Update tech_docs/linux/pci_passthrough.md

tech_docs/linux/pci_passthrough.md

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+# Simple PCI(e) Passthrough Setup Guide
+
+## What is PCI(e) Passthrough?
+PCI(e) passthrough lets you give a virtual machine (VM) direct control over a physical device like a graphics card or network card. The VM gets native performance, but the host can't use that device anymore.
+
+## Before You Start - Check These Requirements
+
+### 1. Hardware Check
+- **CPU**: Must support IOMMU (Intel VT-d or AMD-Vi)
+- **Motherboard**: Must support IOMMU 
+- **Device**: The PCI device you want to pass through
+
+### 2. Quick Compatibility Test
+Run this command to see if IOMMU is working:
+```bash
+dmesg | grep -e DMAR -e IOMMU -e AMD-Vi
+```
+You should see something like "DMAR: IOMMU enabled" or "AMD-Vi: Interrupt remapping enabled"
+
+## Step-by-Step Setup
+
+### Step 1: Enable IOMMU in BIOS
+1. Reboot and enter BIOS/UEFI settings
+2. Look for and enable:
+   - **Intel**: "VT-d" or "Intel Virtualization Technology for Directed I/O"
+   - **AMD**: "AMD-Vi" or "IOMMU" (often enabled by default)
+3. Save and exit
+
+### Step 2: Enable IOMMU in Linux (Intel only)
+If you have an Intel CPU with older kernel, add this to your kernel command line:
+```
+intel_iommu=on
+```
+
+**Optional performance boost**: Add this for both Intel and AMD:
+```
+iommu=pt
+```
+
+### Step 3: Load Required Kernel Modules
+Add these lines to `/etc/modules`:
+```
+vfio
+vfio_iommu_type1
+vfio_pci
+```
+
+Then update initramfs:
+```bash
+update-initramfs -u -k all
+```
+
+### Step 4: Find Your Device
+List all PCI devices:
+```bash
+lspci -nn
+```
+
+Look for your device and note its ID (like `01:00.0`) and vendor:device codes (like `[10de:1d01]`).
+
+### Step 5: Reserve Device for Passthrough
+Create `/etc/modprobe.d/vfio.conf` and add:
+```
+options vfio-pci ids=VENDOR:DEVICE
+```
+
+Replace `VENDOR:DEVICE` with your actual codes (e.g., `10de:1d01` for an NVIDIA GPU).
+
+**Alternative method** - Blacklist the host driver:
+```bash
+# For NVIDIA GPUs
+echo "blacklist nouveau" >> /etc/modprobe.d/blacklist.conf
+
+# For AMD GPUs  
+echo "blacklist amdgpu" >> /etc/modprobe.d/blacklist.conf
+echo "blacklist radeon" >> /etc/modprobe.d/blacklist.conf
+
+# For Intel integrated graphics
+echo "blacklist i915" >> /etc/modprobe.d/blacklist.conf
+```
+
+### Step 6: Update and Reboot
+```bash
+update-initramfs -u -k all
+reboot
+```
+
+### Step 7: Verify Setup
+Check that your device is ready:
+```bash
+lspci -nnk
+```
+
+Look for your device - it should show `Kernel driver in use: vfio-pci` or no driver listed.
+
+### Step 8: Add Device to VM
+**Via Web Interface:**
+1. Go to your VM's Hardware tab
+2. Click "Add" → "PCI Device"
+3. Select your device
+4. For GPUs, check "Primary GPU" and "PCIe" options
+
+**Via Command Line:**
+```bash
+qm set VMID -hostpci0 01:00.0,pcie=on,x-vga=on
+```
+
+## GPU-Specific Tips
+
+### For Best GPU Compatibility:
+- Use **q35** machine type
+- Use **OVMF (UEFI)** instead of SeaBIOS if your GPU supports it
+- Enable **PCIe** mode instead of PCI
+
+### Common GPU Issues:
+
+**Black screen in VM console**: This is normal! Connect a monitor directly to the GPU or use remote desktop software inside the VM.
+
+**Error 43 (NVIDIA)**: Try these kernel parameters:
+```bash
+echo "options kvm ignore_msrs=1" > /etc/modprobe.d/kvm.conf
+```
+
+**Audio crackling**: Enable MSI in the guest:
+```bash
+echo "options snd-hda-intel enable_msi=1" >> /etc/modprobe.d/snd-hda-intel.conf
+```
+
+## Troubleshooting
+
+### Device Not Available
+- Check IOMMU groups: `pvesh get /nodes/NODENAME/hardware/pci --pci-class-blacklist ""`
+- Try moving the card to a different PCI slot
+- Ensure the device isn't being used by the host
+
+### System Won't Boot
+If you have issues after changes:
+1. Boot from recovery
+2. Remove the problematic config files
+3. Run `update-initramfs -u -k all`
+4. Reboot
+
+### Still Not Working?
+- Update your motherboard BIOS to the latest version
+- Check if your specific hardware combination is supported
+- Consider trying the ACS override patch (advanced users only)
+
+## Quick Reference Commands
+
+| Task | Command |
+|------|---------|
+| Check IOMMU status | `dmesg \| grep -e DMAR -e IOMMU` |
+| List PCI devices | `lspci -nn` |
+| Check device driver | `lspci -nnk` |
+| Update initramfs | `update-initramfs -u -k all` |
+| Add device to VM | `qm set VMID -hostpci0 01:00.0` |
+
+Remember: After any module or kernel parameter changes, always run `update-initramfs -u -k all` and reboot!
+
+---
+
 PCI(e) Passthrough
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