The plan looked textbook. PCIe passthrough an NVIDIA RTX A4000 from a Proxmox 8 host into an Ubuntu 24.04 VM, giving a Filecoin sealing worker direct GPU access for the compute phases that need it. VT-d on the Xeon. IOMMU enabled at the kernel line. OVMF BIOS on the VM. vfio-pci bound to the device IDs. Machine type set to q35 for PCIe support. Standard sequence, followed by every guide, ordered exactly as those guides ordered it.
Then qm start returned, and the VM did not come up.
Not a kernel panic. Not a host lockup. Not a "device not found" error surfaced to the operator. QEMU exited from the host's perspective and left nothing behind — no running guest, no console output, no obvious pointer at what had failed. The Proxmox web UI showed the VM as stopped, exactly as it had before I hit start.
The errors were there. They were just in a different terminal. dmesg -w on the host, running in a second window, produced a burst of IRQ allocation failures and vfio interrupt remapping errors scrolling past at VM startup, then silence. The host itself was completely fine throughout. Nothing was wrong with the box. Only the VM layer was broken, and it was broken in a way that read more like a misconfigured VM than a hardware conflict — which is exactly why the real cause took three sessions to identify.
