The error pointed at the GPU. The culprit was the HBA. — a Proxmox passthrough recovery

The plan for setting up an NVIDIA RTX A4000 GPU passthrough on a Proxmox 8 host into an Ubuntu 24.04 VM was executed flawlessly, providing direct GPU access for specific computational tasks. However, despite following the standard procedure and adhering to the guides, the VM failed to start, leaving no indication of what went wrong. The host appeared to function correctly, displaying the VM as stopped, similar to its status before initiation.

Dmesg logs revealed IRQ allocation failures and vfio interrupt remapping errors during VM startup. Initially, the error was attributed to the GPU, but it was later discovered that the culprit was the Host Bus Adapter (HBA). The A4000 GPU was intended to accelerate a specific phase of the Filecoin sealing process, which involved GPU-eligible TreeRC computations. This GPU was crucial for completing tree building computations significantly faster than the CPU could handle.

The investigation into the VM startup issue spanned three sessions over a week, with each session eliminating plausible explanations without yielding the actual cause. In the first session, the focus was on verifying GPU binding and the correctness of the VM configuration. The GPU was confirmed to be properly bound to vfio-pci, and the VM configuration appeared correct.

Session two delved into examining driver blacklists, IOMMU group mappings, and kernel module updates, all of which were found to be correctly configured. However, the IOMMU group listing revealed an unexpected presence of an IBM SAS2008 HBA in the same group as the A4000 GPU. This shared-group membership posed a challenge, as all devices within an IOMMU group are treated as an atomic unit during passthrough.

The shared-group membership became the key factor in the passthrough failure, a detail that initially went unnoticed but later proved instrumental in resolving the issue.