140 lines
4.5 KiB
Nix
140 lines
4.5 KiB
Nix
# This module creates a virtual machine from the NixOS configuration.
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# Building the `config.system.build.vm' attribute gives you a command
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# that starts a KVM/QEMU VM running the NixOS configuration defined in
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# `config'. The Nix store is shared read-only with the host, which
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# makes (re)building VMs very efficient. However, it also means you
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# can't reconfigure the guest inside the guest - you need to rebuild
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# the VM in the host. On the other hand, the root filesystem is a
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# read/writable disk image persistent across VM reboots.
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{config, pkgs, ...}:
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let
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vmName = config.networking.hostName;
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options = {
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virtualisation.diskImage =
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pkgs.lib.mkOption {
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default = "./${vmName}.qcow2";
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description =
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''
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Path to the disk image containing the root filesystem.
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The image will be created on startup if it does not
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exist.
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'';
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};
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};
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# Shell script to start the VM.
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startVM =
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''
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#! ${pkgs.stdenv.shell}
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export PATH=${pkgs.samba}/sbin:$PATH
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NIX_DISK_IMAGE=''${NIX_DISK_IMAGE:-${config.virtualisation.diskImage}}
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if ! test -e "$NIX_DISK_IMAGE"; then
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${pkgs.qemu_kvm}/bin/qemu-img create -f qcow2 "$NIX_DISK_IMAGE" 512M || exit 1
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fi
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# -no-kvm-irqchip is needed to prevent the CIFS mount from
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# hanging the VM on x86_64.
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${pkgs.qemu_kvm}/bin/qemu-system-x86_64 \
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-no-kvm-irqchip \
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-net nic,model=virtio -net user -smb / \
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-drive file=$NIX_DISK_IMAGE,if=virtio,boot=on \
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-kernel ${config.system.build.system}/kernel \
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-initrd ${config.system.build.system}/initrd \
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$QEMU_OPTS \
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-append "$(cat ${config.system.build.system}/kernel-params) init=${config.system.build.bootStage2} systemConfig=${config.system.build.system}"
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'';
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in
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{
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require = options;
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# All the modules the initrd needs to mount the host filesystem via
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# CIFS. Also use paravirtualised network and block devices for
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# performance.
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boot.initrd.extraKernelModules =
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["cifs" "virtio_net" "virtio_pci" "virtio_blk" "virtio_balloon" "nls_utf8"];
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boot.initrd.extraUtilsCommands =
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''
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# We need mke2fs in the initrd.
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cp ${pkgs.e2fsprogs}/sbin/mke2fs $out/bin
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'';
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boot.initrd.postDeviceCommands =
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''
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# Set up networking. Needed for CIFS mounting.
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ipconfig 10.0.2.15:::::eth0:none
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# If the disk image appears to be empty (fstype "unknown";
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# hacky!!!), run mke2fs to initialise.
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eval $(fstype /dev/vda)
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if test "$FSTYPE" = unknown; then
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mke2fs -t ext3 /dev/vda
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fi
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'';
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# Mount the host filesystem via CIFS, and bind-mount the Nix store
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# of the host into our own filesystem. We use mkOverride to allow
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# this module to be applied to "normal" NixOS system configuration,
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# where the regular value for the `fileSystems' attribute should be
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# disregarded for the purpose of building a VM test image (since
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# those filesystems don't exist in the VM).
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fileSystems = pkgs.lib.mkOverride 50 {}
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[ { mountPoint = "/";
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device = "/dev/vda";
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}
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{ mountPoint = "/hostfs";
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device = "//10.0.2.4/qemu";
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fsType = "cifs";
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options = "guest,username=nobody";
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neededForBoot = true;
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}
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{ mountPoint = "/nix/store";
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device = "/hostfs/nix/store";
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options = "bind";
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neededForBoot = true;
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}
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# Mount the host's Nix database. This allows read-only Nix
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# operations in the guest to work properly.
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{ mountPoint = "/nix/var/nix/db";
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device = "/hostfs/nix/var/nix/db";
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options = "bind";
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}
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];
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# Starting DHCP brings down eth0, which kills the connection to the
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# host filesystem and thus deadlocks the system.
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networking.useDHCP = false;
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networking.defaultGateway = "10.0.2.2";
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system.build.vm = pkgs.runCommand "nixos-vm" {}
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''
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ensureDir $out/bin
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ln -s ${config.system.build.system} $out/system
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ln -s ${pkgs.writeScript "run-nixos-vm" startVM} $out/bin/run-${vmName}-vm
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'';
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# sendfile() is currently broken over CIFS, so fix it here for all
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# configurations that use Apache.
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services.httpd.extraConfig =
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''
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EnableSendFile Off
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'';
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# When building a regular system configuration, override whatever
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# video driver the host uses.
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services.xserver.videoDriver = pkgs.lib.mkOverride 50 {} "vesa";
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services.xserver.defaultDepth = pkgs.lib.mkOverride 50 {} 16;
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}
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