Preface

I’ve been using Linux seriously for 11 months now. From Debian to Arch, Linux has become part of my daily life.

People around me use QQ and WeChat. I still need them to send files and pictures, but I don’t want to work in Windows, so I installed VMware Workstation to run a virtual machine.

At first, running Windows 10 on Debian was smooth. After switching to Arch, it often stuttered because ntfs3g consumed too much CPU. Later I switched the host-side partition filesystem for the VM to Ext4, which helped a bit. After removing the built-in apps from Windows 10, things became much better.

KVM is short for Kernel-based Virtual Machine. It’s an open-source system virtualization module and a full virtualization solution for x86 hardware platforms on Linux.

QEMU is a powerful open-source emulator: it can emulate systems and a wide range of hardware.

guide_fig

With KVM acceleration, QEMU can achieve much better performance. I had never used QEMU before, so I spent a lot of time getting started and digging into details. I learned a lot in the process and gained a deeper understanding of virtualization.

IBM DeveloperWorks (CN) is full of high-quality content, along with official docs and wikis for many products. The built-in man pages are also very detailed.

It’s thanks to developers’ great contributions that I can work and have fun on Linux so conveniently. I’m also a bit ashamed that I’m not yet in a position to contribute back.

Installation

To make learning easier, I decided not to use a GUI at first. (Later I realized I was too naive: in a real environment there’s far more configuration than I expected, and it’s much more complex—CPU, keyboard, USB, audio, etc. are all tricky to get right.)

sudo pacman -S qemu

But installing a GUI frontend is still a good idea. I tried qtemu and qemu-launcher; aqemu feels more usable.

yaourt aqemu

Adding a Disk

Creating a disk image

QEMU supports many image formats, including VMDK, VDI, VHD (vpc), VHDX, qcow1, and QED. You can manage them with qemu-img.

In practice, raw and qcow2 are most commonly used.

raw is a raw disk image format. It’s the fastest, but it allocates the disk space immediately.

qcow2 is a bit slower, but it doesn’t need to allocate space upfront and supports many features such as snapshots, rollback, encryption, and compression.

For example, create a raw disk image:

qemu-img create -f raw Kali_Linux.img 20G

Or create a qcow2 image with 2MB cluster size and full preallocation:

qemu-img create -f qcow2 -o cluster_size=2M,preallocation=full Kali_Linux.qcow2 4G

Format conversion

qemu-img also supports converting disk image formats. In some cases, you need to migrate disk images.

For example, I wanted to compare Kali Linux performance under KVM vs VMware, so I needed to convert a VMDK into a disk image format that’s more suitable for QEMU.

Convert vmdk to raw

qemu-img convert -f vmdk -O raw Kali\ 2016\ 64-bit.vmdk Kali_Linux.img

Convert vmdk to qcow2

qemu-img convert -f vmdk -O qcow2 Kali\ 2016\ 64-bit.vmdk Kali_Linux.qcow2

Enabling hardware-assisted virtualization

Make sure VT is enabled in the BIOS. My machine is Intel-based, so I added intel_iommu=on to the kernel parameters. For AMD, use amd_iommu=on.

IOMMU essentially allows mapping hardware devices directly into a VM, addressing peripheral device support and improving hardware utilization.

Edit /boot/grub/grub.cfg:

linux   /boot/vmlinuz-linux root=UUID=6bb8eda5-d588-4e6a-8f5e-bbc856cc9f96 rw quiet intel_iommu=on

After rebooting, verify that IOMMU is enabled:

gorgias@3vil ~> dmesg | grep -e DMAR -e IOMMU
[    0.000000] DMAR: IOMMU enabled

Install a graphics driver—here I’ll try VMware’s first. After installing, the mouse-cursor issue is fixed, but when entering the desktop you may see a few seconds of screen corruption.

sudo pacman -S xf86-video-vmware xf86-input-vmmouse

Try launching the VM with these parameters: use the Kali_Linux.qcow2 disk image, 2GB memory, VMware VGA, emulated Q35 chipset, KVM acceleration, and enable IOMMU.

qemu-system-x86_64 /home/gorgias/media/Lancer/KVM/Kali\ Linux/Kali_Linux.qcow2 -enable-kvm -m 2048 -vga vmware -machine q35,accel=kvm,type=q35 -device intel-iommu

It feels quite smooth.

demo

QEMU monitor

QEMU provides a monitor for inspecting a VM’s runtime state.

In the QEMU window, press Ctrl + Alt + 2 (not on the numpad) to switch to the QEMU monitor. If it doesn’t work, check whether the shortcut conflicts with something else.

You can also add -monitor dev when launching the VM. For example, -monitor stdio allows standard input as the monitor command source.

In the monitor, run info kvm to check whether the KVM module is enabled: