run-qemu

Requirements

• mkosi
  • e.g. dnf install mkosi
• qemu-system-x86_64
• rsync
• dracut
• enabled virtualization (KVM)
• nopasswd sudo preferred, or run as root, or enter passwords into the prompt several times
• argbash to generate the argument parser lib (using parser_generator.m4)

mkosi

Finding an mkosi version compatible with your operating system version and particular test configuration can sometimes be challenging. First, try the version packaged with your operating system. This will indirectly install mkosi dependencies (which have surprisingly not changed that much across mkosi versions). If that OS-provided mkosi version does not work for you, check the long mkosi section below.

Installation

• symlink the run_qemu.sh script into somewhere in your PATH
  • e.g. ln -s ~/git/run_qemu/run_qemu.sh ~/bin/run_qemu.sh
• Note: Supporting files in this repo are required to be in the same location as the script, after any symlink resolution. Copying just the script itself will not work.
• Bash Completion
  • Copy or symlink the run_qemu file into the default bash completions dir
  • The completions directory can be found using: pkg-config --variable=completionsdir bash-completion

Usage Notes

• Run this from the top level of a Linux kernel tree

  • e.g. run_qemu.sh --cxl --git-qemu

• The script can/will:

  • Build the kernel with whatever .config is present (It is up to the user to manage the .config)
  • Create a rootfs image with the chosen distro using mkosi
  • Perform some basic setup on the rootfs, including installing the kernel, utilities (such as ndctl), and other convenience operations such as copying .ssh/*.pub keys for easy access, and your ~/.bashrc etc.
  • Boot qemu with the newly compiled kernel provided on the qemu command line, and using the rootfs image above
  • Various options influence the qemu command line generated - there are options to select NUMA config, NVDIMMs, NVME devices, CXL devices etc.

• More detailed CLI help is available with run_qemu.sh --help

• Once qemu starts, in nographic mode, the Linux console 'takes over' the terminal. To interact with it, the following are useful:

  • Ctrl-a c : switch between the qemu monitor prompt (qemu) and console
  • Ctrl-a x : kill qemu and exit

• mkosi creates a package cache in mkosi.cache/ If a cache is present, it will always use only that, and never go over the network even if newer packages are available. To force re-fetching everything, remove this directory, or --rebuild=wipe which removes the builddir entirely.

• Which qemu to use can be overridden from the environment: qemu=/path/to/qemu/build/qemu-system-x86_64 ./run_qemu.sh [options]

• List of variables that have overrides via env:

  • qemu
  • gdb
  • distro
  • rev
  • builddir
  • ndctl

• To use the 'hostfwd' network, put this in your .ssh/config:

  Host rq
    Hostname localhost
    User root
    Port 10022
    StrictHostKeyChecking no
    UserKnownHostsFile /dev/null
  

  And then ssh rq. You may need to open port 10022 on any local firewalls. Then, the dpipe and sshfs programs can be used to let the guest access host files. For instance, as found in man sshfs: dpipe /usr/lib/ssh/sftp-server = ssh rq sshfs :$HOME/CXL /root/CXL -o passive "sshfs -o passive" uses stdin and stdout instead of the guest connecting back to the host over the network. If you don't have dpipe, try the slightly less efficient: socat EXEC:/usr/lib/ssh/sftp-server SHELL:"'ssh rq sshfs :$HOME/CXL /root/CXL -o passive'" Supposedly faster file sharing options like 9P or virtiofs exist too but they require much more complex configuration. sshfs generally works out of the box.

• The root password for the guest VM is root by default but note many Linux distributions restrict remote root access in various ways. The serial console automatically logs in, and a password isn't required.

CXL Usage

The script enables generating a sane QEMU commandline for instantiating a basic CXL topology. Since QEMU support for CXL isn't yet upstream, --git-qemu is additionally required. The CXL related options are:

• --cxl: Enables a simple CXL topology with:
  • single host bridge
    • 512M window size at 0x4c00000000
    • Bus #52
  • single root port
  • single Type 3 device
    • Persistent 256M
  • simple label storage area
• --cxl-debug: Add any and all flags for extra debug (kernel and QEMU)
• --cxl-hb: Turn q35 into a CXL capable Host bridge. Don't use this option unless you're working on support for this.
• --cxl-test-run: Attempt to do a sanity test of the kernel and QEMU configuration.

DAX Usage

• --dax-debug: Add any and all flags for extra debug of dax modules (kernel)