Support for bootc-style containers

[cgwalters]

Moved from #1263 and apple/containerization#505

See also bootc-dev/bcvk#21 and bootc-dev/bcvk#214

The goal is to go from container build (or just a container pull) with a bootc style container (where the kernel/initramfs/UKI is embedded inside the container) to a "full VM" (and here we also expect to use systemd).

In a nutshell there are two paths for this:

• Have an external tool fetch content from the apple/container storage; in my research this is going to be relatively easy
• Expand the use case for this project to support this more directly (a nontrivial scope increase, but since this project is at the heart about containers and VMs...there is necessarily some duplication if an external tool does it while reusing the container storage etc. from here)

[jtligon]

Howdy @cgwalters! @egernst had a FOSDEM talk about starting up a linux vm quickly. I think being able to turn that sample code into a macOS tool that spins up a bootc container as a VM, is what we're looking to do. My question is where would that macOS binary live in bcvk? or in apple/container to make it Mac native and container native

[jglogan]

@cgwalters @jtligon I'm not the expert for this one (that'd be @dcantah), but I think the main impediment at this point is what Containerization provides (and what it doesn't provide) for booting VMs. Today it's focused strictly on starting up a microVM - direct boot kernel that start a really lightweight guest agent that's capable of running systemd as the primary workload, and can optionally provide basic init features. After that, it can spawn additional processes to support container exec.

The kernels we use have no module support don't use an initramfs.

For exploration, it's probably best to try integrating bcvk against containerization, and seeing if you can extend containerization to suit your needs. You can use cctl as an inspiration for the basic code flow.

I don't think there's much to be gained by working at the container level until you can work out if it can be done with just containerization. It you get to that point, I'd say "why not both" - bcvk can use containerization directly, and you can implement a similar feature as a container CLI plugin, perhaps.

Any questions, just drop em here. Good luck!

[afbjorklund]

• [Request]: Build bootc containers, and run bootc containers in a vm #210