this builds a kernel-less nixos image for running with arbitrary kernels in qemu

build:

nix-build
# copy the image out so we can use it with qemu
install -m644 result/nixos.qcow2 qemu-image.img

build your kernel image we assume your linux checkout is at ../linux/

run qemu:

qemu-system-x86_64 -s \
    -kernel ../linux/arch/x86/boot/bzImage \
    -hda qemu-image.img \
    -append "root=/dev/sda console=ttyS0 nokaslr" \
    -enable-kvm \
    -nographic \
    -m 2048

and if you want it to halt boot until gdb is connected, add -S:

qemu-system-x86_64 -s -S \
    -kernel arch/x86/boot/bzImage \
    -hda qemu-image.img \
    -append "root=/dev/sda console=ttyS0 nokaslr" \
    -enable-kvm \
    -nographic \
    -m 2048

modified from suggestions found at https://nixos.wiki/wiki/Kernel_Debugging_with_QEMU