# KVM (Kernel-based Virtual Machine) Cheat Sheet

## Installation

### Check KVM Support
```bash
# Check if CPU supports virtualization
egrep -c '(vmx|svm)' /proc/cpuinfo

# Check if KVM modules are loaded
lsmod | grep kvm
```

### Install KVM and Related Tools
```bash
# Debian/Ubuntu
sudo apt update
sudo apt install qemu-kvm libvirt-daemon-system libvirt-clients bridge-utils virt-manager

# RHEL/CentOS/Fedora
sudo dnf install @virtualization
```

## User Management

```bash
# Add user to libvirt group
sudo usermod -aG libvirt $USER
sudo usermod -aG kvm $USER

# Restart libvirtd service
sudo systemctl restart libvirtd
```

## VM Management with virsh

### Basic VM Operations
```bash
# List all VMs
virsh list --all

# Start a VM
virsh start vm_name

# Gracefully shutdown a VM
virsh shutdown vm_name

# Force shutdown a VM
virsh destroy vm_name

# Reboot a VM
virsh reboot vm_name

# Delete a VM (and its storage)
virsh undefine vm_name --remove-all-storage

# Suspend/resume a VM
virsh suspend vm_name
virsh resume vm_name
```

### VM Information
```bash
# Get VM information
virsh dominfo vm_name

# View VM XML configuration
virsh dumpxml vm_name

# Edit VM XML configuration
virsh edit vm_name

# Display VM console
virsh console vm_name
```

### VM Creation
```bash
# Create VM from an XML file
virsh define vm_config.xml

# Clone an existing VM
virt-clone --original source_vm --name new_vm --auto-clone
```

## Storage Management

### Storage Pools
```bash
# List storage pools
virsh pool-list --all

# Define new storage pool from XML
virsh pool-define pool_config.xml

# Start storage pool
virsh pool-start pool_name

# Enable autostart for pool
virsh pool-autostart pool_name

# Refresh storage pool
virsh pool-refresh pool_name
```

### Storage Volumes
```bash
# List volumes in a pool
virsh vol-list pool_name

# Create volume in a pool
virsh vol-create-as pool_name vol_name 10G

# Delete a volume
virsh vol-delete vol_name --pool pool_name

# Resize a volume
virsh vol-resize vol_name new_size --pool pool_name
```

## Network Management

### Virtual Networks
```bash
# List all networks
virsh net-list --all

# Start a network
virsh net-start network_name

# Enable autostart for network
virsh net-autostart network_name

# View network XML
virsh net-dumpxml network_name

# Edit network configuration
virsh net-edit network_name
```

## Snapshots

### VM Snapshots
```bash
# Create a snapshot
virsh snapshot-create-as vm_name snapshot_name --description "description"

# List snapshots
virsh snapshot-list vm_name

# View snapshot details
virsh snapshot-info vm_name snapshot_name

# Revert to snapshot
virsh snapshot-revert vm_name snapshot_name

# Delete snapshot
virsh snapshot-delete vm_name snapshot_name
```

## Performance Monitoring & Tuning

```bash
# Show VM resource usage
virt-top

# Display VM I/O statistics
virsh domstats vm_name --block

# Set VM memory
virsh setmem vm_name 2G --config --live

# Set VM vCPUs
virsh setvcpus vm_name 2 --config --live
```

## Backup & Migration

### Backup
```bash
# Export VM definition
virsh dumpxml vm_name > vm_backup.xml

# Create disk backup
sudo dd if=/var/lib/libvirt/images/vm_disk.qcow2 of=/backup/vm_disk.qcow2 bs=1M
```

### Migration
```bash
# Live migration (shared storage)
virsh migrate --live vm_name qemu+ssh://destination_host/system

# Migration with storage
virsh migrate --live --copy-storage-all vm_name qemu+ssh://destination_host/system
```

## Troubleshooting

```bash
# Check libvirt service status
sudo systemctl status libvirtd

# View VM logs
virsh domblklist vm_name
sudo tail -f /var/log/libvirt/qemu/vm_name.log

# Check VM process status
ps aux | grep vm_name

# Debug connection issues
virsh -c qemu:///system
```

## Advanced Features

### CPU Pinning
```bash
# Set CPU affinity
virsh vcpupin vm_name vcpu_number host_cpu_list
```

### Nested Virtualization
```bash
# Enable nested virtualization (Intel)
echo "options kvm-intel nested=1" | sudo tee /etc/modprobe.d/kvm-intel.conf
sudo modprobe -r kvm_intel && sudo modprobe kvm_intel

# Enable nested virtualization (AMD)
echo "options kvm-amd nested=1" | sudo tee /etc/modprobe.d/kvm-amd.conf
sudo modprobe -r kvm_amd && sudo modprobe kvm_amd
```

### QEMU Guest Agent
```bash
# Install in guest (Debian/Ubuntu)
sudo apt install qemu-guest-agent
sudo systemctl start qemu-guest-agent

# Get guest information
virsh qemu-agent-command vm_name '{"execute":"guest-info"}'
```

## Image Management

```bash
# Convert between image formats
qemu-img convert -f raw -O qcow2 source.img target.qcow2

# Resize an image
qemu-img resize vm_disk.qcow2 +10G

# Create a new image
qemu-img create -f qcow2 new_disk.qcow2 20G

# Inspect image information
qemu-img info vm_disk.qcow2
```