QEMU

QEMU🔗

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emulation tools and standard virtual hardware: processors, network interfaces, graphics cards, …
can be used to create VMs, especially for old or uncommon hardware (ARM, PowerPC, SPARC, …) but emulation is slow
if processor supports virtualisation, QEMU can use KVM to directly access the systems hardware with near native performance

Commands🔗

Images🔗

qemu-img: create and handle images ^qemu-img

# create disk image (thin provisioning)
qemu-img create <disk image>.qcow2 -f qcow2 100G
qemu-img resize 80G <disk image>.qcow2
qemu-img resize +10G <disk image>.qcow2
# overlay
qemu-img info diskimage.qcow2
qemu-img rebase -b /new/path/backing.qcow2 overlay.qcow2

growing disk: expand filesystem after
shrinking disk: resize filesystem first
disk operations are risky - backup!

qemu-nbd: mount and inspect images as network storage devices (nbd) that can then be mounted the regular way ^qemu-nbd

# verify nbd kernel module is loaded
sudo modprobe lbd
# or
lsmod | grep nbd
# connect disk image as nbd
sudo qemu-nbd --conect /dev/nbd0 /path/to/disk.qcow2
sudo mount /dev/nbd0 /mnt/point
# dismountd and isconnect
sudo umount /mnt/point
sudo qemu -d /dev/nbd0

System: create and run VMs🔗

qemu-system-<architecture>: create VM

qemu-system-x86_64 \                     # Create a guest with x86_64 architecture
    -enable-kvm \                        # Use KVM instead of CPU emulation
    -cpu host \                          # Give guest same model CPU as host
    -smp 4 \                             # 4 cores, symmetric/shared-memory multiprocessing[^smp]
    -m 4096 \                            # provide the VM 4GB of RAM
    -k en-us \                           # keyboard layout
    -vnc :0 \                            # make display available at port 5900 on host
    -drive file=<disk image>,if=virtio \ # use a file for the system's hard disk with paravirtualisation
    -cdrom <iso image> \                 # present disk image as CD-ROM
    -boot d \                            # boot from CD-ROM device
    -usbdevice table                     # improve mousing

CPU architecture: SMP[^SMP] or ¹
Access display through VNC viewer remote desktop viewer. 0 refers to last digit of port 5900, use 1, 2 and so forth for higher ports in case of multiple guests ^vnc
boot device (letters from DOS)
- d: CD-ROM device
- c: first hard drive
after installation, start machine by dropping last 2 arguments to boot from disk

verify guest system hardware, e.g. with

# General info
hostnamectl
# Output
# Virtualization: kvm
# CPU info
lscpu
# device info
lspci
# usb info
lsusb
# memory info
lsmem

monitor and debug with option -monitor stdio, enter help for available commands
- change_vnc_password together with -vnc :0,password=on
- system_powerdown

Display🔗

Use VNC as described above or if guest has GUI, local video can be used alternatively, providing much improved performance.

-display options: GTK, SDL, VNC, curses, none

qemu-system \
    -display gtk \
    -vga std \
    ...
# -display
#   none : attach no dispoolay (still has a video card that can be connected to)
# curses : text-based console, not suitable for GUI
# vnc    : over network, flexible but poor performance
# sdl    : Simple DirectMedia Layer (local graphical window) suitable for non-Linux hosts
# gtk    : GNOME ToolKit (local graphical window)
#   gtk and sdl support OpenGL with ,gl=on
#
# -vga
#  std   : standard, moderate resolutions, low VRAM

See Images for creating and inter-operating with disk image files.

File-sharing between guest and host: Plan 9 Filesystem Protocol (9P)

qemu-system-x86_64 \
    ...
    -virtfs local,path=$PWD/shared,mount_tag=shared,security_model=mapped
    # local: other options exist but are more for developing/testing
    # security_model=mapped : short for mapped xattr, file ownership derived from user running guest

On guest mount with

# use mount_tag previously defined with qemu
sudo mount -t 9p -o trans=virtio $mount_tag $mount_point -o version=9p2000.L

Check extended attributes on host with getfattr -d shared/file.txt.

Devices🔗

Host hardware can be passed through to guest.

USB🔗

v1.1, v2 or v3 (using xHCI if supported)
option -usbdevice uses USB 1.1

set up bus and connect device

# find usb device on host: bus, device/port, vendor id, product id
lsusb
# Create guest bus and connect device:
# Accessing host USB devices requires root or setting up udev for user.
# NOTE: -display gtk did not work for me with sudo (missing display permissions?), used -vnc :0 for this
sudo qemu-system \
    ... \
    -vnc :0 \
    -device qemu-xhci,id=xhci \
    -device usb-host,bus=xhci.0,vendorid=0x$vendorid,productid=0x$productid
# or access USB port (bus and device in lsusb output)
    -device usb-host,bus=xhci.0,hostbus=1,hostport=5

if device is unplugged/not plugged in, QEMU keeps checking for it
to make device available to host again, guest has to be shut down

Networking🔗

user-mode networking on private NAT

Role	IP
DHCP	10.0.2.2
DNS (forwarding)	10.0.2.3
SMB	10.0.2.4
Guest	10.0.2.15

References🔗

See glossary ↩
See glossary ↩