The Mapping That Wasn't a Directory: Uncovering Proxmox PCI Passthrough Configuration in the Quest for GPU P2P

Introduction

In the middle of a complex debugging session aimed at enabling Peer-to-Peer (P2P) DMA between eight NVIDIA RTX PRO 6000 Blackwell GPUs running inside a Proxmox virtual machine, a seemingly mundane user message arrives. Message [msg 381] contains the output of several diagnostic commands run on the Proxmox host — commands that the AI assistant had carefully crafted to probe the system's PCI mapping configuration. What makes this message significant is not its length (it is brief) but the payload of information it delivers: it reveals the exact structure of Proxmox's PCI passthrough mapping, exposes an incorrect assumption about the file system layout, and provides the raw BAR (Base Address Register) addresses for all eight physical GPUs. For the ongoing investigation into why nvidia-smi topo reports PHB instead of the desired NODE/SYS topology, this message is a crucial piece of the puzzle.

The Message in Full

The user, operating on the Proxmox host machine kpro6, executes a series of commands that the assistant had requested in the previous round ([msg 380]):

root@kpro6:~# # Check mapping directory contents
ls -la /etc/pve/mapping/pci/
# Try alternate locations
find /etc/pve -name "*.cfg" -path "*mapping*" 2>/dev/null
find /etc/pve -name "pro6000*" 2>/dev/null
# Also check the datacenter config for mappings
grep -r "pro6000" /etc/pve/ 2>/dev/null | head -20
ls: cannot access '/etc/pve/mapping/pci/': No such file or directory
/etc/pve/mapping/pci.cfg
/etc/pve/mapping/pci.cfg:pro6000
/etc/pve/nodes/kpro6/qemu-server/128.conf:hostpci0: mapping=pro6000,pcie=1
/etc/pve/nodes/kpro6/qemu-server/128.conf:hostpci1: mapping=pro6000,pcie=1
/etc/pve/nodes/kpro6/qemu-server/128.conf:hostpci2: mapping=pro6000,pcie=1
/etc/pve/nodes/kpro6/qemu-server/128.conf:hostpci3: mapping=pro6000,pcie=1
/etc/pve/nodes/kpro6/qemu-server/128.conf:hostpci4: mapping=pro6000,pcie=1
/etc/pve/nodes/kpro6/qemu-server/128.conf:hostpci5: mapping=pro6000,pcie=1
/etc/pve/nodes/kpro6/qemu-server/128.conf:hostpci6: mapping=pro6000,pcie=1
/etc/pve/nodes/kpro6/qemu-server/128.conf:hostpci7: mapping=pro6000,pcie=1
root@kpro6:~# for gpu in 01:00.0 11:00.0 61:00.0 71:00.0 81:00.0 91:00.0 e1:00.0 f1:00.0; do
  uuid=$(cat /sys/bus/pci/devices/0000:$gpu/resource 2>/dev/null | head -1)
  echo "Physical $gpu: BAR0=$(lspci -v -s $gpu 2>/dev/null | grep 'Memory at' | head -1)"
done
Physical 01:00.0: BAR0= Memory at 73000000000 (64-bit, prefetchable) [size=64M]
Physical 11:00.0: BAR0= Memory at 83000000000 (64-bit, prefetchable) [size=64M]
Physical 61:00.0: BAR0= Memory at 39000000000 (64-bit, prefetchable) [size=64M]
Physical 71:00.0: BAR0= Memory at 29000000000 (64-bit, prefetchable) [size=64M]
Physical 81:00.0: BAR0= Memory at 107000000000 (64-bit, prefetchable) [size=64M]
Physical 91:00.0: BAR0= Memory at 117000000000 (64-bit, prefetchable) [size=64M]
Physical e1:00.0: BAR0= Memory at cd000000000 (64-bit, prefetchable) [size=64M]
Physical f1:00.0: BAR0= Memory at bd000000000 (64-bit, prefetchable) [size=64M]

Why This Message Was Written: The Context and Motivation

To understand why this message exists, we must trace back through the preceding conversation. The overarching goal of this session is to deploy the GLM-5-NVFP4 large language model across eight RTX PRO 6000 Blackwell GPUs and achieve optimal inference performance. A critical bottleneck had been identified: cross-GPU communication latency. In a multi-GPU inference setup, tensor parallelism (TP) requires frequent all-reduce operations that shuffle activations between GPUs. When these transfers have high latency (measured at ~13 microseconds for small messages), the entire inference pipeline suffers.

The assistant had been systematically working through the problem. Earlier in the conversation ([msg 365]–[msg 369]), it investigated whether P2P DMA — direct GPU-to-GPU memory transfers that bypass host memory entirely — could be enabled. After extensive research into NVIDIA's documentation, VFIO internals, and the specific hardware topology of the ASUS ESC8000A-E13 motherboard (which places each GPU on its own dedicated PCIe root complex), the conclusion was sobering: true hardware P2P is fundamentally impossible in this configuration, regardless of software tricks.

However, the assistant identified a secondary path to improvement. On bare metal, nvidia-smi topo showed a topology where GPUs 0–3 (on NUMA node 0) connected via NODE and GPUs 4–7 (on NUMA node 1) connected via NODE, with cross-socket connections showing SYS. In the VM, all GPUs showed PHB — a flat topology that gave NCCL no information about locality. The assistant hypothesized that by properly configuring NUMA affinity and virtual PCIe topology in the VM, it might restore the NODE/SYS distinction, allowing NCCL to optimize its communication patterns.

This message is the direct result of that hypothesis. The assistant needed to understand how Proxmox maps physical GPUs to virtual hostpci devices, specifically which physical PCI addresses correspond to which NUMA nodes, so it could construct a virtual topology that mirrors the bare-metal layout. The commands in this message were designed to uncover that mapping.

How Decisions Were Made: The Diagnostic Strategy

The assistant's diagnostic strategy reveals a systematic approach to reverse-engineering a virtualized PCIe topology. The commands issued in this message fall into two categories:

Category 1: Locating the Proxmox PCI mapping configuration. The assistant had previously assumed that the pro6000 resource mapping was stored in a file within a directory like /etc/pve/mapping/pci/. This assumption was based on Proxmox documentation conventions where resource mappings are often stored as individual files. The first command — ls -la /etc/pve/mapping/pci/ — was designed to explore this directory. The result (No such file or directory) immediately falsified this assumption, forcing the fallback commands using find and grep to locate the actual configuration.

Category 2: Identifying physical GPU BAR addresses. The second set of commands iterates over the eight known physical PCI addresses of the GPUs (01:00.0, 11:00.0, 61:00.0, 71:00.0, 81:00.0, 91:00.0, e1:00.0, f1:00.0) and extracts their BAR0 memory addresses. This information serves two purposes: it confirms that all eight GPUs are properly enumerated and accessible on the host, and it provides a fingerprint that can be cross-referenced with the VM's PCI layout to determine the physical-to-virtual mapping order.

The choice to use lspci -v to extract BAR addresses rather than reading from /sys/bus/pci/devices/ directly (as the original uuid command attempted) shows adaptive problem-solving — the original approach using cat /sys/bus/pci/devices/0000:$gpu/resource was producing empty output (the head -1 would capture nothing useful from the resource file), so the assistant pivoted to a more reliable method.

Assumptions Made by the User and Agent

Several assumptions underpin this message, and some of them proved incorrect:

1. The mapping directory structure assumption. The assistant assumed that /etc/pve/mapping/pci/ was a directory containing individual .cfg files for each PCI mapping. In reality, the mapping configuration is a single file at /etc/pve/mapping/pci.cfg — a flat file, not a directory. This is a subtle but important distinction. The find command with -path "*mapping*" successfully located pci.cfg, but the initial ls command failed because the directory doesn't exist. This incorrect assumption caused a minor detour but was quickly corrected by the fallback commands.

2. The assumption that the mapping would reveal physical-to-virtual ordering. The assistant hoped that examining the pro6000 mapping configuration would reveal which physical PCI addresses map to which hostpci indices in the VM. However, the grep output shows that all eight hostpci0–7 entries simply reference mapping=pro6000,pcie=1 — they all use the same mapping resource. The actual mapping between the pro6000 resource and specific physical PCI addresses is defined in /etc/pve/mapping/pci.cfg, but the content of that file (the actual mapping of PCI addresses to the resource name) is not shown in this message. The user only grepped for "pro6000" references, which shows where the mapping is used, not where it's defined.

3. The assumption that BAR addresses would help identify GPU ordering. The BAR0 addresses provide a unique memory fingerprint for each GPU, but without knowing which BAR addresses appear in the VM's PCI layout, this information alone cannot establish the mapping. The assistant would need to cross-reference these host BAR addresses with the VM's PCI device BARs — a task that would require additional commands inside the VM.

4. The user's assumption about command syntax. The user faithfully executed the commands as provided, including the somewhat awkward uuid=$(cat /sys/bus/pci/devices/0000:$gpu/resource 2>/dev/null | head -1) line, which was the assistant's first attempt at extracting GPU identifiers. This particular approach was flawed — the resource file in sysfs contains BAR resource information in a format that head -1 doesn't usefully capture. The user didn't question this or adapt the command; they simply ran what was asked, and the output shows empty UUID lines (the echo command only printed the BAR0 line because the uuid variable was empty). This demonstrates a pattern where the user acts as a reliable command executor, trusting the assistant's diagnostic strategy even when individual commands are imperfect.

Input Knowledge Required to Understand This Message

To fully grasp the significance of this message, one needs:

Knowledge of Proxmox VE architecture. Understanding that Proxmox uses a file-based configuration system stored in /etc/pve/ (which is actually a clustered filesystem, pmxcfs), and that PCI passthrough is configured through hostpci entries in VM configuration files and resource mappings in /etc/pve/mapping/pci.cfg. The concept of "resource mappings" — where a named mapping (like pro6000) abstracts the physical PCI addresses — is specific to Proxmox and not found in standard KVM/QEMU.

Knowledge of PCIe topology and NUMA. The physical PCI addresses (01:00.0, 11:00.0, etc.) encode the PCI bus/device/function hierarchy. The fact that these GPUs are on buses 01, 11, 61, 71, 81, 91, e1, and f1 indicates they are spread across multiple PCIe root complexes — a key finding that explains why P2P is difficult. The BAR0 memory addresses (like 73000000000, 83000000000) show the memory-mapped I/O regions assigned to each GPU, and their wide separation confirms the GPUs are in distinct address domains.

Knowledge of the ongoing debugging context. Without knowing that the assistant had been investigating NUMA affinity and virtual PCIe topology for the past several messages, this message would appear as a disconnected set of system queries. The reader must understand that this is part of a larger diagnostic arc aimed at improving NCCL communication performance.

Output Knowledge Created by This Message

This message produces several concrete findings:

Finding 1: The Proxmox PCI mapping is a single file, not a directory. The path /etc/pve/mapping/pci.cfg is a file, and it contains the pro6000 resource definition. This is important because it means the mapping configuration is simpler than expected — there isn't a separate file per mapping.

Finding 2: All eight GPUs use the same mapping resource. The VM configuration (128.conf) shows all eight hostpci0–7 entries referencing mapping=pro6000,pcie=1. This means the pro6000 resource must contain a list of all eight physical PCI addresses, and Proxmox assigns them to hostpci indices in the order they appear in that list (or by some other deterministic rule).

Finding 3: The eight GPUs have distinct BAR0 address ranges. The BAR0 addresses are:

The Thinking Process Visible in the Message

Although this is a user message (not an assistant message with explicit reasoning), the thinking process is visible through the structure of the commands themselves. The assistant that crafted these commands was thinking several steps ahead:

Step 1: Locate the mapping definition. The assistant reasoned that to understand the physical-to-virtual GPU mapping, it first needed to find where Proxmox stores the pro6000 resource definition. The cascade of commands — ls, then find with different patterns, then grep — shows a fallback strategy: try the expected path first, then search more broadly.

Step 2: Fingerprint the physical GPUs. The assistant recognized that even without the mapping file contents, it could potentially determine the mapping empirically by comparing GPU identifiers (UUIDs or BAR addresses) between host and VM. The for loop iterates over all eight known GPU PCI addresses and attempts to extract identifying information.

Step 3: Cross-reference with NUMA topology. Earlier in the conversation ([msg 372]), the user had established which GPUs belong to which NUMA node: 01:00.0, 11:00.0, 61:00.0, 71:00.0 are on NUMA 0, while 81:00.0, 91:00.0, e1:00.0, f1:00.0 are on NUMA 1. By combining this NUMA information with the BAR addresses and the eventual mapping to VM hostpci indices, the assistant hoped to construct a virtual topology that mirrors the bare-metal NUMA layout.

The fact that the uuid extraction failed (the resource file approach was incorrect) reveals a limitation in the assistant's knowledge of Linux sysfs internals. The /sys/bus/pci/devices/0000:$gpu/resource file contains BAR resource information in a binary format, not a UUID string. A more effective approach would have been to read the device/serial sysfs entry or use nvidia-smi with the PCI bus ID. This is a minor mistake in the diagnostic strategy, but it doesn't fatally undermine the investigation because the BAR addresses provide sufficient identification.

Mistakes and Incorrect Assumptions

Several mistakes and incorrect assumptions are evident in this message:

The directory vs. file confusion. The assistant's assumption that /etc/pve/mapping/pci/ is a directory was incorrect. In Proxmox VE, the PCI mapping configuration is stored in a single file /etc/pve/mapping/pci.cfg, not in a directory of individual mapping files. This is a minor but telling error — it suggests the assistant was extrapolating from general Linux conventions (where configuration directories are common) without specific knowledge of Proxmox's implementation.

The flawed UUID extraction. The command cat /sys/bus/pci/devices/0000:$gpu/resource 2>/dev/null | head -1 does not extract a UUID. The resource file in sysfs contains the PCI BAR resource map in a format that doesn't include UUIDs. This command would produce either binary data or nothing useful. The fact that the user's output shows only the BAR0 lines (and no UUID lines) confirms this — the uuid variable was empty, so the echo statement only printed the BAR0 portion.

The incomplete grep. The command grep -r "pro6000" /etc/pve/ found references to pro6000 in the VM config file and in pci.cfg, but it only showed the lines containing the match, not the full content of pci.cfg. The line /etc/pve/mapping/pci.cfg:pro6000 tells us that pro6000 appears in that file, but not what the mapping definition actually is. The assistant would need to read the full file contents to get the physical PCI address list.

The assumption that the mapping would reveal ordering. Even if the full contents of pci.cfg were known, the mapping from physical PCI addresses to VM hostpci indices is not necessarily straightforward. Proxmox may assign GPUs to hostpci slots in the order they appear in the mapping, or it may use a different assignment strategy. The assistant would need to verify the mapping empirically by comparing GPU identifiers inside and outside the VM.

Conclusion

Message [msg 381] is a deceptively simple piece of a much larger puzzle. On its surface, it is just the output of a few diagnostic commands — a user dutifully executing instructions on a Proxmox host. But within the context of the ongoing investigation into GPU P2P performance, it represents a critical data-gathering step. It reveals the structure of Proxmox's PCI mapping configuration, provides BAR address fingerprints for all eight GPUs, and exposes several assumptions that needed correction.

The message also illustrates a common pattern in AI-assisted system debugging: the assistant proposes a diagnostic strategy, the user executes it, and the results either confirm or challenge the assistant's mental model of the system. Here, the results challenged the assistant's model of Proxmox's file system layout while providing the raw data needed to reconstruct the physical-to-virtual GPU mapping. The investigation would continue in subsequent messages, building on the foundation laid by this one.