The Validation Playbook: Diagnosing GPU P2P Topology After an ACS Disable Attempt
In the high-stakes world of multi-GPU inference serving, few bottlenecks are as frustrating as the ones imposed not by hardware limits, but by virtualization boundaries. When you have eight NVIDIA RTX PRO 6000 Blackwell GPUs — each with 96GB of VRAM, connected via PCIe Gen5 x16 — the expectation is blistering inter-GPU communication. But in a Proxmox VM, those expectations collide with the reality of VFIO, IOMMU groups, and Access Control Services (ACS). Message 410 in this opencode session captures a pivotal moment: the system is rebooting after a BIOS change to disable ACS, and the assistant provides a comprehensive validation playbook to determine whether the change actually worked. This message is a masterclass in systematic diagnostics, and understanding its structure, assumptions, and reasoning reveals deep insights about GPU virtualization, PCIe topology, and the art of debugging at the hardware-software boundary.
The Message in Full
The assistant writes:
Good, while we wait for the system to come back up, here are the commands to run on the Proxmox host after reboot to validate everything:
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## 1. Check IOMMU Groups (most important — shows if ACS change worked)
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```bash # Show all IOMMU groups with NVIDIA GPUs for d in /sys/kernel/iommu_groups//devices/; do n=$(echo $d | cut -d/ -f5) dev=$(lspci -nns ${d##*/} 2>/dev/null) echo "$dev" | grep -qi nvidia && echo "IOMMU Group $n: $dev" done
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# Compare: before ACS disable, each GPU was in its own group (42,61,28,10,101,117,90,72) # After ACS disable, GPUs on same socket should merge into fewer groups ```
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If ACS disable worked, you'll see multiple GPUs in the same IOMMU group instead of 8 separate groups.
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## 2. Verify IOMMU is in passthrough mode
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```bash cat /proc/cmdline | grep -o 'iommu=[^ ]*' # Should show: iommu=pt
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dmesg | grep -i 'iommu\|AMD-Vi' | grep -i 'mode\|passthrough\|translat' | head -5 ```
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## 3. Check PCIe link speeds (confirms Gen5)
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``bash # Check actual PCIe link speed for each GPU for gpu in 01:00.0 11:00.0 61:00.0 71:00.0 81:00.0 91:00.0 e1:00.0 f1:00.0; do speed=$(lspci -vvv -s $gpu 2>/dev/null | grep "LnkSta:" | head -1) echo "GPU $gpu: $speed" done ``
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Should show Speed 32GT/s, Width x16 for PCIe Gen5 x16.
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## 4. Check Data Fabric / power settings (requires kernel access)
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```bash # Check if DF C-states are disabled (look for cpupower or msr) # P-state info cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor 2>/dev/null cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq 2>/dev/null
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# Check ASPM status lspci -vvv -s 01:00.0 2>/dev/null | grep -i "ASPM\|LnkCtl:"
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# Check NUMA topology numactl --hardware ```
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## 5. Quick P2P check from host (without starting VM)
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``bash # Verify VFIO is still bound to GPUs lspci -k -s 01:00.0 | grep -i driver # Should show: Kernel driver in use: vfio-pci ``
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## 6. Then start the VM and check inside
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Once the host is validated, start the VM: ``bash qm start 128 ``
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Then inside the VM (once SSH is available), the key test: ``bash nvidia-smi topo -p2p r nvidia-smi topo -m ``
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If we seeOKinstead ofNSin the P2P matrix, we've won.
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---
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Paste the output of commands 1-3 when the system is up and I'll analyze whether the ACS change took effect.
Context: The Long Road to P2P
To understand why this message exists, we must trace the journey that led to it. The session began in Segment 0 with a full ML environment setup on Ubuntu 24.04 — NVIDIA drivers, CUDA Toolkit 13.1, PyTorch, flash-attn, and ultimately the deployment of the GLM-5-NVFP4 model using SGLang across eight GPUs. But performance was bottlenecked by cross-GPU communication. In Segment 1, the team battled NaN crashes during decode. By Segment 2, they had established a baseline throughput but identified a critical bottleneck: PCIe P2P latency in the Proxmox VM environment.
Segment 3 (the current segment) is where the investigation went deep into virtualization internals. The team discovered that all eight GPUs showed NS (Not Supported) in the nvidia-smi topo -p2p matrix, meaning Peer-to-Peer DMA was entirely disabled. This forced all inter-GPU communication through a staged GPU→host→GPU path, adding approximately 13 microseconds of latency per transfer — a crippling overhead for the all-reduce operations that underpin tensor parallelism in large model inference.
The investigation peeled back layers: host PCIe topology, IOMMU groups, VM chipset migration from i440FX to Q35, BAR allocation fixes, and sysfs NUMA affinity configuration. Each step brought the team closer to the root cause, but P2P remained stubbornly NS. The critical insight came in messages 403-408: ACS (Access Control Services) — a PCIe capability that enforces isolation between downstream devices — was keeping each GPU in its own IOMMU group. Since VFIO requires ACS-based isolation for security, and each GPU was on its own PCIe root complex (a design inherent to the ASUS ESC8000A-E13 motherboard's AMD EPYC platform), the GPUs could never share an IOMMU group, and thus could never perform direct DMA to each other.
The user's question in message 403 — "pcie acs - wouldn't disabling permit p2p? or is it more hw level thing?" — was the breakthrough moment. The assistant's detailed explanation in message 404-408 confirmed that disabling ACS could merge IOMMU groups, potentially enabling P2P. The user went into the BIOS, disabled ACS, and rebooted the system. Message 410 is the assistant's response while waiting for that reboot to complete.
Why This Message Was Written: The Reasoning and Motivation
The primary motivation for message 410 is temporal opportunism. The system is rebooting — a process that can take several minutes on enterprise server hardware with 8 GPUs, 256GB of RAM, and extensive BIOS initialization. Rather than waiting idly, the assistant uses this window to prepare a comprehensive validation plan. This is a classic engineering productivity technique: when you have a long-running operation (reboot, compile, data transfer), use that time to prepare the next steps so you can execute immediately when the operation completes.
But there's a deeper motivation: epistemic uncertainty about the BIOS change. The assistant does not know for certain that disabling ACS will work as expected on this specific hardware platform. The ASUS ESC8000A-E13 with dual AMD EPYC 9335 processors has a complex PCIe topology. The assistant's earlier analysis (message 404) noted that "each GPU is alone on its own root complex" and that "ACS only controls forwarding within a PCIe hierarchy. Since there's no shared PCIe bridge between any two GPUs, ACS has nothing to do." This creates genuine uncertainty: will ACS disable actually merge IOMMU groups, or is the isolation baked into the hardware topology at a deeper level?
The validation commands are designed to resolve this uncertainty at multiple levels of the stack. By checking IOMMU groups first, the assistant can immediately confirm whether the fundamental prerequisite for P2P has been met. If the groups haven't merged, there's no point proceeding to the other checks — the ACS change didn't work as expected, and the team needs a different approach.
How Decisions Were Made: The Architecture of the Validation Plan
The validation plan in message 410 follows a carefully structured decision tree, organized from most fundamental to most derivative. This is not a random collection of diagnostic commands — it's a hierarchical verification framework.
Step 1: IOMMU Groups is placed first because it is the necessary condition for P2P in a VFIO environment. If ACS disable successfully merged GPUs into shared IOMMU groups, then VFIO will map them into the same address space, and the IOMMU will allow direct DMA between them without trapping through host memory. The assistant provides a specific script that filters for NVIDIA devices across all IOMMU groups, and even includes the "before" group numbers (42, 61, 28, 10, 101, 117, 90, 72) as a reference baseline. This attention to detail — providing the exact previous state — shows deep understanding of the diagnostic process: you can't interpret a result without knowing what changed.
Step 2: IOMMU Passthrough Mode verifies that the host kernel is running with iommu=pt (passthrough mode). This was configured earlier in the session (message 398) and is essential for minimizing IOMMU translation overhead. In passthrough mode, the IOMMU maps devices directly to guest physical addresses with minimal translation, whereas in traditional mode, every DMA transaction requires a full translation table walk. The assistant checks both the kernel cmdline and the dmesg logs, providing redundancy in case one source is unavailable.
Step 3: PCIe Link Speeds confirms that the GPUs are actually running at PCIe Gen5 x16 (32 GT/s). This is a sanity check: if the BIOS change or reboot caused the GPUs to negotiate at a lower link speed, any P2P improvements would be masked by bandwidth limitations. The assistant lists all eight GPU BDF (Bus:Device.Function) addresses explicitly — 01:00.0 11:00.0 61:00.0 71:00.0 81:00.0 91:00.0 e1:00.0 f1:00.0 — which were discovered through earlier topology mapping.
Step 4: Data Fabric and Power Settings is the most speculative check. The assistant recommends checking DF C-states, APBDIS, ASPM, and NUMA topology — settings that were discussed in message 402 as potentially contributing to the 13µs latency floor. However, the assistant doesn't provide specific commands to check DF C-states directly (noting "requires kernel access" and "look for cpupower or msr"), which reflects the difficulty of accessing AMD-specific power management counters from standard Linux tools. This step is included as a "while you're there" diagnostic rather than a critical path item.
Step 5: VFIO Driver Binding is a quick pre-flight check before starting the VM. If the ACS disable somehow caused the GPU to bind to a different driver (e.g., nvidia instead of vfio-pci), the VM won't be able to access it. This is a pragmatic check that prevents wasting time on a VM that won't boot properly.
Step 6: The Ultimate Test — starting the VM and running nvidia-smi topo -p2p r and nvidia-smi topo -m. This is the final validation. The assistant frames it with cautious optimism: "If we see OK instead of NS in the P2P matrix, we've won." The word "if" is significant — it acknowledges that the ACS disable might not work as hoped, given the hardware topology constraints.
Assumptions Made by the Assistant
Message 410 rests on several assumptions, some explicit and some implicit:
Assumption 1: ACS disable will merge IOMMU groups. This is the core hypothesis being tested. The assistant assumes that ACS is the mechanism keeping GPUs in separate IOMMU groups, and that disabling it will cause GPUs on the same socket to merge. However, as noted in message 404, each GPU is on its own PCIe root complex — ACS only controls forwarding on bridges and switches that have multiple downstream devices. If there's no shared bridge, ACS has nothing to control, and disabling it may have no effect on IOMMU grouping.
Assumption 2: The user has BIOS access and successfully changed the ACS setting. The assistant is operating on the user's report that they would try disabling ACS. There's no confirmation that the BIOS actually has an ACS toggle, that it was set correctly, or that the system booted with the new setting. The validation commands are designed to provide this confirmation.
Assumption 3: The host is reachable after reboot. The assistant provides commands to run on the Proxmox host, assuming SSH access or console access will be available. Earlier messages (404-405) showed connectivity issues (ssh: connect to host 10.1.230.175 port 22: No route to host), so this assumption is somewhat fragile.
Assumption 4: The VM ID is 128. The assistant uses qm start 128 without explanation. This VM ID was established earlier in the session and is assumed to be the correct identifier for the 8-GPU inference VM.
Assumption 5: nvidia-smi topo -p2p r is the definitive test. The assistant treats the P2P matrix as the ground truth for whether P2P DMA is working. While nvidia-smi is authoritative for NVIDIA's driver-level view of P2P, there could be scenarios where the matrix shows OK but actual DMA performance doesn't improve (e.g., if the IOMMU still intercepts despite the driver thinking P2P is enabled).
Mistakes and Incorrect Assumptions
The most significant potential mistake in this message is the over-reliance on ACS disable as a solution given the hardware topology. The assistant's own analysis in message 404 stated:
"Each GPU is on its own root complex. There's no shared PCIe switch or bridge between any pair of GPUs. ACS only controls forwarding on bridges/switches — when there's no shared bridge, ACS has nothing to do."
This directly contradicts the assumption that ACS disable will help. The assistant reconciles this by arguing that ACS disable might cause the IOMMU to merge groups even across root complexes — but this is speculative. The AMD EPYC IOMMU (AMD-Vi) groups devices by their PCIe hierarchy, and if each GPU is on a separate root port, they may remain in separate groups regardless of ACS settings.
A second potential issue is the omission of a rollback plan. The validation commands are purely diagnostic — they tell you what happened, but not what to do if the result is negative. If IOMMU groups haven't merged, what's the next step? The assistant doesn't provide contingency commands or alternative approaches in this message. The user is left to paste output and wait for the assistant's analysis, which introduces latency into the debugging cycle.
Third, the PCIe link speed check (Step 3) is misleading in a VM context. The assistant says the GPUs should show Speed 32GT/s, Width x16 for Gen5. But as noted in message 398, inside the VM the GPUs show Gen1 x16 because "the Gen1 is just QEMU virtual link, not real." The command in Step 3 runs on the host, not the VM, so this is correct — but the assistant doesn't explicitly clarify this distinction, which could confuse the user if they try running the command inside the VM.
Input Knowledge Required
To fully understand message 410, the reader needs knowledge spanning several domains:
PCIe Topology and Addressing: The BDF notation (e.g., 01:00.0) represents Bus:Device.Function in PCIe addressing. The eight GPU addresses listed correspond to specific physical slots on the ASUS motherboard, mapped through earlier lspci exploration.
IOMMU and VFIO Internals: Understanding that IOMMU groups are the mechanism by which VFIO isolates devices for passthrough, and that devices in the same group can DMA to each other without host intervention, is essential to interpreting the validation commands.
ACS and PCIe Security: Access Control Services is a PCIe capability defined in the PCIe Base Specification that controls whether peer-to-peer transactions are forwarded or blocked at bridges and root ports. VFIO requires ACS to enforce device isolation.
AMD EPYC Architecture: The dual-socket AMD EPYC platform has multiple PCIe root complexes per socket, connected via the Data Fabric (Infinity Fabric). Each root complex has its own set of root ports, and devices on different root complexes communicate through the Data Fabric rather than a shared PCIe switch.
NVIDIA's P2P Detection: nvidia-smi topo -p2p shows the driver's assessment of whether direct GPU-to-GPU DMA is possible. The matrix values (P2P, NS, OK, NODE, SYS, PHB) indicate different levels of connectivity and topology proximity.
Linux sysfs and Diagnostic Tools: Familiarity with /sys/kernel/iommu_groups/, lspci, dmesg, numactl, and /proc/cmdline is necessary to understand what each command does.
Output Knowledge Created
Message 410 creates several forms of output knowledge:
A Reusable Validation Framework: The six-step validation plan is a template that can be applied to any GPU virtualization setup. The hierarchical structure — check prerequisites before checking outcomes — is a general principle for hardware debugging.
Baseline State Documentation: By providing the "before" IOMMU group numbers (42, 61, 28, 10, 101, 117, 90, 72), the assistant creates a documented baseline against which the ACS change can be measured. This is crucial because IOMMU group numbers can change between reboots due to device enumeration order.
Decision Criteria: The message establishes clear pass/fail criteria for each check. IOMMU groups should show multiple GPUs per group. IOMMU mode should show iommu=pt. Link speed should show 32GT/s. VFIO driver should show vfio-pci. The P2P matrix should show OK. These criteria transform vague hopes ("maybe P2P will work now") into testable hypotheses.
A Shared Mental Model: By framing the validation as a collaborative effort ("Paste the output of commands 1-3 when the system is up and I'll analyze"), the assistant creates a shared diagnostic process. The user is not just a passive recipient of instructions but an active participant in data collection and analysis.
The Thinking Process Visible in the Reasoning
The assistant's thinking process, visible in the structure and content of message 410, reveals several cognitive patterns:
Hierarchical Decomposition: The assistant breaks the validation problem into layers — IOMMU groups (software/firmware boundary), IOMMU mode (kernel configuration), PCIe link (hardware negotiation), power management (fabric configuration), driver binding (device assignment), and finally application-level P2P test. This layered approach ensures that if the final test fails, the team can isolate which layer is responsible.
Temporal Optimization: The assistant uses the reboot wait time productively, preparing commands that can be executed immediately when the system comes up. This reflects an engineering mindset that values throughput and minimizes idle time.
Explicit Expectation Setting: The assistant provides specific expected outputs for each command — iommu=pt, Speed 32GT/s, Width x16, Kernel driver in use: vfio-pci, OK instead of NS. These expectations serve as test assertions, making it easy to spot deviations.
Cautious Optimism: The framing of Step 6 — "If we see OK instead of NS in the P2P matrix, we've won" — balances hope with realism. The "if" acknowledges uncertainty, while "we've won" provides motivation and a clear goal.
Progressive Scope: The commands start with a focused script (IOMMU groups), expand to system-wide checks (PCIe, power), and culminate in the VM test. This progressive expansion prevents information overload while building toward the ultimate validation.
Conclusion
Message 410 is a masterful example of diagnostic engineering under uncertainty. It transforms a waiting period into productive preparation, provides a hierarchical validation framework that can isolate failures at any layer of the stack, and establishes clear success criteria for a speculative BIOS change. Whether the ACS disable ultimately succeeds or fails, the validation commands in this message will provide definitive evidence, enabling the team to either celebrate victory or pivot to the next approach with minimal delay.
The deeper lesson is about the nature of debugging at the hardware-software boundary. In virtualization environments, the layers of abstraction — PCIe topology, IOMMU groups, VFIO passthrough, guest kernel, NVIDIA driver — create a complex system where a failure at any layer can manifest as a symptom at any other layer. Message 410 demonstrates a systematic approach to navigating this complexity: start with the most fundamental prerequisite, verify it unambiguously, and build upward from there. It's a methodology that applies far beyond this specific GPU P2P problem, and it's what makes this message worth studying in detail.