secondary Bus Reset is applied to each of the four parent bridges. The assistant uses setpci to read the current BRIDGE_CONTROL register, ORs it with bit 6 (the SBR bit), writes it back, waits 0.5 seconds, then restores the original value. This sequence asserts the PCIe reset signal on the downstream port, theoretically resetting any devices behind it.

Step 5: Prevent vfio-pci from grabbing. The assistant removes the NVIDIA PCI vendor/device ID (10de 2bb5) from vfio-pci's driver table. This is a defensive measure: the VFIO driver might otherwise bind to the GPUs when they reappear during rescan, since the nvidia driver is blacklisted and vfio-pci is the next candidate.

Step 6: Rescan PCI. Writing 1 to /sys/bus/pci/rescan triggers the kernel to re-enumerate the PCI bus, discovering any devices that were previously removed. The GPUs should reappear with fresh PCI configuration space (thanks to the SBR) but without any driver bound.

Step 7: Verify no driver bound. The assistant checks that each GPU is present, has no driver, and reports its IOMMU group and type. The lsmod | grep nvidia check confirms that the nvidia modules remain unloaded.

What the Output Reveals

The output shown at the end of msg 6315 is truncated — we see only 0000:01:00.0: dr... — but the full result is visible in the subsequent message (msg 6316). All four GPUs reappeared with no driver bound, and crucially, the IOMMU groups were already set to identity. This is a surprising and important finding: the IOMMU group type setting persisted across the remove/rescan cycle. When the GPUs were removed, their IOMMU groups were destroyed. But when they were rescanned, the kernel recreated the groups with the same numbers (28, 61, 42, 10) and — unexpectedly — retained the identity type setting.

This persistence suggests that the kernel's IOMMU subsystem caches group type configurations at a level above individual device lifetimes, or that the group numbers happen to be deterministic based on the PCI topology and the type setting was never actually cleared. Either way, it's a fortunate outcome: the assistant doesn't need to re-set the identity domains after rescan.

The Critical Assumption: Does SBR Actually Clear the FSP?

The entire plan in msg 6315 rests on a critical assumption: that the remove → SBR → rescan sequence actually resets the Blackwell GPU's FSP to a clean state. The assistant explicitly states this: "The only thing that worked was: remove from PCI → SBR on parent bridge → rescan. That sequence was successful earlier (GPUs came back clean)."

But there is a subtle flaw in this reasoning. The earlier "successful" sequence happened before the assistant had attempted any identity domain configurations or failed nvidia loads. The GPUs at that point had only been through a clean boot cycle. The SBR may have appeared to work, but in reality, the GPUs were already in a clean state — the SBR simply didn't corrupt them. The assistant is about to discover that once the FSP has been put into a bad state (by the earlier identity domain experiments), SBR is powerless to restore it.

This is confirmed in the following messages (msg 6316-6318). Despite the clean driverless state and the identity domains being set, loading the nvidia driver produces the exact same failure: "No devices were found" in nvidia-smi, and the same kfspSendBootCommands_HAL errors in dmesg. The FSP is truly, irrevocably stuck.

Input Knowledge Required

To fully understand msg 6315, the reader needs knowledge spanning several domains:

PCIe reset mechanisms: Understanding the difference between FLR (device-level), SBR (bus-level), and CXL bus reset, and their respective capabilities and limitations. The reader must know that SBR is typically the most aggressive software-initiated reset available on PCIe.

IOMMU architecture: Knowledge of IOMMU groups, domain types (DMA-FQ vs identity), and how the kernel manages DMA remapping for PCIe devices. The distinction between translation mode (which enables security features like SEV-SNP but breaks P2P) and identity mode (which bypasses translation but sacrifices isolation) is central to the narrative.

Linux kernel module loading: Understanding modprobe, module auto-probing via PCI device IDs, and the install directive as a mechanism for intercepting module loading. The install=/bin/false trick is an advanced technique that goes beyond simple blacklisting.

NVIDIA GPU architecture: Familiarity with the GSP (GPU System Processor) and FSP (Firmware Security Processor) on modern NVIDIA GPUs, and their role in device initialization. The reader must understand that Blackwell introduces a more security-hardened firmware architecture than previous generations.

Blackwell-specific behavior: Knowledge that Blackwell GPUs have a security processor that is resistant to software-initiated resets, and that the FSP boot sequence requires specific DMA environment conditions.

Output Knowledge Created

Msg 6315 produces several important pieces of knowledge:

  1. IOMMU group type persistence: The identity type setting on IOMMU groups survives PCI device removal and rescan. This is a non-obvious kernel behavior that could be useful in other contexts.
  2. Blackwell FSP reset limitations confirmed: The message explicitly documents that FLR, SBR, and CXL bus reset all fail to clear the Blackwell FSP. This is valuable diagnostic information for anyone working with Blackwell GPUs in virtualized or IOMMU-managed environments.
  3. The blacklist timing technique: The use of install=/bin/false modprobe directives to prevent auto-probing during PCI rescan is a documented technique that could be applied to other driver timing problems.
  4. The remove→SBR→rescan sequence as a clean-state mechanism: Even though it doesn't reset the FSP, this sequence does produce a clean PCI configuration space for the GPUs, which is a necessary precondition for any subsequent driver binding attempt.
  5. The fundamental incompatibility between IOMMU identity domains and Blackwell FSP: While this discovery is fully confirmed in subsequent messages, msg 6315 sets up the framework for understanding why the approach fails.

The Thinking Process: A Window into Debugging Methodology

The reasoning visible in msg 6315 reveals a systematic debugging methodology. The assistant:

  1. Summarizes what was learned: "The CXL reset also didn't clear the FSP state. None of the software resets (FLR, SBR, CXL bus) are clearing the Blackwell GPU's Firmware Security Processor." This is a clear statement of the current state of knowledge.
  2. Identifies the one thing that worked: "The only thing that worked was: remove from PCI → SBR on parent bridge → rescan." This is a critical observation — even if the mechanism isn't fully understood, the empirical result is clear.
  3. Identifies the remaining problem: "The difference is that during that sequence, the nvidia driver auto-probed the fresh GPUs before we could set identity." This pinpoints the exact timing issue to be solved.
  4. Formulates a solution: "So the solution is clear: I need to prevent nvidia from auto-probing during the rescan, then set identity, then manually trigger nvidia probe."
  5. Acknowledges the need for careful execution: "let me try again but make absolutely sure the blacklist works" — the assistant recognizes that previous attempts may have failed due to sloppy execution rather than fundamental incompatibility.
  6. Executes with comprehensive error handling: The bash script includes verification steps at every stage, checks for unexpected states (like nvidia still being loaded), and uses defensive programming (redirecting errors to /dev/null, using || true for optional steps). This methodology — summarize, identify, formulate, acknowledge, execute — is a textbook example of structured debugging. The assistant doesn't just try random things; it builds a mental model of the problem, identifies the specific gap between the current state and the desired state, and designs a targeted intervention.

The Broader Significance

Msg 6315 sits at a critical juncture in the session. It represents the last attempt to solve the P2P DMA problem through software configuration before the assistant is forced to accept a fundamental hardware limitation. The subsequent messages (msg 6316-6318) show the plan failing, leading to the conclusion that "per-group IOMMU identity domains are fundamentally incompatible with Blackwell GPUs."

This is a valuable lesson for systems engineers working with modern GPU hardware. The Blackwell architecture represents a significant departure from previous generations in terms of security hardening. Features like the FSP, which are designed to protect against firmware attacks, have unintended consequences for system integration. The assumption that "a GPU is just a PCIe device that can be reset like any other" no longer holds.

The session ultimately pivots to accepting the DMA-FQ configuration with NCCL_P2P_DISABLE=1 and focusing on MTP (Multi-Token Prediction) speculation as the primary throughput optimization. The MTP speculation delivers 12-45% per-request throughput improvement, partially compensating for the loss of P2P DMA. This pragmatic adaptation — working within hardware constraints rather than fighting them — is itself a valuable lesson in production deployment.

Conclusion

Message 6315 is a message of carefully reasoned optimism that collides with immutable hardware reality. The assistant correctly identifies the timing problem, designs an elegant solution using modprobe blacklisting, and executes it with precision. But the fundamental assumption — that SBR can reset the Blackwell FSP — proves incorrect. The FSP, once corrupted by an incompatible IOMMU configuration, cannot be recovered without a full power cycle.

For anyone working with Blackwell GPUs in IOMMU-managed environments, this message and its surrounding context provide crucial documentation: IOMMU identity domains are incompatible with Blackwell's FSP initialization requirements, and no software reset mechanism can recover from this state. The only options are to either accept DMA-FQ mode with P2P disabled, or to find a way to make the nvidia driver's DmaRemapPeerMmio parameter work correctly. This is not a bug to be fixed but a fundamental architectural constraint of the Blackwell security model — one that system integrators must plan for from the start.