README: init

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+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 to 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.