Fun little thing I have been working on: teach systemd to boot directly into a disk image downloaded via HTTP within the initrd.
In v257 systemd learnt the ability to download disk images at boot via systemd-import-generator, both DDIs and tarballs, and place them in /var/lib/machines/, /var/lib/portables/, /var/lib/confexts, /var/lib/extensions/. The goal was to provide a way to provision any of these resources automatically at boot. But now that we have this, we can take it a step further:
download the root disk image itself with this. There were a bunch of missing bits to make this nice though:
First of all, for raw disk images we need to attach them to a loopback block device, to make them mountable. Easy-peasy, systemd-dissect --attach already delivers that.
Then, for tar disk images we need to bind mount the downloaded and unpacked image to /sysroot/ (which is where the rootfs goes before we transition into it).
Net result of this: I can now point my UEFI to a single URL where it will load the UKI from. A few seconds later the initrd will pick up the rootfs from the same source, and boot it up. Magic!
Why all this though?
It's mostly to tighten my test loop a bit, for physical devices. So here's what this entails:
1. You build your image with mkosi one your development machine, and ask it to serve your image as HTTP. In other words: `mkosi -f serve`.
2. You boot into the target machine once, and register an EFI variable that enables HTTP boot from your development machine. Simply do `kernel-bootcfg --add-uri=http://192.168.47.11:8081/image.efi --title=testloop --boot-order=0`, using @kraxel's wonderful tool.
Lennart Poettering
cyplo