uvms/README.md

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) and [cloud-hypervisor](https://github.com/cloud-hypervisor/cloud-hypervisor).

Prior Art
---

### 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.
- [valpackett/munix](https://git.clan.lol/clan/munix)
- ...

### People

Following the right people might 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.
- [valpackett](https://val.packett.cool)
- yureka
- [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).
- ...

### 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.
  The alternative is virtiofsd, which is potentially more RAM-hungry, and which significantly
  limits bandwidth at least when used with explicitly enabling DAX (`microvm.nix`).

  In practice the virtio-blk way 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`

Yes. Soon.

#### `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,
and Nix is slow.
When using `microvm.nix` you write, for example, each TAP's `hwaddr` by hand, and then 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.

The golang tool relies on obscure Nix CLI options and "channels" (as in tarballs).