The Reset: Restoring Blackwell GPUs After an IOMMU Identity Domain Experiment Goes Wrong

Introduction

In the course of a deep systems-engineering session targeting NVIDIA Blackwell GPUs, message 6324 represents a critical inflection point — a moment of strategic retreat after a series of escalating failures. The assistant, having spent many messages attempting to enable IOMMU identity domains on four NVIDIA RTX PRO 6000 Blackwell GPUs to restore P2P DMA, finds itself in a state where the nvidia driver is not loaded, the GPUs are invisible to nvidia-smi, and the entire carefully constructed inference serving stack is down. This message is the assistant's decision to abandon the current line of experimentation and restore the known-working baseline before attempting anything further.

The Context: A Multi-Message Descent Into Failure

To understand message 6324, one must appreciate the journey that led to it. The assistant had been trying to solve a fundamental problem: on a Proxmox host with SEV-SNP IOMMU enabled, the 8 Blackwell GPUs were split between an LXC container and a VM. The 4 GPUs assigned to the LXC were running under the nvidia driver with DMA-FQ (DMA with Flush Queue) IOMMU type, but P2P (peer-to-peer) DMA between GPUs was broken — NCCL_P2P_DISABLE=1 was required to prevent hangs and corruption.

The assistant hypothesized that switching the IOMMU groups from DMA-FQ to identity mode would allow P2P DMA to work, since identity mode bypasses IOMMU translation entirely for those devices. This is a well-known technique in high-performance computing: if devices can DMA directly to each other's physical addresses without IOMMU remapping, P2P bandwidth increases and latency decreases.

However, a devastating discovery was made: the Blackwell GPU's Firmware Security Processor (FSP) — the microcontroller that initializes the GPU's security and management functions — requires IOMMU translation mode during its boot sequence. When the IOMMU group is set to identity before the nvidia driver probes the GPU, the FSP fails to initialize with error code 0x177. This is not a recoverable error: the FSP state persists across all software-level resets, including Function Level Reset (FLR), Secondary Bus Reset (SBR), and even CXL bus resets. The only way to clear it is to physically power-cycle the system.

The assistant had been trying various workarounds: setting identity domains before the nvidia driver loads, using modprobe blacklists to prevent auto-probing, and attempting to unbind/rebind the driver. Each attempt left the GPUs in a progressively worse state, culminating in message 6323 where modprobe nvidia failed with "No such device" — because the GPUs had been removed from the PCI bus and the nvidia module refuses to load without any matching PCI devices.

The Subject Message: A Pragmatic Reset

Message 6324 opens with a simple, almost understated observation: "OK nvidia isn't even loaded right now." This single sentence encapsulates the entire predicament. The assistant has been operating under the assumption that the nvidia driver was loaded (perhaps from a previous successful recovery), but the failed modprobe in the prior message revealed that it was not. The assistant is now in a state where:

  1. The nvidia driver is not loaded
  2. The GPUs may or may not be present in the PCI bus
  3. The IOMMU groups may or may not be set to identity
  4. The FSP on the Blackwell GPUs may be in a corrupted state from all the failed probe attempts The assistant's decision is immediate and decisive: restore the working state first. The reasoning is clear: without a known-good baseline, further experimentation is meaningless. You cannot test whether identity domains work if the GPUs don't even show up in nvidia-smi.

The Recovery Sequence

The bash script executed in this message is a distilled version of a recovery sequence that was discovered to work earlier in the conversation (see [msg 6320]). The sequence is:

  1. Remove the modprobe blacklist: rm -f /etc/modprobe.d/temp-nvidia-blacklist.conf — this ensures the nvidia driver can auto-load when PCI devices appear.
  2. Remove any existing dead GPU sysfs entries: The script attempts echo 1 > /sys/bus/pci/devices/$gpu/remove for each GPU. This is a cleanup step to ensure no stale PCI device entries remain. Interestingly, all four remove operations fail with "No such file or directory" — confirming that the GPUs were already removed from the PCI bus (likely from the previous message's experiment).
  3. Secondary Bus Reset (SBR) on all parent bridges: For each of the four PCI bridges that host the GPUs, the script toggles the SBR bit in the bridge's control register. It sets bit 6 (0x40) in BRIDGE_CONTROL, waits 2 seconds, then restores the original value. This triggers a secondary bus reset on the downstream PCI bus, which should cause any devices behind that bridge to re-initialize. A 15-second wait follows to allow the GPUs' FSP to complete its boot sequence.
  4. PCI rescan: echo 1 > /sys/bus/pci/rescan triggers the kernel to re-enumerate all PCI buses. Since the nvidia driver is not blacklisted, the kernel's PCI subsystem will match the newly discovered NVIDIA devices against the nvidia driver's device ID table and automatically load and bind the driver.
  5. Verification: nvidia-smi confirms all 4 GPUs are back: "NVIDIA RTX PRO 6000 Blackwell Server Edition" on bus addresses 01:00.0, 11:00.0, 61:00.0, and 71:00.0.

The Critical Insight: Why This Works

The recovery sequence works because of a subtle but crucial property of the nvidia driver's probe mechanism. As discovered in [msg 6321], when the nvidia module is already loaded in memory and new PCI devices appear during a rescan, the driver's probe function handles the fresh device correctly. This is different from the case where the module loads simultaneously with device discovery (e.g., at boot time), where the probe path may be different.

The assistant had previously discovered that loading nvidia before the rescan is essential. In [msg 6323], the assistant tried to load nvidia before rescan, but modprobe nvidia failed because there were no matching PCI devices in the system at that moment. The nvidia driver's module initialization apparently checks for the presence of supported devices and refuses to load if none are found.

In this message, the assistant doesn't explicitly load nvidia — it relies on the kernel's automatic driver binding during rescan. This works because the rescan creates new PCI device entries, which triggers the kernel's driver matching logic, which finds the nvidia driver (either already loaded by udev or loaded on-demand), and binds it to the fresh GPU devices. The result is a clean probe with no lingering FSP corruption.

Assumptions and Their Validity

The assistant makes several assumptions in this message:

Assumption 1: The SBR + rescan sequence will recover the GPUs regardless of prior state. This assumption is validated by the successful output — all 4 GPUs appear in nvidia-smi. However, this was not a guaranteed outcome. Earlier attempts (e.g., [msg 6319]) had shown that even after SBR and rescan, the GPUs could remain in a broken state if the FSP had been corrupted. The key difference is that in this message, the GPUs had been removed from the PCI bus (via the earlier remove operations), and the rescan created entirely new PCI device instances. This "fresh enumeration" appears to be what allows the FSP to re-initialize cleanly.

Assumption 2: The IOMMU groups will default to DMA-FQ after rescan. This is a reasonable assumption since the groups are re-created during rescan and inherit the system's default IOMMU domain type. The assistant does not verify this in the message, but subsequent messages confirm the groups are DMA-FQ.

Assumption 3: The nvidia driver will auto-load during rescan. This depends on the kernel's driver autoprobing mechanism and the absence of blacklists. The assistant had removed the temporary blacklist, and the nvidia driver was not manually loaded, so the kernel's drivers_autoprobe mechanism should handle it. The successful nvidia-smi output confirms this worked.

What This Message Teaches

Message 6324 is a textbook example of a critical systems-engineering principle: when you're deep in unknown territory and things are broken, the first priority is to restore a known-good state. The assistant had been chasing an increasingly complex series of workarounds — modprobe blacklists, driver unbinding, IOMMU group manipulation, multiple SBR cycles — and each attempt was making the situation worse. The realization that "nvidia isn't even loaded" was the signal to stop and reset.

The message also demonstrates the importance of having a reliable recovery mechanism. The remove → SBR → rescan sequence had been discovered and validated earlier in the conversation (see [msg 6320]), and having this proven recovery path allowed the assistant to confidently reset the system without fear of permanent damage. In systems engineering, especially when working with bleeding-edge hardware like Blackwell GPUs, a reliable "panic button" recovery procedure is invaluable.

The Broader Implications

This message sits within a larger narrative about the challenges of deploying NVIDIA Blackwell GPUs in virtualized environments with IOMMU. The fundamental incompatibility between Blackwell's FSP and IOMMU identity domains is a significant constraint: it means that any deployment requiring P2P DMA between Blackwell GPUs must either find an alternative mechanism (like the nvidia driver's DmaRemapPeerMmio parameter) or accept the performance penalty of DMA through the IOMMU.

For the specific deployment in this conversation — an SGLang inference server serving the Qwen3.5-122B-A10B BF16 model across 4 GPUs with tensor parallelism — the loss of P2P DMA is partially mitigated by the MTP (Multi-Token Prediction) speculation optimization, which delivers 12-45% throughput improvement. The assistant has already pivoted to optimizing what can be optimized rather than fighting against hardware limitations.

Conclusion

Message 6324 is a moment of clarity after a long period of escalating complexity. The assistant recognizes that the current state is broken, identifies the minimal recovery procedure, executes it cleanly, and verifies success. It is a reminder that sometimes the most productive thing you can do in a debugging session is to stop debugging and restore the baseline. The message is short — a single bash command with some output — but it represents a critical decision point: the decision to step back, reset, and approach the problem from a position of stability rather than chaos.