2024-08-07/01: add "Quick bbits: is Crostini a micro VM?"

2024-08-07/01-quick-bits-is-crostini-a-microvm.md

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+# Quick bits: is Crostini a micro VM?
+
+**Disclaimer**: I am not a Virtual Machine specialist, this post is mainly the
+conclusion I got after some searching.
+
+Someone asked me in my [previous
+post](/2024-08-05/01-my-favorite-device-is-a-chromebook.md) about my Chromebook
+if Crostini could be considered a micro VM. This was a interesting question, so
+I decided to do another post.
+
+To start, it is really difficult to get a good definition of what a "micro VM"
+is. [Firecracker](https://firecracker-microvm.github.io/) defines itself as a
+micro VM, and describes itself in its website as:
+
+> Firecracker is a virtual machine monitor (VMM) that uses the Linux
+> Kernel-based Virtual Machine (KVM) to create and manage microVMs. Firecracker
+> has a minimalist design. It excludes unnecessary devices and guest
+> functionality to reduce the memory footprint and attack surface area of each
+> microVM. This improves security, decreases the startup time, and increases
+> hardware utilization.
+
+Now looking at Crostini, its heart is a VMM called
+[crosvm](https://crosvm.dev/). It is described in its
+[README](https://chromium.googlesource.com/chromiumos/platform/crosvm/+/HEAD/README.md)
+as:
+
+> crosvm is a virtual machine monitor (VMM) based on Linux’s KVM hypervisor,
+> with a focus on simplicity, security, and speed. crosvm is intended to run
+> Linux guests, originally as a security boundary for running native
+> applications on the ChromeOS platform. Compared to QEMU, crosvm doesn’t
+> emulate architectures or real hardware, instead concentrating on
+> paravirtualized devices, such as the virtio standard.
+
+Similar descriptions right? Actually Firecracker website says it "started from
+Chromium OS's Virtual Machine Monitor, crosvm, an open source VMM written in
+Rust". So I would say it is safe to say crosvm itself is a micro VM.
+
+But
+[Crostini](https://www.chromium.org/chromium-os/developer-library/guides/containers/containers-and-vms/)
+itself is a combination of virtualization AND containerization. Basically
+inside the VM it runs a Linux kernel and LXC, that can start arbitrary
+containers inside it. From the Crostini documentation this choice seems to be
+to keep startup times down, and also to increase security (e.g.: in case of a
+security issue inside the container).
+
+This is definitely an interesting choice, since containers allow the overhead
+of each distro that you run inside Crostini to be low, and the main VM itself
+(called
+[Termina](https://chromium.googlesource.com/chromiumos/overlays/board-overlays/+/HEAD/project-termina/))
+should have low overhead too thanks to crosvm.
+
+By the way, if you want to learn more how "devices" works inside a micro VM
+like crosvm, I recommend [this blog
+post](https://prilik.com/blog/post/crosvm-paravirt/) talking about
+paravirtualized devices in crosvm.