A minimal initrd for spawning VMs with virtiofs mounted rootfs.

• Single statically liked initrd binary
• Supports kernels with virtiofs being modular

Install the musl target for static linking:

rustup target add x86_64-unknown-linux-musl

cargo build --release

The statically linked binary will be at target/x86_64-unknown-linux-musl/release/virtintrd.

./mkvirtinitrd target/x86_64-unknown-linux-musl/release/virtintrd /usr/lib/modules/$(uname -r) initrd.img

qemu-system-x86_64 \
    -kernel /boot/vmlinuz-$(uname -r) \
    -initrd initrd.img \
    -append "console=ttyS0" \
    -nographic

Use the --module= option to include additional kernel modules in the initramfs:

./mkvirtinitrd \
    --module=virtio_blk \
    --module=ext4 \
    --module=nvme \
    target/x86_64-unknown-linux-musl/release/virtintrd \
    /usr/lib/modules/$(uname -r) \
    initrd.img

The virtiofs module is always included automatically.

The initrd handles a few options:

• init: If specified, this is the program run as pid 1, defaults to /bin/sh
• 'debug': If specified, debug output is printed
• 'rootfs': If specified, this virtiofs tag is used for the rootfs mount, default is rootfs
• mount=foo: If specified the virtiofs tag foo is mounted read-write at /run/mnt/foo
• mount-ro=foo: If specified the virtiofs tag foo is mounted read-only at /run/mnt/foo

Try the chrootvm script which takes a chroot dir, finds the latest kernel in it (from /usr/lib/modules), dynamically builds a suitable initrd and runs a program in that kernel with the chroot as rootfs.

For example:

./chrootvm /vcs/other/osbuild/output/build /usr/bin/bash

If no progam is specified, the default is to run /bin/sh.

This also supports --mount a-tag /a/path and --mount-ro a-tag /a/path which creates additional virtiofs mounts at /run/mnt/tag.

There is also a --debug option which will make the VM print debug output.