QEMU Virtual Machine Guide

Run a complete VM with just 4 commands

QEMU is a machine emulator and virtualizer. It can emulate entire systems, including CPU, memory, and devices — running any OS on any CPU architecture.

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Modes of Operation

QEMU Without KVM

• QEMU emulates every CPU instruction in software.
• Works on any host CPU, regardless of architecture.
• Very slow performance — good for testing, debugging, or simulation.

QEMU With KVM

• QEMU offloads CPU execution to hardware via KVM (Kernel-based Virtual Machine).
• Performance close to native (up to 10× faster).
• Requires CPU virtualization support:

Vendor	Feature	Description
Intel	VT-x (vmx)	Hardware virtualization for Intel CPUs
AMD	AMD-V (svm)	Hardware virtualization for AMD CPUs

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Create a Virtual Disk

qemu-img create -f qcow2 win11.img 30G

• qcow2 format supports snapshots, compression, and dynamic sizing.

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Install Operating System

qemu-system-x86_64 \
  -enable-kvm \
  -cpu host \
  -m 6G \
  -smp 4 \
  -hda win11.img \
  -boot d \
  -cdrom windows_11.iso

Notes:

• -enable-kvm → Use hardware acceleration.
• -boot d → Boot from CD-ROM for OS installation.
• -smp 4 → Allocate 4 CPU cores.
• -m 6G → Allocate 6 GB RAM.

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Run the Installed OS with SPICE (QXL Display)

qemu-system-x86_64 \
  -enable-kvm \
  -cpu host \
  -m 6G \
  -smp 4 \
  -device qxl-vga \
  -hda win11.img \
  -boot c \
  -spice port=5905,addr=127.0.0.1,disable-ticketing=on \
  -device virtio-serial-pci \
  -chardev spicevmc,id=vdagent,debug=0,name=vdagent \
  -device virtserialport,chardev=vdagent,name=com.redhat.spice.0 \
  -display none

What this does:

• Uses QXL video driver for better graphics performance.
• Enables SPICE remote desktop for high-speed display and clipboard sharing.
• Adds SPICE agent for mouse sync, clipboard, and resolution change.

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Connect to the VM via SPICE

On the Host:

sudo apt install virt-viewer
remote-viewer spice://127.0.0.1:5905

Inside the Guest:

Install SPICE Guest Tools: https://www.spice-space.org/download.html

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Exploring QEMU Features

Disk Options

Option	Description
-hda win10.img	Simple legacy method to attach a disk
-drive file=win10.img,format=qcow2	Recommended modern syntax (supports snapshots)

Use -drive syntax — it’s more flexible and supports multiple disks easily.

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CPU Emulation

You can emulate or expose specific CPU models:

qemu-system-x86_64 -enable-kvm -cpu Haswell -m 6G -smp 4 -hda win11.img -boot c

• -cpu EPYC → emulate AMD CPU
• -cpu Haswell → emulate Intel CPU
• -cpu host → pass host CPU features directly
• You can even emulate 32-bit CPUs on a 64-bit host.

Check all options:

qemu-system-x86_64 -cpu help

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Networking Modes

Mode	Description	Visibility	Example
user	Built-in NAT mode (no root needed)	VM behind NAT	-nic user,model=virtio-net-pci
bridge	Connects VM to host LAN (same network)	Visible on LAN	-nic bridge,br=br0,model=virtio-net-pci
tap	Direct virtual Ethernet interface	Fastest	-netdev tap,id=n0,ifname=tap0 -device virtio-net-pci,netdev=n0

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TUN vs TAP

Device	Layer	Use Case
TUN	Layer 3 (IP)	Used by VPNs for IP tunneling
TAP	Layer 2 (Ethernet)	Used for VMs, bridges, and containers

Think of:

• TAP → virtual Ethernet cable
• TUN → virtual IP pipe

Create a TAP device manually:

sudo ip tuntap add dev tap0 mode tap user $USER

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Port Forwarding (for SSH or HTTP)

Example:

-device virtio-net-pci,netdev=n0  -netdev user,id=n0,hostfwd=tcp::2222-:22

Maps:

• Host port 2222 → Guest port 22

Then you can SSH into the VM:

ssh -p 2222 [email protected]

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Summary

• QEMU without KVM → universal but slow (pure software).
• QEMU with KVM → near-native performance (hardware-accelerated).
• QXL + SPICE → better graphics, mouse sync, clipboard sharing.
• qcow2 → supports snapshots and compression.
• VirtIO → faster disk and network drivers.
• Create and run a VM → with just 4 commands