The Blackwell FSP Defeats Another Reset Attempt: A Deep Dive into GPU P2P DMA Troubleshooting

Introduction

In the high-stakes world of enterprise GPU deployment, few challenges are as frustrating as a hardware-level firmware lock that survives every software reset you throw at it. Message <msg id=6381> captures a pivotal moment in an ongoing battle to restore P2P DMA (Peer-to-Peer Direct Memory Access) between NVIDIA Blackwell RTX PRO 6000 GPUs on an AMD EPYC system running Proxmox. The assistant, having already spent dozens of rounds diagnosing and attempting to work around a fundamental incompatibility between Blackwell's Firmware Security Processor (FSP) and IOMMU identity domains, executes a desperate 10-step procedure involving Secondary Bus Reset (SBR), PCI device removal and rescan, and a careful choreography of driver blocking and reloading. This message is a masterclass in low-level Linux PCI debugging, but also a tragic lesson in the limits of software-level GPU reset.

Context: The Road to This Moment

To understand why message <msg id=6381> matters, we need to understand what led to it. The system in question is a Proxmox host with 8 NVIDIA Blackwell RTX PRO 6000 GPUs, split across two NUMA domains. Four GPUs (NUMA0) are assigned to an LXC container running SGLang for model inference, while the other four (NUMA1) are bound to vfio-pci for a SEV-SNP VM. The challenge has been enabling P2P DMA between the NUMA0 GPUs — a critical optimization for multi-GPU tensor parallelism in large language model serving.

The root cause of the P2P problem traces back to the AMD IOMMU configuration. The system requires amd_iommu=on for SEV-SNP (Secure Encrypted Virtualization) support in the VM, which means IOMMU is in full translation mode. In this mode, GPU P2P DMA breaks because the IOMMU translates peer addresses incorrectly. The standard fix is to set specific IOMMU groups to "identity" mode, which bypasses translation for those devices. This approach has been used successfully on other GPU architectures.

However, Blackwell GPUs introduced a new complication: the FSP (Firmware Security Processor), a hardware security module that initializes during the nvidia driver's first probe. The FSP establishes specific DMA mappings through the kernel's DMA API, and these mappings apparently require IOMMU translation mode to be active. When IOMMU identity mode is set after the FSP has initialized, the FSP enters a corrupted state — and crucially, this state survives all software-level resets: FLR (Function Level Reset), SBR (Secondary Bus Reset), CXL bus reset, and even driver module unload.

The assistant had previously created a boot-time script (gpu-iommu-identity.sh) to set identity domains before the nvidia driver loaded. After a reboot to test this approach, the assistant discovered that the boot ordering was wrong — the gpu-vfio-split.service ran after the identity service and grabbed all GPUs for vfio-pci. After fixing the order and manually running the identity script, the GPUs appeared to be on the nvidia driver with identity domains, but nvidia-smi reported "No devices were found." The GSP was dead.

Message 6381: The Forced Reset Attempt

Message <msg id=6381> opens with the assistant recognizing a critical bug in its own logic:

Need to fix the idempotency check — it should also verify nvidia-smi works. Let me update the check and force the reset.

This is a moment of self-correction. The previous script (in <msg id=6378>) had an idempotency check that simply verified driver=nvidia and type=identity in sysfs. But as the assistant discovered in <msg id=6380>, these sysfs entries can show the correct values even when the GPU's firmware is completely non-functional. The check was checking the kernel's view of the device, not the hardware's actual state. The assistant now understands that a working nvidia driver requires both correct sysfs state and a responsive GPU firmware.

The assistant's decision to "force the reset" is significant. Rather than trying to fix the idempotency check incrementally, it decides to run the full SBR procedure regardless of the current state. This is a pragmatic choice: the GPUs are already in a broken state (GSP corrupted), so there's nothing to lose by attempting a more aggressive reset.

The 10-Step Reset Procedure

The bash script that follows is a remarkable piece of low-level Linux PCI surgery. Let's examine each step and the reasoning behind it.

Step 1: Unload nvidia

for gpu in $GPUS; do
    echo "$gpu" > /sys/bus/pci/drivers/nvidia/unbind 2>/dev/null || true
done
sleep 1
rmmod nvidia_drm 2>/dev/null; rmmod nvidia_uvm 2>/dev/null; rmmod nvidia_modeset 2>/dev/null; rmmod nvidia 2>/dev/null

The assistant first unbinds each GPU from the nvidia driver, then removes all nvidia kernel modules in reverse dependency order. The || true on the unbind command is important — it prevents the script from aborting if a GPU is already unbound. The sleep 1 gives the driver time to complete its teardown. The module removal order matters: nvidia_drm depends on nvidia_modeset, which depends on nvidia_uvm, which depends on nvidia. Removing them in reverse dependency order prevents errors.

Step 2: Remove GPUs from PCI Subsystem

for gpu in $GPUS; do
    echo 1 > /sys/bus/pci/devices/$gpu/remove 2>/dev/null && echo "  $gpu removed" || echo "  $gpu: already gone"
done

Writing 1 to the remove sysfs file triggers a PCI hot-remove operation. This tells the kernel to forget about the device entirely — it won't appear in lspci or any PCI enumeration until the bus is rescanned. The assistant's reasoning here is that a complete PCI removal might force the GPU into a more thorough reset state than a simple driver unbind.

Step 3: Secondary Bus Reset (SBR)

for bridge in $BRIDGES; do
    bridge_ctl=$(setpci -s $bridge BRIDGE_CONTROL 2>/dev/null)
    new_val=$(printf "%04x" $((0x$bridge_ctl | 0x40)))
    setpci -s $bridge BRIDGE_CONTROL=$new_val
    sleep 0.5
    setpci -s $bridge BRIDGE_CONTROL=$bridge_ctl
    echo "  $bridge done"
done

This is the most technically sophisticated part of the procedure. SBR is a PCIe mechanism where a bridge asserts a reset signal on its secondary bus, forcing all downstream devices to reset. The assistant uses setpci (from the pciutils package) to read the current BRIDGE_CONTROL register, set bit 6 (the Secondary Bus Reset bit), wait 500ms, then clear it.

The bridges targeted (0000:00:01.1, 0000:10:01.1, 0000:60:01.1, 0000:70:01.1) are the PCIe root ports or switch upstream ports that connect to each GPU. Each NUMA0 GPU sits behind one of these bridges.

The 10-second wait after SBR is critical. The assistant's comment says "Waiting for GSP reset..." — the hope is that SBR, being a hardware-level reset that reinitializes the PCIe link and device configuration space, might be sufficient to clear the FSP's locked state. This is the assistant's best theory for why previous resets failed: driver unbind and module unload don't touch the PCIe configuration space, but SBR does.

Step 4: Block nvidia and vfio-pci

echo "install nvidia /bin/false" > /etc/modprobe.d/tmp-nvidia-block.conf
echo "10de 2bb5" > /sys/bus/pci/drivers/vfio-pci/remove_id 2>/dev/null || true

This is a clever piece of orchestration. Before rescanning the PCI bus, the assistant needs to prevent two things:

  1. The nvidia module from automatically binding to the GPUs (because the identity domains aren't set yet)
  2. The vfio-pci driver from grabbing the GPUs (because it has a new_id registration from the vfio-split service) The install nvidia /bin/false modprobe directive is a trick: it tells modprobe that the "install" command for the nvidia module is /bin/false, which always fails. This effectively prevents any automatic loading of the nvidia module. The remove_id on vfio-pci removes the PCI ID match so that vfio-pci won't automatically bind during rescan.

Step 5: Rescan

echo 1 > /sys/bus/pci/rescan
sleep 5

Writing 1 to /sys/bus/pci/rescan triggers a full PCI bus rescan. The kernel re-enumerates all PCI buses, discovers devices that were previously removed, and attempts to bind drivers. With both nvidia and vfio-pci blocked, the GPUs should appear with no driver bound, giving the assistant a clean slate.

Step 6: Check GPU 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")
        group=$(basename $(readlink /sys/bus/pci/devices/$gpu/iommu_group))
        type=$(cat /sys/kernel/iommu_groups/$group/type)
        echo "  $gpu: driver=$driver, group=$group, type=$type"
    else
        echo "  $gpu: NOT FOUND"
    fi
done

This diagnostic step reveals something important: the GPUs are found, but their IOMMU groups have changed. Previously, the groups were 42, 61, 28, 10. After the PCI removal and rescan, they're 28, 61, 42 — and the types are DMA-FQ (translation mode), not identity. The IOMMU group assignment has been reshuffled because the PCI topology was re-enumerated.

This is a subtle but critical point: IOMMU groups are assigned based on PCI topology and the IOMMU hardware's DMA remapping units. When devices are removed and rescanned, they may end up in different groups if the IOMMU hardware groups them differently. The assistant's script assumed the original group numbers would persist, but they didn't.

Step 7: Set Identity Domains

for gpu in $GPUS; do
    if [ -e /sys/bus/pci/devices/$gpu ]; then
        group=$(basename $(readlink /sys/bus/pci/devices/$gpu/iommu_group))
        type=$(cat /sys/kernel/iommu_groups/$group/type)
        if [ "$type" != "identity" ]; then
            echo "identity" > /sys/kernel/iommu_groups/$group/type 2>/dev/null
            echo "  $gpu group $group -> $(cat /sys/kernel/iommu_groups/$group/type)"
        else
            echo "  $gpu group $group already identity"
        fi
    fi
done

Now the assistant sets identity domains on the new groups. This is the first time in this boot cycle that identity mode is being set before the nvidia driver touches the GPUs — which was the original hypothesis for what might work.

Step 8: Remove Block, Load nvidia

rm -f /etc/modprobe.d/tmp-nvidia-block.conf
modprobe nvidia && echo "  nvidia loaded"
sleep 5
modprobe nvidia_modeset; modprobe nvidia_uvm; modprobe nvidia_drm

With identity domains set, the assistant removes the nvidia block and loads the driver. The 5-second sleeps give the driver time to probe all four GPUs and initialize their firmware. This is the moment of truth: will the FSP initialize correctly with identity domains already in place?

Step 9: Re-register vfio-pci

echo "10de 2bb5" > /sys/bus/pci/drivers/vfio-pci/new_id 2>/dev/null || true

This restores the vfio-pci binding for the NUMA1 GPUs (which share the same PCI ID 10de:2bb5). The || true handles the case where the NUMA1 GPUs are already bound.

Step 10: Final Check

The final check runs nvidia-smi and prints the sysfs state for all GPUs. This is where the assistant will learn whether the entire procedure succeeded or failed.

The Output: A Partial Success That Reveals a Deeper Problem

The output shown in the message is truncated, but we can see the critical findings:

  1. nvidia modules unloaded cleanly — no "WARN: still loaded" message.
  2. All 4 GPUs removed from PCI subsystem.
  3. SBR completed on all 4 bridges.
  4. GPUs reappeared after rescan with driver=none (no driver bound).
  5. IOMMU groups changed — the groups are now 28, 61, 42 (only 3 groups visible in the truncated output, but the 4th GPU presumably has a group too).
  6. Type is DMA-FQ — identity mode was not yet set at this point. The message cuts off before showing the results of steps 7-10. To understand what happened next, we need to look at the subsequent messages. In <msg id=6382>, the assistant checks dmesg and finds:
NVRM: GPU0 _kgspBootGspRm: (the GPU is likely in a bad state and may need to be reset)
NVRM: GPU0 RmInitAdapter: Cannot initialize GSP firmware RM
NVRM: GPU 0000:61:00.0: RmInitAdapter failed! (0x62:0x40:2142)

The FSP initialization still failed. Even with identity domains set before the nvidia driver loaded, the Blackwell FSP could not initialize. This is the devastating conclusion: the FSP apparently requires DMA translation mode to be active during its boot sequence, and identity mode — even if set before driver load — breaks this initialization.

What This Message Reveals About the Assistant's Thinking

Message <msg id=6381> is particularly valuable for understanding the assistant's reasoning process because it shows:

1. Recognition of a Bug in Previous Logic

The assistant explicitly identifies that its idempotency check was insufficient. The check verified sysfs state (driver name, IOMMU type) but not actual hardware functionality (nvidia-smi output). This is a classic debugging blind spot: assuming that kernel data structures accurately reflect hardware state.

2. Escalating Aggression in Reset Strategy

The assistant progressively escalates its reset approach across the conversation:

3. Careful Orchestration of Dependencies

The script shows sophisticated understanding of Linux PCI driver interactions:

4. The Assumption That Failed

The critical assumption underlying this entire approach is that SBR would clear the FSP state. The assistant believed that because SBR reinitializes PCIe configuration space and triggers a device-level reset, it would be sufficient to clear whatever lock the FSP had entered. This assumption turned out to be wrong.

The Blackwell FSP appears to maintain its state in a way that survives SBR. This suggests the FSP state is stored in non-volatile memory on the GPU, or in a region of the GPU's internal SRAM that isn't cleared by PCIe resets. The error code 0x62:0x40:2142 from the subsequent dmesg output indicates a firmware initialization failure that persists regardless of reset method.

Input Knowledge Required

To fully understand this message, a reader needs:

  1. Linux PCI subsystem internals: Understanding of /sys/bus/pci/devices/, driver binding/unbinding, PCI hot-remove, and bus rescan.
  2. IOMMU concepts: What DMA translation mode vs. identity mode means, how IOMMU groups work, and why identity mode is needed for P2P DMA.
  3. PCIe reset mechanisms: The difference between FLR (function-level reset), SBR (secondary bus reset), and system reset, and what each one affects.
  4. NVIDIA driver architecture: The GSP (GPU System Processor) and FSP (Firmware Security Processor), their role in GPU initialization, and why they might survive resets.
  5. Blackwell architecture specifics: The fact that Blackwell introduced a new firmware security model that is more stateful than previous generations.
  6. Shell scripting and PCI utilities: setpci, modprobe, rmmod, and the sysfs interface for PCI device management.

Output Knowledge Created

This message produces several important pieces of knowledge:

  1. A documented procedure for SBR-based GPU reset that can be reused or adapted for other GPU troubleshooting scenarios.
  2. Evidence that SBR does not clear Blackwell FSP state, narrowing the possible causes and solutions.
  3. A bug report for the idempotency check pattern: sysfs state alone is insufficient to verify GPU health; nvidia-smi or similar GPU-level diagnostics must also be checked.
  4. IOMMU group reassignment after PCI rescan: a subtle behavior where IOMMU groups can change when devices are removed and re-enumerated.
  5. The definitive finding that IOMMU identity mode is fundamentally incompatible with Blackwell GPUs, regardless of timing relative to driver load.

Mistakes and Incorrect Assumptions

Several assumptions in this message turned out to be incorrect:

  1. SBR would clear FSP state: The most critical assumption. The FSP state survived SBR, meaning it's stored in a domain that PCIe resets don't touch.
  2. Setting identity before driver load would work: Even with identity domains set before the nvidia driver ever touched the GPUs, the FSP still failed to initialize. This suggests the FSP actively requires DMA translation mode during its boot sequence.
  3. IOMMU groups would remain stable after PCI rescan: The groups changed from (42, 61, 28, 10) to (28, 61, 42, ...), requiring the script to dynamically discover the new groups rather than hardcoding them.
  4. The idempotency check was sufficient: The assistant had to learn the hard way that sysfs state doesn't guarantee GPU health.

Conclusion

Message <msg id=6381> represents the high-water mark of the assistant's effort to restore P2P DMA via IOMMU identity domains. The 10-step SBR procedure is technically impressive — it demonstrates deep knowledge of Linux PCI internals, careful orchestration of driver interactions, and systematic escalation of reset strategies. But it also reveals the limits of software-level GPU reset on Blackwell architecture. The FSP's state persistence through SBR is a hardware design choice that no amount of kernel-level manipulation can overcome.

The message is a turning point in the conversation. After this attempt fails (as revealed in subsequent messages), the assistant pivots away from the IOMMU identity approach entirely, concluding that "per-group IOMMU identity domains are fundamentally incompatible with Blackwell GPUs." The remaining optimization path shifts to MTP speculation (which provides 12-45% throughput improvement) and the possibility of making nvidia's DmaRemapPeerMmio=1 parameter work correctly.

For anyone debugging GPU P2P issues on Blackwell systems with IOMMU, this message serves as both a reference for the most aggressive software reset possible and a cautionary tale about the limits of that approach. Sometimes the hardware simply doesn't cooperate, and the best engineering response is to document the dead end and move on to alternative optimizations.