# QEMU (Quick Emulator) Cheat Sheet

## Basic Concepts

QEMU is a generic machine emulator and virtualizer that can run operating systems and programs for one machine on a different machine. It can be used in two main modes:
- Full system emulation: Emulates a complete computer system, including CPUs and peripherals
- User mode emulation: Runs programs compiled for one CPU on another CPU

## Installation

```bash
# Debian/Ubuntu
sudo apt install qemu-system qemu-utils

# RHEL/CentOS/Fedora
sudo dnf install qemu-kvm qemu-img

# Arch Linux
sudo pacman -S qemu qemu-arch-extra

# macOS (using Homebrew)
brew install qemu
```

## Image Management

### Creating Disk Images
```bash
# Create a raw disk image (20GB)
qemu-img create -f raw disk.img 20G

# Create a qcow2 disk image (more efficient, supports snapshots)
qemu-img create -f qcow2 disk.qcow2 20G

# Create a qcow2 with pre-allocation for better performance
qemu-img create -f qcow2 -o preallocation=metadata disk.qcow2 20G
```

### Converting Between Formats
```bash
# Convert from raw to qcow2
qemu-img convert -f raw -O qcow2 disk.img disk.qcow2

# Convert from qcow2 to raw
qemu-img convert -f qcow2 -O raw disk.qcow2 disk.img

# Convert from vmdk (VMware) to qcow2
qemu-img convert -f vmdk -O qcow2 disk.vmdk disk.qcow2
```

### Resizing Disk Images
```bash
# Resize a qcow2 image (increase by 10GB)
qemu-img resize disk.qcow2 +10G

# Note: After resizing, you must also resize the filesystem inside the VM
```

### Image Information
```bash
# Get information about an image
qemu-img info disk.qcow2

# Check disk image for errors
qemu-img check disk.qcow2
```

### Snapshots
```bash
# Create a snapshot
qemu-img snapshot -c snapshot_name disk.qcow2

# List snapshots
qemu-img snapshot -l disk.qcow2

# Restore a snapshot
qemu-img snapshot -a snapshot_name disk.qcow2

# Delete a snapshot
qemu-img snapshot -d snapshot_name disk.qcow2
```

## Basic VM Operation

### Starting a VM
```bash
# Basic VM launch (x86_64 architecture)
qemu-system-x86_64 -hda disk.qcow2 -m 2G

# Launch with KVM acceleration (much faster)
qemu-system-x86_64 -enable-kvm -hda disk.qcow2 -m 2G
```

### Installing an OS
```bash
# Boot from ISO and install to disk
qemu-system-x86_64 -enable-kvm \
  -m 2G \
  -cdrom os-installer.iso \
  -hda disk.qcow2 \
  -boot d
```

### Common Options
```bash
-m 2G                   # Allocate 2GB of memory
-smp 4                  # Use 4 CPU cores
-enable-kvm             # Use KVM hardware acceleration
-boot c|d|n             # Boot from disk (c), CD (d), or network (n)
-cdrom file.iso         # Connect CD-ROM to ISO file
-hda disk.qcow2         # Primary hard disk
-hdb disk2.qcow2        # Secondary hard disk
-cpu host               # Use host CPU model (best performance with KVM)
-display vnc=:0         # Use VNC display (connect to localhost:5900)
-display gtk            # Use GTK display (local window)
-device virtio-net-pci  # Use virtio network device
-nic user,hostfwd=tcp::2222-:22 # User-mode networking with port forwarding
```

## Networking Options

### User-mode Networking (simplest)
```bash
# Basic user-mode networking
qemu-system-x86_64 -enable-kvm -hda disk.qcow2 -m 2G -nic user

# With port forwarding (host port 2222 -> guest port 22)
qemu-system-x86_64 -enable-kvm -hda disk.qcow2 -m 2G \
  -nic user,hostfwd=tcp::2222-:22
```

### Bridged Networking
```bash
# Create a TAP interface (requires root)
sudo ip tuntap add dev tap0 mode tap user $USER
sudo ip link set tap0 up
sudo ip link set tap0 master br0

# Use TAP interface
qemu-system-x86_64 -enable-kvm -hda disk.qcow2 -m 2G \
  -nic tap,ifname=tap0,script=no,downscript=no
```

### Complex Network Configuration
```bash
# Multiple network interfaces
qemu-system-x86_64 -enable-kvm -hda disk.qcow2 -m 2G \
  -nic user,hostfwd=tcp::2222-:22 \
  -nic tap,ifname=tap0,script=no,downscript=no
```

## Storage Options

### Drive Options
```bash
# Connect disk to IDE bus
-hda disk.qcow2

# Connect disk to VirtIO (faster)
-drive file=disk.qcow2,if=virtio

# Read-only disk
-drive file=disk.img,if=virtio,readonly=on

# CD-ROM
-drive file=os.iso,media=cdrom

# Multiple disks with explicit index
-drive file=disk1.qcow2,if=virtio,index=0 \
-drive file=disk2.qcow2,if=virtio,index=1
```

### Using Physical Devices
```bash
# Use a physical disk (requires root)
sudo qemu-system-x86_64 -enable-kvm -m 2G -hda /dev/sdb

# Use a physical CD/DVD drive
qemu-system-x86_64 -enable-kvm -m 2G -cdrom /dev/cdrom -hda disk.qcow2
```

## Display and Input Options

### Display Options
```bash
# Local GUI window
-display gtk

# VNC server
-display vnc=:0  # Connect to localhost:5900

# Headless (no display)
-display none

# SPICE remote desktop protocol
-spice port=5930,addr=127.0.0.1,disable-ticketing
```

### Remote Access
```bash
# Connect to a QEMU VNC server
vncviewer localhost:5900

# Connect to a QEMU SPICE server
remote-viewer spice://localhost:5930
```

## Advanced Options

### CPU and Memory
```bash
# Pin vCPUs to physical cores
-smp 4,cores=2,threads=2,sockets=1 \
-cpu host,topoext \
-numa node,nodeid=0,cpus=0-1,mem=2G \
-numa node,nodeid=1,cpus=2-3,mem=2G

# Huge pages for better performance
-mem-path /dev/hugepages

# Memory hotplug
-m 4G,slots=3,maxmem=16G
```

### Device Passthrough (VFIO)
```bash
# Pass through a PCI device to VM
qemu-system-x86_64 -enable-kvm -m 4G \
  -hda disk.qcow2 \
  -device vfio-pci,host=01:00.0
```

### QEMU Monitor
```bash
# Access QEMU monitor (runtime control)
-monitor stdio

# Common monitor commands:
# info status     - Show VM status
# info cpus       - Show CPU info
# device_add      - Hot-add a device
# device_del      - Hot-remove a device
# system_powerdown - Send ACPI shutdown
# system_reset    - Reset the VM
# quit            - Quit QEMU
```

### QEMU + Libvirt
```bash
# Get QEMU command line from libvirt XML
virsh domxml-to-native qemu-argv --domain vm_name

# Dump QEMU monitor commands for VM
virsh qemu-monitor-command vm_name --hmp 'info status'
```

## Script Examples

### Basic VM Launch Script
```bash
#!/bin/bash
qemu-system-x86_64 \
  -name "My VM" \
  -enable-kvm \
  -cpu host \
  -smp 4 \
  -m 4G \
  -drive file=disk.qcow2,if=virtio \
  -drive file=os.iso,media=cdrom \
  -nic user,hostfwd=tcp::2222-:22 \
  -display gtk
```

### Headless Server VM
```bash
#!/bin/bash
qemu-system-x86_64 \
  -name "Server VM" \
  -enable-kvm \
  -cpu host \
  -smp 2 \
  -m 2G \
  -drive file=server.qcow2,if=virtio \
  -nic user,hostfwd=tcp::2222-:22,hostfwd=tcp::8080-:80 \
  -display none \
  -daemonize
```

## Debugging and Troubleshooting

```bash
# Trace all QEMU operations (very verbose)
qemu-system-x86_64 -d in_asm -D qemu.log [other options]

# Enable QEMU debug console
qemu-system-x86_64 -monitor stdio [other options]

# Get QEMU version and supported features
qemu-system-x86_64 --version

# List available emulated machines
qemu-system-x86_64 -machine help

# List available CPU models
qemu-system-x86_64 -cpu help
```

## QEMU Guest Agent

```bash
# Enable the QEMU Guest Agent
qemu-system-x86_64 -enable-kvm \
  -hda disk.qcow2 \
  -m 2G \
  -device virtio-serial \
  -chardev socket,path=/tmp/qga.sock,server,nowait,id=qga0 \
  -device virtserialport,chardev=qga0,name=org.qemu.guest_agent.0

# Install Guest Agent inside VM (Linux guest)
# sudo apt install qemu-guest-agent
# sudo systemctl start qemu-guest-agent
```