如何在NixOS MicroVM中配置可用的nixpkgs频道？

我在已安装Nix包管理器的Debian系统上，用microvm创建并运行NixOS虚拟机。虚拟机内的NIX_PATH环境变量默认设置为：

nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos:nixos-config=/etc/nixos/configuration.nix:/nix/var/nix/profiles/per-user/root/channels

但虚拟机里/nix/var/nix/profiles/per-user/目录是空的，上述引用的nixpkgs路径根本不存在。请问怎么配置虚拟机，让它构建完成后拥有一个可用的、和构建虚拟机所用的nixpkgs频道一致的引用？

我看到过几个类似问题，但不确定是否适用：

• 如何在configuration.nix中声明式添加NixOS unstable频道
• NixOS：在configuration.nix中设置默认频道

{
  description = "NixOS in MicroVMs";

  inputs.microvm.url = "github:astro/microvm.nix";
  inputs.microvm.inputs.nixpkgs.follows = "nixpkgs";

  outputs = { self, nixpkgs, microvm }:
    let
      system = "x86_64-linux";
      user = "REDACTED";
      vmname = "haskell";
    in {
      defaultPackage.${system} = self.packages.${system}.my-microvm;

      packages.${system} = {
        my-microvm = self.nixosConfigurations.my-microvm.config.microvm.declaredRunner;
      };

      nixosConfigurations = {
        my-microvm = nixpkgs.lib.nixosSystem {
          inherit system;
          modules = [
            microvm.nixosModules.microvm
            ({pkgs, ... }: {
              environment.systemPackages = [
                pkgs.ghc
                pkgs.git
                (pkgs.haskell-language-server.override { supportedGhcVersions = [ "94" ]; })
                pkgs.stack
              ];
              microvm = {
                forwardPorts = [
                  { from = "host"; host.port = 2222; guest.port = 22; }
                ];
                hypervisor = "qemu";
                interfaces = [
                  { type = "user"; id = "usernet"; mac = "00:00:00:00:00:02"; }
                ];
                mem = 4096;
                shares = [ {
                  proto = "9p";
                  tag = "ro-store";
                  source = "/nix/store";
                  mountPoint = "/nix/.ro-store";
                } {
                  proto = "virtiofs";
                  tag = "hometest";
                  source = "~/.local/share/microvm/vms/${vmname}/shares/home";
                  mountPoint = "/home/${user}";
                  socket = "/run/user/1000/microvm-virtiofsd_vm_home";
                }
                ];
                socket = "/run/user/1000/microvm-control.socket";
                vcpu = 3;
                volumes = [];
                writableStoreOverlay = "/home/${user}/nix-rw-store";
              };
              networking.hostName = vmname;
              nix.settings = {
                extra-experimental-features = ["nix-command" "flakes"];
                trusted-users = [user];
              };
              security.sudo = {
                enable = true;
                wheelNeedsPassword = false;
              };
              services.getty.autologinUser = user;
              services.openssh = {
                enable = true;
              };
              users.users.${user} = {
                extraGroups = [ "wheel" "video" ];
                group = "user";
                isNormalUser = true;
                openssh.authorizedKeys.keys = [
                  "ssh-rsa REDACTED"
                ];
                password = "";
              };
              users.users.root.password = "";
              users.groups.user = {};
            })
          ];
        };
      };
    };
}

要让虚拟机内的NIX_PATH指向构建时使用的nixpkgs，同时创建对应的频道引用，有两种实用方法：

方法1：声明式设置NIX_PATH并绑定构建用的nixpkgs #

在虚拟机配置模块中，直接覆盖environment.variables.NIX_PATH，并将nixpkgs指向系统构建时使用的pkgs路径，同时通过nix.channels声明默认频道：

({pkgs, ... }: {
  # 保留原有其他配置...
  environment.variables.NIX_PATH = {
    nixpkgs = pkgs.path;
    nixos-config = "/etc/nixos/configuration.nix";
  };
  # 声明与构建版本一致的系统频道
  nix.channels = {
    nixos = pkgs.path;
  };
  # 让root用户的频道链接指向系统频道
  nix.channelSources = {
    root = [ "nixos" ];
  };
})

配置后，虚拟机内的NIX_PATH会直接指向构建时使用的nixpkgs在/nix/store中的有效路径，系统也会自动创建/nix/var/nix/profiles/per-user/root/channels/nixos的链接，解决路径不存在的问题。

方法2：利用Flakes配置默认nixpkgs源（推荐） #

因为你已经在用Flakes，更贴合工作流的方式是通过Nix配置指定默认源，同时兼容NIX_PATH：

({pkgs, ... }: {
  # 保留原有其他配置...
  nix.settings = {
    extra-experimental-features = ["nix-command" "flakes"];
    trusted-users = [user];
    # 允许nix命令使用NIX_PATH兼容模式
    use-nix-path = true;
    # 配置默认nixpkgs源为构建时的版本
    registry.nixpkgs.flake = pkgs.path;
  };
  # 可选：为普通用户设置NIX_PATH
  users.users.${user}.environmentVariables = {
    NIX_PATH = "nixpkgs=${pkgs.path}:nixos-config=/etc/nixos/configuration.nix";
  };
})

这种方式确保虚拟机内使用的nixpkgs和构建时完全一致，无需手动管理频道链接。

验证配置 #

启动虚拟机后，执行以下命令验证效果：

echo $NIX_PATH
ls -l /nix/var/nix/profiles/per-user/root/channels/

应能看到NIX_PATH中的nixpkgs指向/nix/store内的有效路径，且channels目录下存在nixos的链接。

内容的提问来源于stack exchange，提问作者Thomas Koch