https://pve.proxmox.com/wiki/PCI_Passthrough
- check this
'Legacy boot' or CSM: For GPU passthrough it can help to disable this, but keep in mind that PVE has to be installed in UEFI mode, as it will not boot in BIOS mode without this enabled. The reason for disabling this is that it avoids legacy VGA initialization of installed GPUs, making them able to be re-initialized later, as required for passthrough. Most useful when trying to use passthrough in single GPU systems.
https://forum.proxmox.com/threads/gpu-passthrough-with-rtx-3090-doesnt-work-dmar-errors.120677/