μVMs === Slowly comprehending [spectrum-os](https://spectrum-os.org) and [microvm.nix](https://github.com/microvm-nix/microvm.nix), by reproducing bits and pieces using [NixOS](https://nixos.org), [systemd](https://nixos.org/manual/nixos/stable/options#opt-systemd.services), and [cloud-hypervisor](https://github.com/cloud-hypervisor/cloud-hypervisor). Prior Art --- ### People Following the right people is one of the most effective ways to maintain bibliography, a form of "importance sampling". The following are the people whose work, at the time of writing, I already know to watch out for. Whom am I missing? - [Alyssa Ross](https://github.com/alyssais) - Via [spectrum.org]https://spectrum-os.org/bibliography.html) and [Nixpkgs](https://github.com/NixOS/Nixpkgs). - [Demi Marie](https://demimarie.github.io): - Via [spectrum-devel](https://spectrum-os.org/lists/archives/spectrum-devel/2ff17d00-6603-46ae-9eb0-d4fa179db86c@gmail.com/) and Qubes. - [Thomas Leonard](https://roscidus.com/blog/blog/2021/03/07/qubes-lite-with-kvm-and-wayland) - Via [spectrum-discuss](https://spectrum-os.org/lists/archives/spectrum-discuss/CAG4opy8BZn2pXDRBHOjcENFBHJON1LoG7A8GPdP0Wt_3KLaHyw@mail.gmail.com/), [qubes-lite, and `wayland-proxy-virtwl`](https://roscidus.com/blog/blog/2021/03/07/qubes-lite-with-kvm-and-wayland). - [Astro](https://spaceboyz.net/~astro/): - Via [microvm.nix](https://github.com/microvm-nix/microvm.nix) - [Jean-Philippe](https://github.com/jpouellet): - Via [Nixpkgs](https://github.com/NixOS/Nixpkgs), [Qubes](https://doc.qubes-os.org/en/latest/developer/general/devel-books.html), and [Genode](https://genode.org/about/challenges). - [Joanna Rutkowska](https://blog.invisiblethings.org/papers): - Via [Qubes](https://doc.qubes-os.org/en/latest/developer/general/devel-books.html) [OS](https://theinvisiblethings.blogspot.com/2012/09/how-is-qubes-os-different-from.html). - ... ### Projects - Obvious: Qubes, Spectrum, Genode, ... - Google ChromeOS and [AWS Firecracker](https://firecracker-microvm.github.io): - rust-vmm, [crosvm](https://crosvm.dev/book), and [sommelier](https://chromium.googlesource.com/chromiumos/platform2/+/master/vm_tools/sommelier); - [Asahi muvm](https://github.com/AsahiLinux/muvm): microvms using [libkrun](https://github.com/containers/libkrun) and virtio-gpu "native context". - [AppVM](https://github.com/jollheef/appvm): apparently available in Nixpkgs under `nixos/modules/virtualisation/appvm.nix`. Based on (NixOS,) qemu and libvirt. - ... ### Timeline The following are the questions I'd like to eventually answer about how virtualization happened: - [ ] Was Qubes the first attempt at isolating e.g. peripheral and network devices using virtualization? - [ ] Was Chromium OS the first and (or) the main driver for paravirtualized devices? - [ ] ... ### Choices The following are some of the current "am I holding this right?" questions: - [ ] Memory: it is said that Linux always needs some swap in order not to be weird; what is the general scheme for allocating memory and swap to the hypervisor and the guests? > Most of the day I'm using a laptop with 8GiB of RAM; this necessitates frequent OOMs, > even when not running VMs. - [x] It seems that hotplug memory is generaly preferred ot ballooning. - [x] It seems that we want the hotplug "banks" to not be too small, as to avoid fragmentation. - [ ] Zswap or zram on the hypervisor? On the guest? - [ ] How much swap to allocate on the hypervisor? On the guest? - [ ] Startup time: [lightvm](https://dl.acm.org/doi/10.1145/3132747.3132763) claims boot time of 2.3ms; can we ever achieve comparable numbers with NixOS and systemd? What is the first bottleneck? - [ ] Guest-to-guest communication: in order to implement configurations similar to Whonix (e.g. Tor in a separate VM), guests need to be able to talk to each other directly, without exposing the hypervisor to their traffic. Generally, I've heard of three solutions to guest-to-guest communication: - NAT via hypervisor, - [MACVTAP](https://microvm-nix.github.io/microvm.nix/interfaces.html#type--macvtap), - and [vhost-device-vsock](https://github.com/rust-vmm/vhost-device/blob/main/vhost-device-vsock/README.md#sibling-vm-communication). I've only ever implemented the first. The second is something of a bridge, but the packets (frames?) never enter the hypervisor's network stack. I've only recently learned about the third: mentioned by Alyssa in the spectrum matrix chat. I do not entirely understand how it fits into the bigger picture. - [ ] Filesystems: Virtio-blk appears to be the way to allocate persistent storage for VMs that require it. In practice this means allocating a zvol or a contiguous file on the hypervisor, to be exposed to the guest as a block device. One suspicion I have is that allocating a CoW filesystem (e.g. xfs, btrfs) on top of another CoW filesystem (e.g. zfs, as in xfs-on-zvol) may have non-trivial implications for fragmentation, depending on parameters like the chunksizes. On the other hand, use-cases like `/nix/store` offer serious deduplication opportunities, and I'm generally not using `snix-store` just yet. - [ ] ... ### Why Not? #### `spectrum-os` ...is in active development and not advertised as user-ready yet. Spectrum OS appears to be a balance-shifting project, building up towards a principled solution, which must require patience... It does not, for example, reuse NixOS systemd modules, but uses s6 instead. #### `microvm.nix` Is inherently static. A cynical spin on `microvm.nix` would be, and I mean it with utmost respect, that it's a glorified qemu flags generator, written in Nix. When using `microvm.nix` you write, for example, each TAP's `hwaddr` by hand, and the rebuild the "runner script". When using the "fully-declarative mode" you also engangle the guest's and the hypervisor's life cycles, and double the NixOS evaluation time. Microvm-nix ships support for a wide selection of different hypervisors, but you may only care about e.g. `cloud-hypervisor`. An instructive reference implementation and a convenient entry point, `microvm.nix` may not be a direct or complete answer to the question "what does a life cycle of a microvm-deployed service look like". #### appvm I only noticed the option in `man configuration.nix` a few days ago, so I just never tried. Long-term I'd definitely prefer not to use qemu.