aeondave/qemu

QEMU

QEMU is the default public tool for cross-architecture user-mode execution and full-system virtual machines.

Choose mode

| Need | Mode | |---|---| | Run one Linux/BSD ELF with syscalls forwarded to host | qemu-<arch> user-mode | | Run a process inside an extracted rootfs | qemu-<arch>-static in chroot/proot/container | | Boot kernel/initrd/rootfs/disk/firmware | qemu-system-<arch> | | Debug kernel, bootloader, or process before start | -S -gdb ... or user-mode -g | | Expose serial/UART | -serial stdio, -serial unix:..., -chardev ... | | Expose guest services | -netdev user,hostfwd=tcp:127.0.0.1:<host>-:<guest> |

User-mode patterns

qemu-arm -L ./sysroot ./target arg1
qemu-mipsel -L ./rootfs -strace ./usr/sbin/httpd
QEMU_LD_PREFIX=./rootfs qemu-aarch64 ./bin/app
qemu-riscv64 -g 1234 -L ./rootfs ./bin/app

If an existing file reports "No such file or directory", check the ELF interpreter and shared libraries:

readelf -lW ./target | grep 'Requesting program interpreter'
readelf -dW ./target | grep NEEDED

Use -strace for syscall evidence. Use -g <port> plus gdb-multiarch when register/memory proof is needed.

Full-system patterns

qemu-system-x86_64 \
  -kernel bzImage \
  -initrd initramfs.cpio.gz \
  -append "console=ttyS0 panic=-1" \
  -serial stdio -display none

qemu-system-mipsel \
  -M malta -kernel vmlinux \
  -drive file=rootfs.ext2,format=raw \
  -append "root=/dev/sda console=ttyS0" \
  -netdev user,id=n0,hostfwd=tcp:127.0.0.1:8080-:80 \
  -device e1000,netdev=n0 \
  -nographic

Discover available hardware:

qemu-system-arm -machine help
qemu-system-arm -device help
qemu-system-aarch64 -cpu help

Do not assume the virt machine or virtio devices match vendor firmware. Old router kernels often need e1000, rtl8139, IDE, SD, or board-specific devices.

Debugging

qemu-system-x86_64 -S -gdb tcp:127.0.0.1:1234 ...
gdb vmlinux
(gdb) target remote 127.0.0.1:1234

Use nokaslr or a deterministic load setup when setting breakpoints by symbol address. For boot sectors, set the GDB architecture to i8086.

Storage and images

qemu-img info disk.img
qemu-img convert -O raw vendor.vmdk rootfs.raw
qemu-img resize scratch.qcow2 +256M

Prefer read-only source images and copy-on-write scratch layers for experiments. Record image format explicitly; guessing raw vs qcow2/vmdk causes misleading boot failures.

Validation signals

• user-mode syscall trace reaches the behavior under study
• serial console prints the expected boot/init line
• GDB can halt at a relevant symbol/address
• guest service responds through a localhost-bound forward
• disk or filesystem mutation is visible in a scratch image

Failure triage

• No output: wrong console, machine, DTB, storage root, or guest waiting before serial init.
• Kernel panic: capture the full first panic line and root-device hints.
• Network dead: check NIC model, guest IP, service process, firewall, and hostfwd syntax.
• Illegal instruction: wrong CPU model or missing extension.
• User-mode loader error: wrong sysroot/interpreter/libc version.