# GPU Passthrough (VFIO) Notes

Host/guest checklist for GPU passthrough (or virtual GPU) when building Debian VMs in Proxmox / virt-manager style setups.

---

## Host: Pick GPU and enable passthrough

List GPUs on the host:

```bash
lspci | egrep -i 'vga|3d|display'
```

Note the PCI ID of the card you want to pass, e.g. `0000:01:00.0`.

On the host, enable IOMMU and passthrough (Proxmox/virt-manager style):

- Add appropriate kernel params (example Intel):

```text
intel_iommu=on iommu=pt
```

- Reboot and verify:

```bash
dmesg | egrep -i 'iommu|dmar'
```

Bind the GPU to `vfio` (or equivalent passthrough driver) on the host so the VM can own it exclusively.

---

## Host: Create the VM with a VGA/GPU device

Create a new VM (Proxmox or virt-manager):

- **Firmware**: UEFI/OVMF if possible
- **Machine type**: Q35/modern
- **CPU**: host-passthrough (or similar)

Add GPU to the VM:

- Add PCI device: select your GPU (and its audio function if present)
- For a purely virtual GPU instead (no passthrough), pick Virtio GPU or QXL as the display adapter

Add display channel:

- If you want a normal graphical console, use SPICE (virt-manager) or Proxmox’s standard console display
- Add a Channel (spice) device for clipboard/resize when using SPICE

---

## Guest: Verify what GPU the VM sees

Inside the guest OS (your Debian VM):

```bash
lspci | egrep -i 'vga|3d|display'
lsmod | egrep 'nvidia|amdgpu|radeon|virtio_gpu'
```

Interpretation:

- `NVIDIA Corporation ...` → real NVIDIA passthrough
- `Advanced Micro Devices, Inc. [AMD/ATI] ...` → AMD passthrough
- `Red Hat, Inc. Virtio GPU` or `QXL` → virtual GPU only

---

## Guest: Install and test drivers

### If NVIDIA passthrough

Install driver and tools:

```bash
sudo apt update
sudo apt install nvidia-driver-535 nvidia-smi
```

(Adjust version to what Debian suggests.)

Test:

```bash
nvidia-smi
watch -n 1 nvidia-smi
```

The watch line must be `watch -n 1 nvidia-smi`.

Quick 3D check:

```bash
sudo apt install mesa-utils
glxgears
```

GPU utilization should rise in `nvidia-smi` while `glxgears` runs.

### If AMD passthrough

Inspect and ensure `amdgpu` is used:

```bash
sudo lshw -c video
lsmod | grep amdgpu
```

Install tools:

```bash
sudo apt install mesa-utils
# optionally:
sudo apt install amdgpu-tools  # or ROCm SMI tools if available
```

Monitor:

```bash
watch -n 1 sudo amd-smi
```

(Or the ROCm SMI CLI in your distro.)

### If only Virtio/QXL is present

- You have a virtual GPU, not the physical one.
- You can still use `glxgears` and basic acceleration, but host GPU usage is not controlled directly from inside this VM.
- To use the physical card, change the VM to PCI passthrough as above.

---

## Guest: Console and SPICE/GUI behavior

- With SPICE display and the spice channel present, dynamic resolution and clipboard should work once guest tools are installed.
- If a graphical display manager (e.g. LightDM) misbehaves, you can force a TTY-only boot:

```bash
sudo systemctl set-default multi-user.target
sudo systemctl disable --now lightdm.service
```

This gives you a reliable text console, and you can work purely over SSH for GPU workloads.