The Blackwell GSP/FSP Impasse: When GPU Firmware Refuses to Let Go

Introduction

In the intricate dance between operating system kernels, device drivers, and GPU firmware, few problems are as stubborn as the one confronted in message 6378 of this opencode session. The message, written by an AI assistant in response to a user's simple request to "check P2P status" after a reboot, documents a pivotal moment in a long-running battle to enable peer-to-peer DMA (P2P) between NVIDIA Blackwell GPUs while maintaining SEV-SNP virtual machine support. What emerges is a deep exploration of a fundamental hardware limitation: the Blackwell GPU's Firmware Security Processor (FSP) cannot survive a driver rebind when the IOMMU is in identity mode, and no software-level reset can clear this state.

This article examines message 6378 in detail—its reasoning, its assumptions, its mistakes, and the knowledge it creates. The message represents a turning point where the assistant recognizes that a simpler approach cannot work and escalates to an aggressive PCI-level reset strategy, only to ultimately discover that even this is insufficient against Blackwell's firmware architecture.

Context: The P2P DMA Restoration Saga

To understand message 6378, one must understand the broader context. The system in question is a Proxmox host running Ubuntu 24.04 with 8 NVIDIA RTX PRO 6000 Blackwell GPUs. Four of these GPUs (on NUMA node 0) are assigned to a container running SGLang for LLM inference, while the other four (on NUMA node 1) are passed through via VFIO to a SEV-SNP confidential VM. The challenge is that SEV-SNP requires the IOMMU to remain in full translation mode (DMA-FQ), but GPU P2P DMA requires the IOMMU to be in identity mode for the GPU groups involved.

Earlier in the session ([msg 6368]), the assistant had discovered that per-IOMMU-group identity domain switching is supported on kernel 6.14 and AMD IOMMU, and had successfully set the four NUMA0 GPU groups to identity mode at runtime. However, a critical blocker emerged: the Blackwell GPU's FSP (Firmware Security Processor) retains locked state across all software-level resets—function-level reset (FLR), secondary bus reset (SBR), CXL bus reset, and module unload. The only viable path was to set identity domains at boot, before the nvidia driver first initializes the GPUs.

The assistant created a boot-time service (gpu-iommu-identity.service) that would run early in the boot sequence to set identity domains, followed by the nvidia driver loading onto those GPUs. The user rebooted to test this approach ([msg 6369]).

Message 6378: The Discovery of Failure

Message 6378 opens with a stark realization: "Same GSP FSP issue." The assistant has just checked the system state after the reboot (<msg id=6371-6377>) and found that while the identity domains were correctly set to identity for all four NUMA0 GPU groups, the GPUs themselves were bound to vfio-pci instead of nvidia. This happened because the boot ordering was wrong—gpu-vfio-split.service (which registers the GPU PCI ID with vfio-pci) ran after gpu-iommu-identity.service, causing all unbound GPUs matching the 10de:2bb5 PCI ID to be grabbed by vfio-pci.

The assistant attempted a manual fix by unbinding the GPUs from vfio-pci and rebinding them to nvidia ([msg 6375]), but this resulted in nvidia-smi reporting "No devices were found" ([msg 6377]). The dmesg log showed the familiar GSP/FSP errors:

NVRM: GPU0 kfspDumpDebugState_GB100: NV_PGSP_MAILBOX(1) = 0xbadf4100
NVRM: GPU0 gpuHandleSanityCheckRegReadError_GH100: Possible bad register read

This is the same error pattern that had been encountered earlier in the session. The nvidia driver's initial probe at boot had established a GSP/FSP session on each GPU. When the assistant unbound the GPUs from vfio-pci and attempted to rebind them to nvidia, the driver tried to establish a new GSP/FSP session, but the firmware was still holding locked state from the initial boot-time initialization. The FSP refused to reinitialize, leaving the GPUs in a wedged state.

The Reasoning: Why This Happens

The assistant's analysis in message 6378 is precise and well-reasoned:

"The fundamental issue: on Blackwell, the nvidia driver's initial probe at boot establishes the GSP/FSP session. Any subsequent unbind/rebind fails because the FSP retains locked state. The vfio-pci driver doesn't have this problem because it doesn't initialize the GPU's firmware."

This is the key insight. The VFIO driver is a "dumb" driver—it doesn't initialize the GPU's firmware, it just exposes the PCIe device to the VM. So unbinding from vfio-pci is clean. But the nvidia driver is "smart"—it initializes the GSP/FSP, and once that initialization happens, the firmware state is locked. Any subsequent attempt to reinitialize fails because the FSP detects that it's already been initialized and refuses to re-enter the boot sequence.

The assistant correctly identifies the root cause: the nvidia module auto-loads during boot (via systemd-modules-load.service or similar), probes the GPUs, and establishes GSP/FSP sessions before the identity domain service has a chance to run. By the time the identity script executes, the GPUs are already initialized, and any unbind/rebind kills the GSP.

The Proposed Solution: A Radical Escalation

Having recognized that the simple approach of unbinding and rebinding cannot work, the assistant proposes a much more aggressive strategy. The new version of gpu-iommu-identity.sh attempts to:

  1. Unbind the GPUs from whatever driver they're on (nvidia or vfio-pci)
  2. Set identity domains on the IOMMU groups
  3. If the GPUs were on nvidia, perform a full PCI-level reset sequence: - Unload all nvidia kernel modules (nvidia_drm, nvidia_uvm, nvidia_modeset, nvidia) - Remove the GPU devices from the PCI bus via echo 1 &gt; /sys/bus/pci/devices/$gpu/remove - Perform a Secondary Bus Reset (SBR) on the parent PCI bridges by setting bit 6 (0x40) of the Bridge Control register via setpci - Wait 10 seconds for the GSP to reset - Temporarily block both vfio-pci and nvidia from grabbing the GPUs during rescan - Rescan the PCI bus - Set identity domains on the newly created IOMMU groups - Remove the nvidia block and load the nvidia modules in the correct order - Re-register the vfio-pci new_id for the NUMA1 GPUs This is a sophisticated approach that demonstrates deep knowledge of Linux PCI subsystem mechanics. The use of setpci to toggle the Secondary Bus Reset bit on the parent bridges is a particularly advanced technique—it's the hardware-level equivalent of power-cycling the PCIe slot.

Assumptions Made in This Message

The assistant makes several assumptions, some explicit and some implicit:

Assumption 1: SBR can clear the GSP/FSP state. The script performs a Secondary Bus Reset on the parent PCI bridges with the expectation that this will force the GPU firmware to fully reset. This assumption turns out to be incorrect—as the analyzer summary for this segment reveals, the Blackwell FSP survives SBR. The FSP's locked state is stored in non-volatile memory or in a region that is not cleared by a PCI bus reset.

Assumption 2: PCI remove + rescan creates a clean slate. The script removes the GPU devices from the PCI bus and then rescans, expecting that the GPUs will be re-enumerated as if from a cold boot. However, the FSP state persists across device removal and re-enumeration because it's stored in the GPU's internal firmware memory, not in the PCI configuration space.

Assumption 3: Module unloading order matters. The script carefully unloads nvidia modules in reverse dependency order (nvidia_drm → nvidia_uvm → nvidia_modeset → nvidia) and reloads them in forward order. This is correct for clean module management but irrelevant if the FSP state is persistent.

Assumption 4: The identity domains will survive the PCI rescan. The script sets identity domains after the rescan, anticipating that new IOMMU groups will be created. This is actually correct—IOMMU groups are re-created on rescan—but the FSP issue makes it moot.

Assumption 5: Blocking nvidia via modprobe install hook prevents auto-loading. The script creates a temporary /etc/modprobe.d/tmp-nvidia-block.conf with install nvidia /bin/false to prevent nvidia from auto-loading during the rescan. This is a clever trick but assumes the modprobe configuration is honored during the rescan process.

The Mistake: Underestimating Blackwell's Firmware Persistence

The fundamental mistake in this message—and it's a mistake that the assistant could not have avoided without deeper knowledge of Blackwell's firmware architecture—is the assumption that any software-level reset can clear the GSP/FSP state. The analyzer summary for segment 41 states definitively:

"Blackwell's FSP requires DMA translation mode during initialization, and no software-level reset (FLR, SBR, CXL bus reset) can clear this state."

This means that even the aggressive PCI remove + SBR + rescan approach cannot work. The FSP state is persistent across all software-level resets. The only way to clear it would be a full power cycle of the GPU (removing power from the PCIe slot), which is not possible through software on a standard server platform.

The assistant's reasoning is logical and follows from the available evidence, but it underestimates the depth of the firmware persistence. The error code 0x177 (mentioned in the analyzer summary) indicates that the FSP boot sequence fails specifically because the DMA mappings it expects are not present in identity mode. This is not a transient state that can be cleared by a reset—it's a fundamental incompatibility between the FSP's initialization requirements and the IOMMU identity mode.

Input Knowledge Required

To fully understand message 6378, one needs knowledge of:

Linux PCI subsystem: Understanding of /sys/bus/pci/devices/, driver unbinding (driver/unbind), device removal (remove), bus rescan (rescan), and driver probing (drivers_probe). The assistant uses all of these mechanisms fluently.

IOMMU and DMA remapping: Understanding of IOMMU groups, domain types (identity vs DMA-FQ), and how they affect DMA transactions. The assistant knows that P2P DMA requires identity domains and that SEV-SNP requires translation mode.

NVIDIA GPU architecture: Understanding of the GSP (GPU System Processor) and FSP (Firmware Security Processor) on Blackwell GPUs. The assistant knows that the nvidia driver initializes the GSP/FSP during probe and that this initialization is destructive (cannot be repeated).

PCIe bridge management: Understanding of Secondary Bus Reset (SBR), the Bridge Control register (bit 6 = Secondary Bus Reset), and how setpci can manipulate PCI configuration space. This is advanced knowledge that most Linux administrators would not have.

Kernel module management: Understanding of module dependencies, load/unload ordering, and modprobe configuration (install hooks, blacklisting).

Systemd boot ordering: Understanding of systemd service dependencies (After=, Before=, DefaultDependencies=no) and how to sequence early boot services.

Output Knowledge Created

Message 6378 creates several important pieces of knowledge:

1. A definitive diagnosis of the Blackwell GSP/FSP rebind problem. The message clearly articulates why unbinding and rebinding the nvidia driver on Blackwell GPUs fails: the FSP retains locked state across unbind, and any subsequent rebind attempt fails because the firmware refuses to reinitialize.

2. A comprehensive script for PCI-level GPU reset. While the script ultimately doesn't solve the problem, it represents a thorough attempt at the most aggressive software-level reset possible. The techniques used (PCI remove, SBR via setpci, temporary driver blocking, ordered module reload) are all valid and could be useful in other contexts.

3. The insight that vfio-pci is "safe" because it doesn't initialize firmware. This is an important architectural observation: VFIO passthrough works because the VFIO driver is minimal and doesn't touch the GPU's firmware. This means VFIO can be bound and unbound freely without corrupting GPU state.

4. The correct boot ordering requirement. The assistant identifies that the only viable approach is to set identity domains before nvidia ever touches the GPUs. This requires restructuring the boot sequence so that the identity service runs before nvidia auto-loads.

5. A modprobe-based nvidia blocking technique. The use of install nvidia /bin/false in a temporary modprobe configuration file is a clever way to prevent the nvidia module from loading during a critical window.

The Thinking Process

The message reveals a structured problem-solving approach:

Step 1: Pattern recognition. The assistant immediately recognizes the error as "Same GSP FSP issue" from earlier in the session. This pattern recognition allows it to skip re-diagnosis and focus on solutions.

Step 2: Root cause analysis. The assistant correctly identifies that the nvidia driver's initial boot-time probe is the culprit. The FSP session established at boot cannot be re-established after unbind.

Step 3: Solution space exploration. The assistant considers multiple approaches:

The Broader Significance

Message 6378 is significant beyond its immediate context because it illustrates a fundamental challenge in modern GPU computing: the increasing complexity of GPU firmware. Blackwell's FSP is not just a simple microcontroller—it's a sophisticated security processor that manages encryption, attestation, and secure boot. This complexity means it has persistent state that survives software resets, making it resistant to the kind of driver hot-plugging that was possible with older GPUs.

This has practical implications for GPU virtualization, multi-instance GPU (MIG) partitioning, and any scenario where GPUs need to be dynamically reassigned between host and VM use. The Blackwell architecture essentially requires a cold boot (full power cycle) to change the GPU's driver binding, which is a significant operational constraint.

The message also demonstrates the value of systematic debugging. The assistant doesn't give up when the simple approach fails—it escalates to more aggressive techniques, documents its reasoning, and creates reusable infrastructure (the script) even if the specific approach doesn't ultimately work.

Conclusion

Message 6378 is a masterclass in diagnosing and attempting to work around a fundamental hardware limitation. The assistant correctly identifies the Blackwell GSP/FSP persistence problem, articulates why it occurs, and proposes an aggressive PCI-level reset strategy. While the approach ultimately cannot succeed—Blackwell's FSP is simply too persistent—the reasoning is sound, the implementation is thorough, and the knowledge created (about GSP/FSP behavior, PCI reset techniques, and boot ordering requirements) is valuable.

The message represents the moment when the assistant realizes that P2P DMA restoration via IOMMU identity domains is "definitively blocked" (as the analyzer summary puts it) and begins to pivot toward alternative optimizations. It's a turning point in the session, where a promising approach meets its match in Blackwell's firmware architecture, and the assistant must accept a fundamental constraint of the hardware.