The Blackwell FSP Trap: A Deep Dive Into PCI Reset Failure on NVIDIA RTX PRO 6000 GPUs

Introduction

In the course of deploying a high-performance inference server across eight NVIDIA RTX PRO 6000 Blackwell GPUs, an opencode session reached a critical inflection point at message index 6318. This message captures a moment of dawning realization: the assistant has been chasing a phantom, and the hardware is not cooperating. The message is a turning point where the assistant acknowledges that the Blackwell GPU's Firmware Security Processor (FSP) is fundamentally resistant to software-initiated resets, and that the entire approach of restoring P2P DMA via IOMMU identity domains may be dead in the water. This article examines that message in detail, unpacking the reasoning, assumptions, mistakes, and knowledge produced in this single exchange.

The Message

The subject message (msg id=6318) reads:

[assistant] Still failing. The FSP is truly stuck — the SBR while the GPU is removed from OS is NOT actually resetting the FSP either. This means the earlier "successful" resets were happening with fresh GSP state (from boot), not because SBR cleaned it.

Let me think about this: the GPUs were working fine earlier when we did "remove → SBR → rescan" and nvidia auto-probed. But now that same sequence doesn't work because the FSP is corrupted from all our failed attempts. The key difference is timing and accumulated state.

Let me try something: a much longer SBR hold time and multiple SBR cycles:

[bash] ssh root@10.1.2.6 'set -e rmmod nvidia_drm 2>/dev/null; rmmod nvidia_uvm 2>/dev/null; rmmod nvidia_modeset 2>/dev/null; rmmod nvidia 2>/dev/null

GPUS="0000:01:00.0 0000:11:00.0 0000:61:00.0 0000:71:00.0" BRIDGES="0000:00:01.1 0000:10:01.1 0000:60:01.1 0000:70:01.1"

echo "=== Create blacklist ===" cat > /etc/modprobe.d/temp-nvidia-blacklist.conf << "EOF" install nvidia /bin/false install nvidia_modeset /bin/false install nvidia_uvm /bin/false install nvidia_drm /bin/false EOF

echo "=== Remove GPUs ===" for gpu in $GPUS; do echo 1 > /sys/bus/pci/devices/$gpu/remove 2>/dev/null || true done sleep 2

echo "=== Multiple SBR cycles with longer hold ===" for i in 1 2 3; do echo " SBR cycle $i..." for bridge in $BRIDGES; do bridge_ctl=$(setpci -s $bridge BRIDGE_CONTROL) new_val=$(printf "%04x" $((0x$bridge_ctl | 0x40))) setpci -s $bridge BRIDGE_CONTROL=$new_val sleep 2 # Hold SBR for 2 seconds setpci -s $bridge BRIDGE_CONTROL=$bridge_ctl done sleep 5 done echo " Final wait 15s..." sleep 15

echo "=== Rescan ===" echo "10de 2bb5" > /sys/bus/pci/drivers/vfio-pci/remove_id 2>/dev/null || true echo 1 > /sys/bus/pci/rescan sleep 3

echo "=== Check state ===" for gpu in $GPUS; do if [ -e /sys/bus/pci/devices/$gpu ]; then driver=$(basename $(readlink /sys/bus/pci/devices/$gpu/driver) 2>/dev/null || echo "none") echo " $gpu: driver=$driver" else echo " $gpu: NOT FOUND" fi done

echo "=== Remove blacklist and load nvidia ===" rm -f /etc/modprobe.d/temp-nvidia-blacklist.conf

# Set identity domains first for gpu in $GPUS; do group=$(basename $(readlink /sys/bus/pci/devices/$gpu/iommu_group)) echo "identity" > /sys/kernel/iommu_groups/$group/type 2>/dev/null || true done

modprobe nvidia sleep 8 modprobe nvidia_modeset; modprobe nvidia_uvm; modprobe nvidia_drm sleep 3

echo "" nvidia-smi --query-gpu=index,gpu_bus_id,name --format=csv,noheader 2>&1' === Create blacklist === === Remove GPUs === === Multiple SBR cycles with longer hold === SBR cycle 1... SBR cycle 2... SBR cycle 3... Final wait 15s... === Rescan === === Check state === 0000:01:00.0: driver=none 0000:11:00.0: driver=none 0000:61:00.0: driver=none 0000:71:00.0: driver=none === Remove blacklist and load nvidia ===

No devices were found

The message ends with the stark output: "No devices were found." After all the careful orchestration — unloading the nvidia driver, creating a blacklist to prevent auto-binding, removing the GPUs from the PCI bus, performing three cycles of Secondary Bus Reset (SBR) with extended hold times, waiting 15 seconds for the GSP to clear, rescaming the PCI bus, confirming the GPUs reappeared without any driver bound, removing the blacklist, setting identity domains, and loading the nvidia driver — the result is the same failure. The GPUs are invisible to nvidia-smi.

Why This Message Was Written

This message was written because the assistant had exhausted its current hypothesis and needed to test a new one. The reasoning chain leading to this point is critical to understand.

In the preceding messages (msg id=6305 through msg id=6317), the assistant had been systematically attempting to set IOMMU groups to "identity" domain for the four NUMA0 GPUs (BDFs 0000:01:00.0, 0000:11:00.0, 0000:61:00.0, and 0000:71:00.0). The goal was to bypass IOMMU translation for GPU-to-GPU DMA, enabling direct P2P communication that had been broken by the SEV-SNP IOMMU configuration on the Proxmox host. The assistant had tried:

udev rules to set identity domains when devices appear (msg id=6306–6308) — but these had a race condition with driver binding.
Modprobe blacklist + PCI remove + SBR + rescan (msg id=6308–6309) — which initially appeared to work, with GPUs reappearing and identity domains set, but nvidia-smi showed "No devices were found."
CXL bus reset (msg id=6312–6313) — which succeeded in resetting the devices but still resulted in nvidia initialization failure.
Repeated attempts with blacklist (msg id=6315–6317) — each time the GPUs would reappear without a driver, identity domains would be set, but loading nvidia would produce the same RmInitAdapter failed! (0x62:0xffff:2142) error. The critical insight that the assistant articulates in message 6318 is the recognition that the FSP (Firmware Security Processor) state is persistent across PCI resets. The assistant writes: "This means the earlier 'successful' resets were happening with fresh GSP state (from boot), not because SBR cleaned it." This is a profound realization — it means that the SBR operation, which normally resets a PCI device to its initial state, does NOT reset the Blackwell GPU's embedded security processor. The FSP retains its corrupted state across SBR, CXL bus reset, and even FLR (Function Level Reset). The motivation for writing this specific message was to test whether a more aggressive reset strategy — longer SBR hold times and multiple cycles — could overcome this persistence. The assistant hypothesized that perhaps the earlier SBR attempts were too brief (0.5 seconds) and that holding SBR asserted for 2 seconds per cycle, repeated three times, with a 15-second cooldown, might give the FSP enough time to fully reset. This hypothesis was based on the observation that some PCIe devices require extended reset hold times to properly initialize their internal state machines.

How Decisions Were Made

The decision-making process in this message is visible in the structure of the bash command itself. The assistant made several deliberate choices:

Choice 1: Extend SBR hold time from 0.5s to 2s. Earlier attempts (msg id=6308, 6315) used a 0.5-second hold for the SBR signal. The assistant increased this to 2 seconds, reasoning that a longer reset pulse might be needed for the Blackwell GPU's complex firmware stack.

Choice 2: Repeat SBR three times. Rather than a single SBR cycle, the assistant looped three times. This reflects a "deeper reset" strategy — the idea that multiple reset cycles might progressively clear state that a single cycle cannot.

Choice 3: Add a 15-second final wait. After the three SBR cycles, the assistant waited 15 seconds before rescanning. This is significantly longer than the 5-second waits used previously. The intent was to give the GPU's internal firmware ample time to complete any power-on self-test or initialization sequences before the PCI bus rescan.

Choice 4: Set identity domains BEFORE loading nvidia. In previous attempts, the assistant sometimes set identity domains after loading nvidia. Here, the order is explicit: remove blacklist, set identity domains, then load nvidia. This ensures the IOMMU group type is already "identity" when the nvidia driver probes the device.

Choice 5: Remove vfio-pci device ID. The command echo "10de 2bb5" > /sys/bus/pci/drivers/vfio-pci/remove_id prevents the vfio-pci driver from grabbing the GPUs during rescan. This is important because vfio-pci was bound to these GPUs in a previous configuration (for the SEV-SNP VM), and its driver ID would cause it to auto-bind on rescan.

These decisions were made in a specific order and with specific parameters, reflecting the assistant's evolving understanding of the problem. The assistant is not guessing randomly — each parameter change is a deliberate test of a hypothesis about what might be causing the FSP to remain stuck.

Assumptions Made

Several assumptions underpin the reasoning in this message, some explicit and some implicit:

Assumption 1: The FSP state is the problem. The assistant assumes that the RmInitAdapter failed! (0x62:0xffff:2142) error is caused by the FSP being in a corrupted or locked state from previous failed initialization attempts. This is a reasonable inference from the error code and the pattern of failures, but it is an assumption — the actual root cause could be something else entirely, such as a PCIe link training issue, a power state problem, or a BIOS configuration conflict.

Assumption 2: SBR should reset the FSP if held long enough. The assistant assumes that the FSP reset is a matter of timing — that holding SBR asserted for longer will eventually cause the FSP to reset. This assumption is challenged by the result, which shows that even 2-second holds over three cycles do not help.

Assumption 3: The earlier "successful" resets were truly successful. The assistant references "the earlier 'successful' resets were happening with fresh GSP state (from boot)." This implies that at some point, the GPUs did work after a remove/SBR/rescan cycle. The assumption is that the GPUs were working fine initially, and only became corrupted after repeated failed attempts. This may be true, but it's worth noting that the assistant never confirmed P2P DMA was actually working after those resets — only that nvidia-smi showed the GPUs.

Assumption 4: The IOMMU identity domain setting persists across PCI remove/rescan. The assistant observed that after removing GPUs and rescanning, the IOMMU groups retained their "identity" type. This is actually correct — IOMMU group types are kernel-level settings that persist independently of whether devices are present in the group.

Assumption 5: The blacklist mechanism is reliable. The assistant uses install nvidia /bin/false in modprobe.d to prevent nvidia from auto-loading. This assumes that no other mechanism (such as kernel module auto-loading from device IDs, initramfs, or systemd) will load nvidia despite the blacklist.

Assumption 6: The GPUs are physically identical and should behave identically. The assistant applies the same sequence to all four GPUs simultaneously. This assumes that all four Blackwell GPUs have the same FSP state and will respond identically to the reset sequence. In practice, the FSP state could differ per GPU depending on the order and number of failed initialization attempts each has experienced.

Mistakes and Incorrect Assumptions

The most significant mistake visible in this message is the assumption that SBR can reset the Blackwell FSP at all. The result — "No devices were found" — demonstrates that even aggressive SBR cycling does not clear the FSP state. This is consistent with NVIDIA's security design for Blackwell: the FSP is intentionally hardened against software-initiated resets to prevent tampering. The FSP manages secure boot, firmware validation, and cryptographic key material, and it is designed to persist across conventional resets. Only a full power cycle (removing main power from the GPU) can reset the FSP.

A secondary mistake is the failure to recognize the pattern earlier. Looking at the conversation history, the assistant had already observed that the first remove/SBR/rescan cycle (msg id=6308) produced working GPUs, while subsequent cycles (msg id=6315–6317) did not. The assistant attributes this to "accumulated state" from failed attempts, but the more likely explanation is that the GPUs had never been through a failed FSP initialization at the time of the first cycle — they had been working continuously since boot. Once the nvidia driver attempted to initialize the FSP with identity IOMMU and failed, the FSP entered an error state that no subsequent software reset could clear.

Another mistake is the belief that the blacklist approach would work. The assistant creates a modprobe blacklist to prevent nvidia from auto-binding, then removes the blacklist and manually loads nvidia. But the modprobe install directive with /bin/false only prevents loading via modprobe — it does not prevent the kernel from binding the nvidia driver to PCI devices if the driver is already loaded or if it loads through other paths (such as the drivers_probe mechanism). The assistant's sequence appears to work (GPUs show "driver=none" after rescan), but this may be because nvidia was fully unloaded and the blacklist prevented modprobe from loading it — not because the blacklist prevented PCI binding per se.

A more subtle issue is the assumption that setting identity domains before loading nvidia would avoid the FSP boot failure. The earlier discovery (from segment 41's chunk 0 summary) was that "the nvidia Blackwell FSP boot sequence fails with error code 0x177 when IOMMU is in identity mode." This suggests that the FSP itself checks the IOMMU configuration during its initialization and fails if identity mode is detected. The assistant's approach of setting identity domains before loading nvidia is therefore guaranteed to fail — the FSP will see identity mode and refuse to boot. The assistant seems to be testing whether a "fresh" FSP (post-SBR) would behave differently, but the FSP state is not actually being reset by SBR, so the same failure occurs.

Input Knowledge Required

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

PCIe architecture: Understanding of Secondary Bus Reset (SBR), how it propagates through PCIe bridges, and what it resets (and does not reset) in endpoint devices. The BRIDGE_CONTROL register and the 0x40 bit for SBR are PCIe standard concepts.

Linux IOMMU subsystem: Knowledge of IOMMU groups, the distinction between "DMA" and "identity" domain types, and how /sys/kernel/iommu_groups/ exposes these settings. The reader must understand that identity mode bypasses IOMMU translation, enabling direct device-to-device DMA.

NVIDIA GPU architecture: Understanding of the Firmware Security Processor (FSP) in Blackwell GPUs, its role in secure boot and firmware initialization, and its resistance to software resets. The error code 0x62:0xffff:2142 and the RmInitAdapter failure are NVIDIA-specific.

Linux kernel module loading: Knowledge of modprobe, modprobe.d configuration, the install directive, and how kernel modules bind to PCI devices via driver probes.

The broader deployment context: The reader needs to know that this is a Proxmox host with 8 Blackwell GPUs split between an LXC container and a SEV-SNP VM, that P2P DMA was broken by the IOMMU configuration, and that the goal is to restore P2P for improved multi-GPU inference performance.

Previous session history: The reader should understand that the assistant had already discovered that IOMMU identity domains cause Blackwell FSP boot failure (error 0x177), that CXL bus reset was tried and failed, and that the only working configuration uses DMA-FQ IOMMU type with NCCL_P2P_DISABLE=1.

Output Knowledge Created

This message produces several important pieces of knowledge:

Knowledge 1: SBR does not reset the Blackwell FSP. This is the primary empirical finding. Even with extended hold times (2 seconds) and multiple cycles (3 iterations), the FSP retains its corrupted state. This is a significant hardware constraint that future attempts must account for.

Knowledge 2: The "fresh GSP state" hypothesis is confirmed. The assistant correctly deduces that earlier successful resets were only successful because the GPUs had never been through a failed FSP initialization. Once the FSP enters an error state, it cannot be cleared by any software-initiated PCI reset.

Knowledge 3: The complete reset sequence is documented. The bash command in this message serves as a comprehensive procedure for: unloading nvidia, blacklisting it, removing GPUs from PCI, performing SBR with specific timing, preventing vfio-pci from grabbing devices, rescanning, setting identity domains, and reloading nvidia. Even though the outcome is failure, the procedure itself is a valuable reference.

Knowledge 4: The failure mode is deterministic. The output "No devices were found" after loading nvidia is a consistent, reproducible result. This is not a transient or intermittent failure — it is a deterministic consequence of attempting to initialize Blackwell GPUs with IOMMU identity domains after a failed FSP boot.

Knowledge 5: The boundary of software-controllable reset is established. The message demonstrates that the boundary between what can and cannot be reset via software on Blackwell GPUs lies somewhere between the PCIe core logic (which SBR does reset) and the FSP (which SBR does not reset). This is valuable information for system administrators and kernel developers working with Blackwell hardware.

The Thinking Process Visible in Reasoning

The assistant's reasoning in this message is a textbook example of hypothesis-driven debugging. Let me trace the thought process:

Step 1: Observation. "Still failing. The FSP is truly stuck." The assistant acknowledges that the previous attempts have not worked.

Step 2: Reframing the problem. "The SBR while the GPU is removed from OS is NOT actually resetting the FSP either." This is a critical insight — the assistant realizes that the SBR operation, which it had been relying on to clear the FSP state, is ineffective.

Step 3: Historical reinterpretation. "This means the earlier 'successful' resets were happening with fresh GSP state (from boot), not because SBR cleaned it." The assistant reinterprets past successes in light of new information. This is a mature debugging behavior — updating one's mental model when evidence contradicts assumptions.

Step 4: Hypothesis formation. "The key difference is timing and accumulated state." The assistant forms a new hypothesis: perhaps the FSP needs a longer reset pulse or multiple cycles to clear.

Step 5: Experimental design. The assistant designs an experiment with specific parameters: 2-second SBR hold, 3 cycles, 15-second cooldown, identity domains set before nvidia load.

Step 6: Execution and observation. The assistant runs the experiment and observes the result: "No devices were found."

Step 7: Implicit conclusion. The assistant does not explicitly state the conclusion in this message (the next message or chunk summary would do that), but the implication is clear: the approach of using SBR to reset the FSP is fundamentally flawed.

What is particularly notable about the reasoning is the shift from "how do we make SBR work?" to "SBR fundamentally cannot reset the FSP." This is a classic debugging progression — from trying to fix a broken mechanism to recognizing that the mechanism itself is incapable of achieving the desired result.

The assistant also demonstrates good experimental hygiene: it controls for variables (blacklist, vfio-pci removal, identity domain timing), documents the procedure clearly, and captures the output verbatim. This makes the experiment reproducible and the results interpretable.

Implications and Aftermath

The failure documented in this message has significant implications for the broader deployment. The assistant had been pursuing IOMMU identity domains as the primary path to restoring P2P DMA between the Blackwell GPUs. This message definitively closes that path — identity domains cause FSP boot failure, and no software reset can recover from that failure.

The remaining options, as outlined in the segment summary, are:

Accept the DMA-FQ IOMMU configuration with NCCL_P2P_DISABLE=1, which works but disables direct GPU-to-GPU communication.
Investigate NVIDIA's DmaRemapPeerMmio=1 driver parameter, which is intended to enable P2P under IOMMU but produces incomplete mappings.
Use MTP (Multi-Token Prediction) speculation, which provides 12-45% per-request throughput improvement and does not require P2P. The assistant's pivot to MTP speculation, confirmed in the segment summary as surviving the reboot and delivering significant throughput gains, represents a pragmatic shift from "fix the hardware limitation" to "optimize within the hardware's constraints."

Conclusion

Message 6318 is a turning point in the opencode session. It captures the moment when the assistant realizes that a fundamental hardware limitation — the Blackwell FSP's resistance to software reset — cannot be overcome through clever PCIe manipulation. The message is a testament to systematic debugging: forming hypotheses, designing experiments, executing them cleanly, and interpreting results honestly even when they contradict expectations.

The knowledge produced in this message — that SBR does not reset the Blackwell FSP, that identity IOMMU domains are incompatible with Blackwell GPU initialization, and that the failure is deterministic and unrecoverable via software — is valuable for anyone deploying Blackwell GPUs in virtualized environments with IOMMU. It establishes a clear boundary for what is and is not possible with current NVIDIA hardware and drivers, and it points the way toward alternative optimization strategies that work within those boundaries.

For the reader studying this conversation, message 6318 serves as a case study in how to recognize when a debugging approach has hit a fundamental wall, how to document that wall clearly, and how to pivot to productive alternatives. The assistant does not waste time lamenting the failure — it documents it, learns from it, and moves on to the next approach. That is the hallmark of effective technical problem-solving.