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)

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 your ~/.ssh/id_rsa.pub for easy ssh 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.

• The root password for the guest VM is root. 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)

Kernel config

• Make sure to Turn on CXL related options in the kernel's .config:

$ grep -i cxl .config
CONFIG_CXL_BUS=y
CONFIG_CXL_PCI=m
CONFIG_CXL_MEM_RAW_COMMANDS=y
CONFIG_CXL_ACPI=m
CONFIG_CXL_PMEM=m
CONFIG_CXL_MEM=m
CONFIG_CXL_PORT=y
CONFIG_CXL_SUSPEND=y

The following is a way to check basic sanity within the QEMU guest:

lspci  | grep '3[45]:00'
34:00.0 PCI bridge: Intel Corporation Device 7075
35:00.0 Memory controller [0502]: Intel Corporation Device 0d93 (rev 01)

readlink -f /sys/bus/cxl/devices/mem0
/sys/devices/pci0000:34/0000:34:00.0/0000:35:00.0/mem0

Fedora 39+ Notes

Fedora 39 updates mkosi to 15 (and later) which contains a lot of breaking changes, and indeed breaks various expectations with run_qemu's usage of it.

Fedora has packaged mkosi 14 separately and in parallel to the latest mkosi. Uninstall the default mkosi package, and install 'mkosi14' to continue using run_qemu on Fedora 39 and later.

# dnf remove mkosi
# dnf install mkosi14