The Reconnaissance Probe: Understanding the Starting Point for LXC GPU Passthrough

Introduction

In the long-running effort to deploy the GLM-5-NVFP4 large language model across 8 NVIDIA RTX PRO 6000 Blackwell GPUs, the team had reached a critical inflection point. After spending segments 0 through 3 battling the fundamental limitations of GPU communication inside a KVM virtual machine — where VFIO/IOMMU group isolation prevented Peer-to-Peer (P2P) DMA between GPUs, imposing a punishing ~13µs latency floor — the decision was made to abandon the VM approach entirely. The user's question in [msg 427] — "Would I get better results in Proxmox LXC containers?" — had opened a new path. The assistant's analysis in [msg 428] and detailed setup guide in [msg 433] had laid the theoretical groundwork. Now, in [msg 436], the assistant takes the first concrete step: a three-pronged reconnaissance probe to discover the actual state of both the Proxmox host and the existing LXC container.

This message is the first implementation action in what becomes segment 4 of the session — the LXC container approach to bypass VFIO/IOMMU P2P bottlenecks. It is a deceptively simple message, consisting of three parallel bash tool calls that query the state of two machines. But beneath that simplicity lies a crucial moment of transition from planning to execution, from theory to practice, and from assumption to discovery.

The Message in Full

The assistant issued three SSH commands in parallel:

Command 1 — Probe the Proxmox host's kernel version and read the existing LXC container 129 configuration:

ssh root@10.1.2.6 "uname -r && cat /etc/pve/nodes/kpro6/lxc/129.conf" 2>&1

Command 2 — Probe the Proxmox host for NVIDIA driver presence, device nodes, loaded kernel modules, and modprobe configurations:

ssh root@10.1.2.6 "nvidia-smi 2>&1 | head -30; echo '---'; ls -la /dev/nvidia* 2>&1; echo '---'; lsmod | grep -E 'nvidia|vfio' 2>&1; echo '---'; cat /etc/modprobe.d/vfio*.conf 2>&1; echo '---'; cat /etc/modprobe.d/blacklist*.conf 2>&1" 2>&1

Command 3 — Probe the existing LXC container (llm-two, CT 129) for kernel version, OS release, NVIDIA driver, disk usage, and memory:

ssh root@10.1.230.174 "uname -r && cat /etc/os-release | head -4 && echo '---' && nvidia-smi 2>&1 | head -5 && echo '---' && df -h / && echo '---' && free -h" 2>&1

The results painted a clear picture: the Proxmox host was running kernel 6.8.12-9-pve, had no NVIDIA driver installed (nvidia-smi: command not found, no /dev/nvidia* device nodes), no vfio-pci configuration files, and no blacklist configurations. The LXC container 129 was an unprivileged Ubuntu 24.04 container with 128 cores, 400GB memory, and an 800GB rootfs — but also had no NVIDIA driver. It was a blank slate.

Why This Message Was Written: The Reasoning and Motivation

The motivation for this message is rooted in a fundamental principle of systems engineering: never assume the state of a system you haven't inspected. The assistant had spent the previous messages (particularly [msg 428] and [msg 433]) developing an elaborate plan for LXC-based GPU passthrough, complete with detailed steps for installing NVIDIA drivers on the Proxmox host, configuring device bind-mounts, and setting up the container. But that plan was built on assumptions about the current state of the machines — assumptions that could easily be wrong.

Several critical unknowns existed:

1. Was the NVIDIA driver already installed on the Proxmox host? The assistant's guide in [msg 433] assumed it was not, but the user might have installed it independently, or it might have been installed previously for another purpose.
2. What was the actual configuration of LXC container 129? The assistant knew the container existed (the user had referenced it as "llm-two" in [msg 434]), but its configuration — whether it was privileged or unprivileged, what resources were allocated, what features were enabled — was unknown. The setup guide assumed a freshly created privileged container, but reusing an existing container would be more practical.
3. Were there any vfio-pci bindings or blacklist configurations that would conflict? The guide assumed the user had vfio-pci bindings for the GPUs (from the KVM VM setup), but this was never verified. The presence of such bindings would need to be removed before the NVIDIA driver could claim the GPUs.
4. What kernel was the Proxmox host running? The NVIDIA driver installation depends on matching kernel headers via pve-headers. The exact kernel version determines which headers package to install.
5. What was the state of the LXC container's OS and resources? The container might have been a minimal Ubuntu installation, or it might have had existing ML software. Its disk and memory allocation would determine whether it could handle the model and workload. The message was written to answer all these questions with a single, efficient parallel probe. The assistant's reasoning was: before I can execute any of the planned steps, I need to know the starting point. Three parallel SSH commands will give me the full picture in one round.

How Decisions Were Made

The most notable decision in this message is the choice of parallel execution. All three bash commands were issued in the same assistant round, meaning they ran simultaneously. This is a deliberate design choice in the opencode session architecture: within a single round, multiple tool calls are dispatched in parallel, and the assistant waits for all results before proceeding. The assistant leveraged this to maximize information gathering in minimal time.

The decision to probe three specific areas reveals the assistant's mental model of the system:

• Command 1 targets the container configuration — the static definition that determines how the container is constructed. This is the "source code" of the container setup.
• Command 2 targets the host's GPU-related state — the dynamic runtime state of drivers, device nodes, and kernel modules. This is the "runtime" of the host.
• Command 3 targets the container's internal state — what OS, resources, and software are actually present inside the running container. This is the "runtime" of the container. Together, these three probes cover the entire system stack: host kernel and config → host GPU state → container config → container internal state. It's a comprehensive reconnaissance that leaves no major unknown. Another decision was the choice of commands within each probe. For the host GPU state (Command 2), the assistant chained five separate checks using echo '---' as a delimiter: 1. nvidia-smi — check if NVIDIA driver is installed and functional 2. ls -la /dev/nvidia* — check if device nodes exist (a different indicator from nvidia-smi) 3. lsmod | grep -E 'nvidia|vfio' — check which kernel modules are loaded 4. cat /etc/modprobe.d/vfio*.conf — check for vfio-pci bindings 5. cat /etc/modprobe.d/blacklist*.conf — check for nouveau blacklist This is a well-structured diagnostic sequence that covers: user-facing driver status → kernel device interface → kernel module state → boot-time configuration. Each check provides a different layer of information about the GPU software stack.

Assumptions Made

Several assumptions underpin this message, some explicit and some implicit:

Assumption 1: The SSH credentials and network paths are correct. The assistant assumes that root@10.1.2.6 is the Proxmox host (kpro6) and that root@10.1.230.174 is the LXC container (llm-two, CT 129). This was established in [msg 434] where the user provided these credentials. The assistant trusts this mapping.

Assumption 2: The container configuration file exists at the standard path. The path /etc/pve/nodes/kpro6/lxc/129.conf follows the Proxmox convention for LXC container configs. The assistant assumes Proxmox is using its standard directory structure.

Assumption 3: The container is running and accessible via SSH. The assistant doesn't check whether the container is running before attempting to SSH into it. It assumes the container is booted and has network connectivity.

Assumption 4: The 2>&1 redirect will capture errors properly. The assistant redirects stderr to stdout to ensure error messages (like "command not found") are captured in the output. This is a reasonable assumption for bash.

Assumption 5: The commands will not hang or timeout. The assistant doesn't set a timeout on the SSH commands. It assumes the machines are responsive and the commands will complete quickly.

Assumption 6: The existing container 129 is the one to use. The assistant's plan in [msg 433] suggested creating a new container (CT 200), but the user referenced "llm-two (ct 129)" in [msg 434]. The assistant adapts to use the existing container rather than creating a new one. This is a flexible response to user input.

Mistakes and Incorrect Assumptions

The most significant discovery from this message is that many of the assistant's assumptions about the host state were wrong — but this is precisely why the reconnaissance was necessary, not a mistake in itself.

The assistant assumed vfio-pci bindings existed. In [msg 433], the assistant wrote: "Currently your GPUs are claimed by vfio-pci for the KVM VM passthrough. For LXC, the host's NVIDIA driver needs to own the GPUs directly." The probe revealed no vfio-pci configuration files and no vfio modules loaded. This means the GPUs were not bound to vfio-pci on the host — they were likely already free. This is actually good news for the LXC approach, as it removes a step.

The assistant assumed the NVIDIA driver might already be installed. The probe showed it was not — no nvidia-smi, no device nodes, no nvidia modules. This confirms that the full driver installation will be needed.

The assistant assumed the container was unprivileged. The config file showed unprivileged: 1, confirming the container was unprivileged. The assistant's guide recommended a privileged container for simplicity. This means the container configuration will need to be changed (or a new container created) to support GPU access, since unprivileged containers have restricted device access.

The container has 390GB of RAM. This is a critical finding. The GLM-5-NVFP4 model requires significant GPU memory for 8 GPUs, and the host memory is used for CPU-side operations, NCCL buffers, and model loading. 390GB is substantial but may be tight depending on the model size and batch processing needs.

The container's rootfs is on ZFS (local-zfs:subvol-129-disk-0). This is important because ZFS performance characteristics differ from raw block devices. The 800GB allocation with only 373MB used means there's plenty of space, but the ZFS filesystem may introduce overhead for large model file operations.

One potential oversight: the assistant did not check the GPU PCIe topology from the host's perspective. While the probe checked for NVIDIA drivers and device nodes, it did not run lspci to verify that the GPUs were actually visible on the PCIe bus. This would have been useful to confirm the GPUs are properly detected by the system, even without drivers.

Another subtle issue: the container's kernel is also 6.8.12-9-pve, which is the Proxmox host kernel. This confirms the container shares the host kernel (as expected for LXC), but it also means the container is running a Proxmox kernel rather than a standard Ubuntu kernel. This could affect compatibility with some software that expects a generic Ubuntu kernel.

Input Knowledge Required

To fully understand this message, one needs:

1. Proxmox VE architecture: Understanding that LXC containers share the host kernel, that container configs are stored in /etc/pve/nodes/<node>/lxc/<ctid>.conf, and that the unprivileged flag determines device access capabilities.
2. NVIDIA driver stack knowledge: Understanding that nvidia-smi is the primary diagnostic tool, that /dev/nvidia* device nodes indicate driver installation, and that lsmod reveals loaded kernel modules.
3. SSH and remote execution: Understanding that the assistant is executing commands on remote machines via SSH, and that 2>&1 redirects stderr for error capture.
4. The session history: Understanding why this probe is necessary — the long struggle with VFIO/IOMMU P2P limitations in KVM VMs (segments 0-3), the decision to try LXC, and the setup guide that preceded this message.
5. The GLM-5-NVFP4 model requirements: Understanding that this is a large MoE model requiring 8 GPUs with tensor parallelism, and that P2P DMA performance is critical for the expert-parallel communication patterns.

Output Knowledge Created

This message produced several critical pieces of knowledge:

1. Host kernel version confirmed: 6.8.12-9-pve. This determines which pve-headers package to install for NVIDIA driver DKMS compilation.
2. No NVIDIA driver on host: The host is a clean slate. No driver conflicts to resolve, no vfio bindings to remove, but also no existing infrastructure to leverage.
3. Container 129 is unprivileged: This is a significant finding. The assistant's guide recommended privileged containers. Converting an existing unprivileged container to privileged is possible but requires stopping the container and editing its config. Alternatively, a new privileged container could be created.
4. Container has 390GB RAM and 800GB rootfs: Sufficient resources for the model workload, though the RAM may be tight for CPU-side operations.
5. Container runs Ubuntu 24.04 LTS: Standard Ubuntu, compatible with the ML software stack (PyTorch, CUDA toolkit, SGLang).
6. No existing GPU software in container: Like the host, the container is a clean slate. No CUDA, no PyTorch, no ML libraries.
7. Container uses ZFS storage: The rootfs is on a ZFS subvolume, which may have performance implications for I/O-heavy operations.

The Thinking Process Visible in the Message

While the message itself is just three bash commands with their outputs, the thinking process is visible in the structure of the probes. The assistant is systematically answering a checklist of unknowns:

• What kernel? → Command 1 (uname -r)
• What container config? → Command 1 (cat lxc/129.conf)
• Is NVIDIA driver installed? → Command 2 (nvidia-smi)
• Are device nodes present? → Command 2 (ls -la /dev/nvidia*)
• Are vfio modules loaded? → Command 2 (lsmod)
• Are there vfio bindings? → Command 2 (cat vfio*.conf)
• Is nouveau blacklisted? → Command 2 (cat blacklist*.conf)
• What OS in container? → Command 3 (os-release)
• Is NVIDIA driver in container? → Command 3 (nvidia-smi)
• What disk space? → Command 3 (df -h)
• What memory? → Command 3 (free -h) The parallel execution of all three commands reveals that the assistant is thinking in terms of information independence — none of these probes depends on the results of another, so they can all run simultaneously. This is efficient but also reveals a mental model where the host state and container state are treated as independent variables. The choice to probe the container's OS release (cat /etc/os-release | head -4) rather than just checking nvidia-smi shows the assistant is thinking ahead: knowing the exact Ubuntu version will determine which package repositories to use, which CUDA toolkit version is compatible, and whether the container needs any OS-level configuration changes.

Conclusion

Message [msg 436] is a textbook example of the reconnaissance-first approach to systems engineering. Before making any changes to the Proxmox host or LXC container, the assistant gathers comprehensive state information through three parallel SSH probes. The results confirm that both machines are clean slates — no NVIDIA driver, no vfio bindings, no pre-existing GPU infrastructure. This is both good news (no conflicts to resolve) and bad news (full installation needed from scratch).

The message also reveals the critical finding that container 129 is unprivileged, which will require either conversion to privileged or creation of a new container. This finding will shape the next steps: the assistant will need to either modify the container config to add unprivileged: 0 or create a new privileged container with GPU device mounts.

In the broader narrative of the session, this message marks the transition from planning to execution. The theoretical analysis of LXC vs KVM for GPU P2P is now being tested against reality. The next messages will build on this reconnaissance, installing the NVIDIA driver on the host, configuring the container for GPU access, and ultimately testing whether the LXC approach can deliver the P2P DMA performance that the KVM VM could not.