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QEMU launch options

Written May 25, 2024 4 min read

Command to boot custom or installed kernel with virtio based folder sharing between host and guest.

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Setup QEMU for Kernel GDB Debugging (no display)

You need these to run a compiled kernel and debug it using GDB:

  1. The kernel bzImage
  2. ramdisk image

Tips:

  • To debug it with gdb, run QEMU with option -s (shorthand for gdb -tcp::1234) and -S for starting the CPU in the stopped stated.
  • Load the GDB scripts by adding the add-auto-load-safe-path script to gdbinit. The position of gdbinit can be ~/.gdbinit or ~/.config/gdb/gdbinit.
echo "add-auto-load-safe-path /path/to/linux/scripts/gdb/vmlinux-gdb.py" >> ~/.gdbinit

Steps

  1. Run the default config : make defconfig
  2. Add or set these symbols in .config file of the kernel :
CONFIG_GDB_SCRIPTS=y
CONFIG_DEBUG_INFO=y
CONFIG_DEBUG_KERNEL=y
CONFIG_KALLSYMS=y
CONFIG_RANDOMIZE_BASE=n
  1. You can also run these :
$ ./scripts/config -e DEBUG_INFO -e DEBUG_KERNEL -e DEBUG_INFO_DWARF4
$ ./scripts/config -e DEBUG_SECTION_MISMATCH -e DEBUG_OBJECTS -e DEBUG_OBJECTS_WORK -e DEBUG_VM
$ ./scripts/config -e GDB_SCRIPTS -e HEADERS_INSTALL
  1. Run make -j$(nproc) to build the kernel
  2. Build gdb scripts: make scripts_gdb
  3. Run modules_install: make modules_install
  4. Build the ramdisk image
sudo dracut --kernel-image arch/x86/boot/bzImage --confdir /etc/dracut.conf.d/ -v ramdisk.img <version>
  1. Run QEMU :
qemu-system-x86_64 \
  -kernel $kernel_path/vmlinux \
  -initrd $ramdisk_path/ramdisk.img \
  -append "console=ttyS0 nokaslr" \
  -nographic -enable-kvm -cpu host -m 512 \
  -s -S
  1. Run gdb $KERNEL_SRC/vmlinux in another terminal
  2. Enter target remote localhost:1234 to get started with remote debugging the kernel

Note: Try to use hbreak instead of b in gdb to attach breakpoints early in the kernel

If you are getting memory cannot be accessed during adding a breakpoint, then you must have missed disabling KASLR or Kernel Address Space Layour Randomisation. See CONFIG_RANDOMIZE_BASE config flag.

Setup QEMU for running QEMU with a custom kernel and display

This will help you setup a shared folder between a QEMU guest and the host. This also disables KASLR or Kernel Address Space Layout Randomisation (which is a security feature, but can be disabled for easier debugging - Wikipedia).

Make sure to adjust the parameters to your RAM (-m), file with your_os.qcow2 location and shared_folder parameters with your own shared folder values.

Host

Terminal window
# boot arch-linux image with installed kernel
# to get the tty or the terminal, run,
# $ telnet localhost 4321
$ sudo qemu-system-x86_64 \
    --enable-kvm \
    -cpu host -smp $(nproc) \
    -m 8192 \
    -nographic \
    -device virtio-net,netdev=vmnic -netdev user,id=vmnic \
    -drive file=archlinux.qcow2,media=disk,if=virtio \
    -virtfs local,path=/path/to/shared/folder,mount_tag=shared_folder,security_model=passthrough,id=shared_folder \
    -serial telnet:localhost:4321,server,nowait


# boot custom kernel using bzImage.
# this will open the display in a window, else you can add `-serial` at the
# end same as previous command and then runn using telnet
$ sudo qemu-system-x86_64 \
 --enable-kvm \
 -cpu host -smp $(nproc) \
 -m 8192 \
 -kernel linux-next/arch/x86_64/boot/bzImage \
 -append "rw nokaslr root=/dev/vda1" \
 -device virtio-net,netdev=vmnic -netdev user,id=vmnic \
 -device VGA,vgamem_mb=256 \
 -drive file=debian.qcow2,media=disk,if=virtio \
 -virtfs local,path=/path/to/shared/folder,mount_tag=shared_folder,security_model=passthrough,id=shared_folder

Guest

Terminal window
$ sudo apt install -y qemu-guest-agent
$ sudo mkdir /mnt/shared_folder
$ sudo mount -t 9p -o trans=virtio,version=9p2000.L shared_folder /mnt/shared_folder


# tty settings in case of telnet tty
$ stty rows 30 && stty columns 136

Reference: https://wiki.gentoo.org/wiki/QEMU/Options