The Diagnostic Pivot: Understanding GPU Binding State in a Proxmox Passthrough Workflow

In the middle of a complex deployment session spanning multiple GPU configurations, model serving stacks, and virtualization boundaries, a single diagnostic message at index 5778 stands out as a masterclass in systems-level troubleshooting. The message, issued by the AI assistant in response to a user reporting that nvidia-smi failed with "No supported GPUs were found" after switching back to an LXC container, is deceptively simple on its surface. Beneath the single bash command lies a rich tapestry of reasoning about GPU driver binding, Proxmox PCI passthrough mechanics, kernel module state, and the subtle interplay between virtual machines and containers sharing the same physical hardware.

The Context: A Tangled Web of GPU Ownership

To understand why this message was written, one must first appreciate the operational landscape that preceded it. The user had been running a Kimi-K2.5 INT4 model on a Proxmox host equipped with eight NVIDIA RTX PRO 6000 Blackwell GPUs. This setup lived inside an LXC container (CT 129, named llm-two), where the GPUs were bound to the proprietary NVIDIA driver and fully available for inference workloads. A significant amount of effort had gone into stabilizing this environment — resolving flash-attn build issues, tuning NCCL parameters, enabling hierarchical KV cache, and creating a systemd service for production deployment.

Then came the tangent experiment. The user wanted to pass all eight GPUs through to a VM (VM 131, ml-pipelines) instead. This required the GPUs to be bound to the vfio-pci driver on the host rather than the NVIDIA driver — a fundamental shift in GPU ownership. The assistant guided this transition, discovering that Blackwell GPUs require the open kernel module (nvidia-dkms-590-open) rather than the proprietary one. The VM was successfully configured, the open modules were installed, and the GPUs worked inside the guest.

But then the host rebooted. And when the user tried to switch back to the LXC container, the GPUs were invisible to nvidia-smi. This is the crisis point that message 5778 addresses.

The Message: A Hypothesis in Action

The message opens with a clear diagnostic hypothesis:

"The host rebooted, so the nvidia driver module may not have reloaded properly after the GPUs came back from vfio-pci. Let me check:"

This sentence reveals the assistant's mental model of the situation. The hypothesis is that the host reboot caused the GPU binding state to reset, but that the NVIDIA kernel modules failed to re-bind to the devices during boot. This is a reasonable assumption — after a reboot, the boot sequence should load the NVIDIA driver, which should claim the GPUs. If the driver loaded but the GPUs remained unclaimed, the LXC container would indeed see "No supported GPUs were found."

The assistant then executes a single compound bash command that probes four critical pieces of state simultaneously:

  1. VM status (qm status 131): Is the VM that was using the GPUs still running?
  2. Container status (pct status 129): Is the target LXC container running?
  3. PCI driver binding (lspci -ks 01:00.0): Which kernel driver is currently bound to the first GPU?
  4. NVIDIA module state (lsmod | grep nvidia): Are the NVIDIA kernel modules loaded, and what are their reference counts? This is a beautifully constructed diagnostic probe. Each piece of data is independently valuable, but together they tell a complete story about the state of GPU ownership on the system.

What the Diagnostic Revealed

The results were illuminating:

status: running   (VM 131 is still running)
---
status: running   (Container 129 is running)
---
01:00.0 3D controller: NVIDIA Corporation Device 2bb5 (rev a1)
    Kernel driver in use: vfio-pci
    Kernel modules: nvidiafb, nouveau, nvidia_drm, nvidia
---
nvidia_drm            131072  0
nvidia_uvm           2031616  0
nvidia_modeset       1679360  1 nvidia_drm
nvidia              14745600  2 nvidia_uvm,nvidia_modeset

The critical finding is that VM 131 was still running. Because the VM was active and holding the GPUs via vfio-pci, the NVIDIA driver on the host could not claim them. The lspci output confirms this: the kernel driver in use is vfio-pci, not nvidia. The NVIDIA modules are loaded (as shown by lsmod), but they have zero reference counts from device binding — they're sitting idle, loaded into memory but not attached to any GPU.

The assistant's initial hypothesis was therefore incorrect in its specifics but correct in its general direction. The NVIDIA driver modules had loaded properly. The issue wasn't that they failed to reload — it was that they couldn't reload because the VM was still holding the devices. The GPUs never came back from vfio-pci in the first place.

The Thinking Process: A Chain of Inference

This message reveals a sophisticated chain of reasoning. Let me reconstruct the assistant's thinking:

  1. Symptom observation: The user reports that nvidia-smi inside the LXC container says "No supported GPUs were found." This means the NVIDIA driver stack cannot see any NVIDIA devices.
  2. Hypothesis generation: The host rebooted recently. Perhaps the driver didn't reinitialize properly after the reboot. This is a common failure mode — DKMS modules sometimes fail to build or load on boot, or the boot order might race with GPU initialization.
  3. Hypothesis testing: The assistant designs a probe that will either confirm or refute the hypothesis. The key insight is to check whether the GPUs are still on vfio-pci (meaning the VM is holding them) or whether they're free but the NVIDIA driver failed to claim them.
  4. Result interpretation: The VM is still running, the GPUs are on vfio-pci, and the NVIDIA modules are loaded but idle. This refutes the "driver didn't reload" hypothesis and reveals the true cause: the VM must be stopped before the GPUs can return to the NVIDIA driver.
  5. Implicit next step: The assistant doesn't state it explicitly in this message, but the logical conclusion is that VM 131 needs to be stopped before the LXC container can use the GPUs. The diagnostic data makes this obvious to anyone familiar with Proxmox's PCI passthrough model.

Assumptions and Their Validity

The message operates on several assumptions, most of which are sound:

Assumption 1: The host reboot is the relevant event. This is correct — the user confirmed the host was rebooted, and the timing aligns with the problem appearing.

Assumption 2: The NVIDIA driver modules might not have reloaded. This turned out to be incorrect in the specific sense (the modules did load), but it was a reasonable hypothesis. Driver initialization failures after reboot are common, especially with DKMS-managed modules.

Assumption 3: Checking lspci -ks and lsmod will reveal the binding state. This is correct and reflects solid systems knowledge. The lspci -ks command shows the kernel driver in use for a specific PCI device, while lsmod | grep nvidia shows whether the NVIDIA modules are loaded and their reference counts.

Assumption 4: The first GPU (01:00.0) is representative of all eight. This is a reasonable sampling assumption. If the first GPU is on vfio-pci, the others almost certainly are too, since Proxmox manages them as a group through the PCI mapping.

Input Knowledge Required

To fully understand this message, one needs:

Output Knowledge Created

This message produces several valuable pieces of knowledge:

  1. VM 131 is still running: This is the root cause. The user may have assumed the VM was stopped or didn't realize it was still active.
  2. GPUs are bound to vfio-pci, not nvidia: Confirmation that the passthrough is still active and the host driver cannot access the devices.
  3. NVIDIA modules are loaded but idle: The driver stack is present and functional, just not bound to any devices. This rules out driver installation or build issues.
  4. Container 129 is running: The LXC container is ready to receive the GPUs once they're freed.
  5. The switching workflow needs a step to stop the VM: This is implicit knowledge — the diagnostic reveals that the user must explicitly stop VM 131 before the GPUs will return to the host NVIDIA driver.

The Broader Significance

This message exemplifies a critical pattern in systems engineering: the diagnostic probe that reframes the problem. The assistant started with one hypothesis (driver reload failure) and designed a test that would either confirm it or reveal the true cause. The test didn't confirm the original hypothesis, but the data it returned immediately pointed to the real issue.

This is far more valuable than simply asking "Is the VM still running?" or guessing at the cause. By collecting multiple independent signals simultaneously, the assistant created a complete picture of the GPU binding state in a single round trip. The user can see not just what's wrong, but why — and more importantly, what needs to happen next (stop VM 131, then start the container).

The message also demonstrates the importance of understanding the ownership model of shared hardware resources. GPUs in a Proxmox environment are not simply "available" or "unavailable" — they have a binding state that depends on which VM or process last claimed them. A reboot resets this state, but only if the VM doesn't survive the reboot. In this case, VM 131 was configured to auto-start, so it reclaimed the GPUs before the container could.

Conclusion

Message 5778 is a compact but powerful example of diagnostic reasoning in action. In a single bash command and its results, the assistant transforms an opaque error ("No supported GPUs were found") into a clear, actionable understanding: the VM is still holding the GPUs, and it must be stopped before the LXC container can use them. The message's true value lies not in providing the answer, but in providing the data that makes the answer obvious — a skill that separates effective troubleshooting from guesswork.