4 Using the Quick EMUlator (QEMU)

The Yocto Project uses an implementation of the Quick EMUlator (QEMU) Open Source project as part of the Yocto Project development “tool set”. This chapter provides both procedures that show you how to use the Quick EMUlator (QEMU) and other QEMU information helpful for development purposes.

4.1 Overview

Within the context of the Yocto Project, QEMU is an emulator and virtualization machine that allows you to run a complete image you have built using the Yocto Project as just another task on your build system. QEMU is useful for running and testing images and applications on supported Yocto Project architectures without having actual hardware. Among other things, the Yocto Project uses QEMU to run automated Quality Assurance (QA) tests on final images shipped with each release.

Note

This implementation is not the same as QEMU in general.

This section provides a brief reference for the Yocto Project implementation of QEMU.

For official information and documentation on QEMU in general, see the following references:

4.2 Running QEMU

To use QEMU, you need to have QEMU installed and initialized as well as have the proper artifacts (i.e. image files and root filesystems) available. Follow these general steps to run QEMU:

  1. Install QEMU: QEMU is made available with the Yocto Project a number of ways. One method is to install a Software Development Kit (SDK). See “The QEMU Emulator” section in the Yocto Project Application Development and the Extensible Software Development Kit (eSDK) manual for information on how to install QEMU.

  2. Setting Up the Environment: How you set up the QEMU environment depends on how you installed QEMU:

    • If you cloned the poky repository or you downloaded and unpacked a Yocto Project release tarball, you can source the build environment script (i.e. oe-init-build-env):

      $ cd poky
      $ source oe-init-build-env
      
    • If you installed a cross-toolchain, you can run the script that initializes the toolchain. For example, the following commands run the initialization script from the default poky_sdk directory:

      . poky_sdk/environment-setup-core2-64-poky-linux
      
  3. Ensure the Artifacts are in Place: You need to be sure you have a pre-built kernel that will boot in QEMU. You also need the target root filesystem for your target machine’s architecture:

    • If you have previously built an image for QEMU (e.g. qemux86, qemuarm, and so forth), then the artifacts are in place in your Build Directory.

    • If you have not built an image, you can go to the machines/qemu area and download a pre-built image that matches your architecture and can be run on QEMU.

    See the “Extracting the Root Filesystem” section in the Yocto Project Application Development and the Extensible Software Development Kit (eSDK) manual for information on how to extract a root filesystem.

  4. Run QEMU: The basic runqemu command syntax is as follows:

    $ runqemu [option ] [...]
    

    Based on what you provide on the command line, runqemu does a good job of figuring out what you are trying to do. For example, by default, QEMU looks for the most recently built image according to the timestamp when it needs to look for an image. Minimally, through the use of options, you must provide either a machine name, a virtual machine image (*wic.vmdk), or a kernel image (*.bin).

    Here are some additional examples to help illustrate further QEMU:

    • This example starts QEMU with MACHINE set to “qemux86-64”. Assuming a standard Build Directory, runqemu automatically finds the bzImage-qemux86-64.bin image file and the core-image-minimal-qemux86-64-20200218002850.rootfs.ext4 (assuming the current build created a core-image-minimal image).

      Note

      When more than one image with the same name exists, QEMU finds and uses the most recently built image according to the timestamp.

      $ runqemu qemux86-64
      
    • This example produces the exact same results as the previous example. This command, however, specifically provides the image and root filesystem type.

      $ runqemu qemux86-64 core-image-minimal ext4
      
    • This example specifies to boot an initial RAM disk image and to enable audio in QEMU. For this case, runqemu set the internal variable FSTYPE to “cpio.gz”. Also, for audio to be enabled, an appropriate driver must be installed (see the previous description for the audio option for more information).

      $ runqemu qemux86-64 ramfs audio
      
    • This example does not provide enough information for QEMU to launch. While the command does provide a root filesystem type, it must also minimally provide a MACHINE, KERNEL, or VM option.

      $ runqemu ext4
      
    • This example specifies to boot a virtual machine image (.wic.vmdk file). From the .wic.vmdk, runqemu determines the QEMU architecture (MACHINE) to be “qemux86-64” and the root filesystem type to be “vmdk”.

      $ runqemu /home/scott-lenovo/vm/core-image-minimal-qemux86-64.wic.vmdk
      

4.3 Switching Between Consoles

When booting or running QEMU, you can switch between supported consoles by using Ctrl+Alt+number. For example, Ctrl+Alt+3 switches you to the serial console as long as that console is enabled. Being able to switch consoles is helpful, for example, if the main QEMU console breaks for some reason.

Note

Usually, “2” gets you to the main console and “3” gets you to the serial console.

4.4 Removing the Splash Screen

You can remove the splash screen when QEMU is booting by using Alt+left. Removing the splash screen allows you to see what is happening in the background.

4.5 Disabling the Cursor Grab

The default QEMU integration captures the cursor within the main window. It does this since standard mouse devices only provide relative input and not absolute coordinates. You then have to break out of the grab using the “Ctrl+Alt” key combination. However, the Yocto Project’s integration of QEMU enables the wacom USB touch pad driver by default to allow input of absolute coordinates. This default means that the mouse can enter and leave the main window without the grab taking effect leading to a better user experience.

4.6 Running Under a Network File System (NFS) Server

One method for running QEMU is to run it on an NFS server. This is useful when you need to access the same file system from both the build and the emulated system at the same time. It is also worth noting that the system does not need root privileges to run. It uses a user space NFS server to avoid that. Follow these steps to set up for running QEMU using an NFS server.

  1. Extract a Root Filesystem: Once you are able to run QEMU in your environment, you can use the runqemu-extract-sdk script, which is located in the scripts directory along with the runqemu script.

    The runqemu-extract-sdk takes a root filesystem tarball and extracts it into a location that you specify. Here is an example that takes a file system and extracts it to a directory named test-nfs:

    runqemu-extract-sdk ./tmp/deploy/images/qemux86-64/core-image-sato-qemux86-64.tar.bz2 test-nfs
    
  2. Start QEMU: Once you have extracted the file system, you can run runqemu normally with the additional location of the file system. You can then also make changes to the files within ./test-nfs and see those changes appear in the image in real time. Here is an example using the qemux86 image:

    runqemu qemux86-64 ./test-nfs
    

Note

Should you need to start, stop, or restart the NFS share, you can use the following commands:

  • The following command starts the NFS share:

    runqemu-export-rootfs start file-system-location
    
  • The following command stops the NFS share:

    runqemu-export-rootfs stop file-system-location
    
  • The following command restarts the NFS share:

    runqemu-export-rootfs restart file-system-location
    

4.7 QEMU CPU Compatibility Under KVM

By default, the QEMU build compiles for and targets 64-bit and x86 Intel Core2 Duo processors and 32-bit x86 Intel Pentium II processors. QEMU builds for and targets these CPU types because they display a broad range of CPU feature compatibility with many commonly used CPUs.

Despite this broad range of compatibility, the CPUs could support a feature that your host CPU does not support. Although this situation is not a problem when QEMU uses software emulation of the feature, it can be a problem when QEMU is running with KVM enabled. Specifically, software compiled with a certain CPU feature crashes when run on a CPU under KVM that does not support that feature. To work around this problem, you can override QEMU’s runtime CPU setting by changing the QB_CPU_KVM variable in qemuboot.conf in the Build Directory deploy/image directory. This setting specifies a -cpu option passed into QEMU in the runqemu script. Running qemu -cpu help returns a list of available supported CPU types.

4.8 QEMU Performance

Using QEMU to emulate your hardware can result in speed issues depending on the target and host architecture mix. For example, using the qemux86 image in the emulator on an Intel-based 32-bit (x86) host machine is fast because the target and host architectures match. On the other hand, using the qemuarm image on the same Intel-based host can be slower. But, you still achieve faithful emulation of ARM-specific issues.

To speed things up, the QEMU images support using distcc to call a cross-compiler outside the emulated system. If you used runqemu to start QEMU, and the distccd application is present on the host system, any BitBake cross-compiling toolchain available from the build system is automatically used from within QEMU simply by calling distcc. You can accomplish this by defining the cross-compiler variable (e.g. export CC="distcc"). Alternatively, if you are using a suitable SDK image or the appropriate stand-alone toolchain is present, the toolchain is also automatically used.

Note

Several mechanisms exist that let you connect to the system running on the QEMU emulator:

  • QEMU provides a framebuffer interface that makes standard consoles available.

  • Generally, headless embedded devices have a serial port. If so, you can configure the operating system of the running image to use that port to run a console. The connection uses standard IP networking.

  • SSH servers exist in some QEMU images. The core-image-sato QEMU image has a Dropbear secure shell (SSH) server that runs with the root password disabled. The core-image-full-cmdline and core-image-lsb QEMU images have OpenSSH instead of Dropbear. Including these SSH servers allow you to use standard ssh and scp commands. The core-image-minimal QEMU image, however, contains no SSH server.

  • You can use a provided, user-space NFS server to boot the QEMU session using a local copy of the root filesystem on the host. In order to make this connection, you must extract a root filesystem tarball by using the runqemu-extract-sdk command. After running the command, you must then point the runqemu script to the extracted directory instead of a root filesystem image file. See the “Running Under a Network File System (NFS) Server” section for more information.

4.9 QEMU Command-Line Syntax

The basic runqemu command syntax is as follows:

$ runqemu [option ] [...]

Based on what you provide on the command line, runqemu does a good job of figuring out what you are trying to do. For example, by default, QEMU looks for the most recently built image according to the timestamp when it needs to look for an image. Minimally, through the use of options, you must provide either a machine name, a virtual machine image (*wic.vmdk), or a kernel image (*.bin).

Following is the command-line help output for the runqemu command:

$ runqemu --help

Usage: you can run this script with any valid combination
of the following environment variables (in any order):
  KERNEL - the kernel image file to use
  ROOTFS - the rootfs image file or nfsroot directory to use
  MACHINE - the machine name (optional, autodetected from KERNEL filename if unspecified)
  Simplified QEMU command-line options can be passed with:
    nographic - disable video console
    serial - enable a serial console on /dev/ttyS0
    slirp - enable user networking, no root privileges is required
    kvm - enable KVM when running x86/x86_64 (VT-capable CPU required)
    kvm-vhost - enable KVM with vhost when running x86/x86_64 (VT-capable CPU required)
    publicvnc - enable a VNC server open to all hosts
    audio - enable audio
    [*/]ovmf* - OVMF firmware file or base name for booting with UEFI
  tcpserial=<port> - specify tcp serial port number
  biosdir=<dir> - specify custom bios dir
  biosfilename=<filename> - specify bios filename
  qemuparams=<xyz> - specify custom parameters to QEMU
  bootparams=<xyz> - specify custom kernel parameters during boot
  help, -h, --help: print this text

Examples:
  runqemu
  runqemu qemuarm
  runqemu tmp/deploy/images/qemuarm
  runqemu tmp/deploy/images/qemux86/<qemuboot.conf>
  runqemu qemux86-64 core-image-sato ext4
  runqemu qemux86-64 wic-image-minimal wic
  runqemu path/to/bzImage-qemux86.bin path/to/nfsrootdir/ serial
  runqemu qemux86 iso/hddimg/wic.vmdk/wic.qcow2/wic.vdi/ramfs/cpio.gz...
  runqemu qemux86 qemuparams="-m 256"
  runqemu qemux86 bootparams="psplash=false"
  runqemu path/to/<image>-<machine>.wic
  runqemu path/to/<image>-<machine>.wic.vmdk

4.10 runqemu Command-Line Options

Following is a description of runqemu options you can provide on the command line:

Note

If you do provide some “illegal” option combination or perhaps you do not provide enough in the way of options, runqemu provides appropriate error messaging to help you correct the problem.

  • QEMUARCH: The QEMU machine architecture, which must be “qemuarm”, “qemuarm64”, “qemumips”, “qemumips64”, “qemuppc”, “qemux86”, or “qemux86-64”.

  • VM: The virtual machine image, which must be a .wic.vmdk file. Use this option when you want to boot a .wic.vmdk image. The image filename you provide must contain one of the following strings: “qemux86-64”, “qemux86”, “qemuarm”, “qemumips64”, “qemumips”, “qemuppc”, or “qemush4”.

  • ROOTFS: A root filesystem that has one of the following filetype extensions: “ext2”, “ext3”, “ext4”, “jffs2”, “nfs”, or “btrfs”. If the filename you provide for this option uses “nfs”, it must provide an explicit root filesystem path.

  • KERNEL: A kernel image, which is a .bin file. When you provide a .bin file, runqemu detects it and assumes the file is a kernel image.

  • MACHINE: The architecture of the QEMU machine, which must be one of the following: “qemux86”, “qemux86-64”, “qemuarm”, “qemuarm64”, “qemumips”, “qemumips64”, or “qemuppc”. The MACHINE and QEMUARCH options are basically identical. If you do not provide a MACHINE option, runqemu tries to determine it based on other options.

  • ramfs: Indicates you are booting an initial RAM disk (initramfs) image, which means the FSTYPE is cpio.gz.

  • iso: Indicates you are booting an ISO image, which means the FSTYPE is .iso.

  • nographic: Disables the video console, which sets the console to “ttys0”. This option is useful when you have logged into a server and you do not want to disable forwarding from the X Window System (X11) to your workstation or laptop.

  • serial: Enables a serial console on /dev/ttyS0.

  • biosdir: Establishes a custom directory for BIOS, VGA BIOS and keymaps.

  • biosfilename: Establishes a custom BIOS name.

  • qemuparams=\"xyz\": Specifies custom QEMU parameters. Use this option to pass options other than the simple “kvm” and “serial” options.

  • bootparams=\"xyz\": Specifies custom boot parameters for the kernel.

  • audio: Enables audio in QEMU. The MACHINE option must be either “qemux86” or “qemux86-64” in order for audio to be enabled. Additionally, the snd_intel8x0 or snd_ens1370 driver must be installed in linux guest.

  • slirp: Enables “slirp” networking, which is a different way of networking that does not need root access but also is not as easy to use or comprehensive as the default.

  • kvm: Enables KVM when running “qemux86” or “qemux86-64” QEMU architectures. For KVM to work, all the following conditions must be met:

    • Your MACHINE must be either qemux86” or “qemux86-64”.

    • Your build host has to have the KVM modules installed, which are /dev/kvm.

    • The build host /dev/kvm directory has to be both writable and readable.

  • kvm-vhost: Enables KVM with VHOST support when running “qemux86” or “qemux86-64” QEMU architectures. For KVM with VHOST to work, the following conditions must be met:

    • kvm option conditions must be met.

    • Your build host has to have virtio net device, which are /dev/vhost-net.

    • The build host /dev/vhost-net directory has to be either readable or writable and “slirp-enabled”.

  • publicvnc: Enables a VNC server open to all hosts.