windows/seal: leave nothing on C:, activate on the fresh SID, unmount the CD

Sealed VMs were left with setup scripts on C:\, the config CD mounted,
and Windows unactivated. Rework the first-boot flow so a settled VM
carries no vmix artifacts:

- Activation moves off the throwaway bootstrap's boot 1 into the boot-2
  finalize, which runs as the real account after its fresh SID/profile
  exist -- MAS on that SID is what actually sticks. MAS is kept until
  then, run once, and deleted.
- The static address is set once into the persistent store on the target
  NIC (sealed images already OOBE on the real NIC), so no per-boot task
  and no script survive on disk.
- Boot-2 finalize wipes every vmix-*, MAS_AIO.cmd, config and marker off
  C:\ after use, retires the bootstrap account/profile, and self-deletes.
- The config CD is unmounted for good -- drop its letter and disable the
  mount manager's auto-lettering (D: keeps its explicit assignment).

Non-configMedium generalize (the shared path) is unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0117qMyjpuXsjpVAcpJbFD8g

lib/images/windows/default.nix

@@ -3,6 +3,7 @@ let
   windows = rec {
     drivers = import ./drivers { inherit pkgs system; };
     makeFilesISO = (import ./helpers/makeFilesISO.nix) { inherit pkgs; };
+    makeConfigMedium = (import ./helpers/makeConfigMedium.nix) { inherit pkgs lib makeFilesISO; };
     customizeImage = (import ./helpers/customizeImage.nix) { inherit pkgs lib; };
     customizeImageFold = builtins.foldl' customizeImage;
     templates = (import ./templates) { inherit pkgs lib system drivers makeFilesISO; };
@@ -14,14 +15,20 @@ let
   win10 = (import ./win10) { inherit pkgs lib system windows; };
   win11 = (import ./win11) { inherit pkgs lib system windows; };
 
-  # Recursively add .generalize to every derivation leaf in the image tree
+  # Recursively add .generalize and .seal to every derivation leaf in the tree
   addGeneralize = val:
     if val ? _vmixOsType then
-      val // { generalize = args:
+      val // {
+        generalize = args:
           let
             templateArgs = builtins.removeAttrs args [ "vncDisplay" ];
             displayArgs = lib.optionalAttrs (args ? vncDisplay) { inherit (args) vncDisplay; };
           in windows.customizeImage val (windows.templates.generalize templateArgs // displayArgs);
+        seal = args:
+          let
+            templateArgs = builtins.removeAttrs args [ "vncDisplay" ];
+            displayArgs = lib.optionalAttrs (args ? vncDisplay) { inherit (args) vncDisplay; };
+          in windows.customizeImage val (windows.templates.seal templateArgs // displayArgs);
       }
     else if builtins.isAttrs val then
       lib.mapAttrs (_: addGeneralize) val

lib/images/windows/helpers/makeConfigMedium.nix

@@ -0,0 +1,59 @@
+# Per-VM config medium for a sealed Windows image (see templates.seal).
+#
+# A sealed image carries no per-VM data. The values that differ between VMs --
+# hostname, the static address the guest asserts, timezone, desktop tint -- are
+# written here as a PowerShell data file and packed into a tiny ISO. config.nix
+# attaches it as a read-only CD-ROM; the image's baked first-boot scripts
+# (vmix-load-config.cmd finds it, then dot-source it) apply the values. So one
+# sealed store path is shared by every VM, and only this cheap ISO is per-VM.
+#
+# Usage:
+#   makeConfigMedium {
+#     name = "win-config";
+#     hostname = "panda-win";
+#     staticIP = { address = "10.10.10.26"; prefixLength = 24;
+#                  gateway = "10.10.10.1"; dns = [ "10.10.10.1" ]; };
+#     timezone = "E. South America Standard Time";
+#     bgColor  = "#856558";
+#   }
+{ pkgs, lib, makeFilesISO, ... }:
+{
+  name ? "vmix-config",
+  hostname ? "",
+  # The per-VM account. The sealed image carries a generic bootstrap account
+  # (only there to carry OOBE); on first boot this real account is created from
+  # here, gets the SID-bound profile on D:\Users\<username>, and the bootstrap
+  # is retired. Distinct per VM -- nothing about the account is shared/baked.
+  username ? "",
+  password ? "",
+  # { address; prefixLength; gateway; dns = [ ... ]; }
+  staticIP ? null,
+  timezone ? null,
+  # Solid desktop background as a hex string, e.g. "#856558". Converted to the
+  # registry's decimal "R G B" on the target, in vmix-apply-config.ps1.
+  bgColor ? null,
+}:
+let
+  dnsList = lib.optionalString (staticIP != null)
+    (lib.concatMapStringsSep "," (s: "'${s}'") staticIP.dns);
+
+  # Consumed by dot-sourcing (. C:\vmix-config.ps1), so it only assigns
+  # variables. Anything not set here is simply absent, and the baked scripts
+  # guard on that ($VmixIpAddress being null skips the static-IP assignment).
+  configPs1 = pkgs.writeText "vmix-config.ps1" ''
+    # vmix per-VM config -- generated, read by the sealed image's baked scripts.
+    $VmixHostname     = '${hostname}'
+    ${lib.optionalString (username != "") "$VmixUsername     = '${username}'"}
+    ${lib.optionalString (username != "") "$VmixPassword     = '${password}'"}
+    ${lib.optionalString (staticIP != null) ''
+    $VmixIpAddress    = '${staticIP.address}'
+    $VmixPrefixLength = ${toString staticIP.prefixLength}
+    $VmixGateway      = '${staticIP.gateway}'
+    $VmixDns          = @(${dnsList})''}
+    ${lib.optionalString (timezone != null) "$VmixTimeZone     = '${timezone}'"}
+    ${lib.optionalString (bgColor != null) "$VmixBgColor      = '${bgColor}'"}
+  '';
+in
+# makeFilesISO strips the store-hash prefix, so this lands at the ISO root as
+# exactly vmix-config.ps1 -- which is what vmix-load-config.cmd scans for.
+makeFilesISO { inherit name; files = [ configPs1 ]; }

lib/images/windows/templates/default.nix

@@ -39,6 +39,24 @@ in rec {
   # Generalize (sysprep + OOBE). Pass seal=true for hardware deployment.
   generalize = import ./generalize.nix args;
 
+  # Seal: a generic OOBE-deferred base whose per-VM data is not baked but
+  # delivered at deploy time on a config medium (helpers/makeConfigMedium.nix).
+  # One sealed store path is shared by every VM; each VM's first boot mints its
+  # own SID and builds the whole profile on the relocated data volume (D:).
+  #
+  # The baked account is a generic bootstrap that only exists to carry OOBE to a
+  # logon -- the real, per-VM account (username/password) comes from the config
+  # medium, and the bootstrap is retired on the target. So nothing per-VM is
+  # baked. RDP and locale stay caller args.
+  seal = templateArgs: generalize ({
+    delayOobeRun = true;
+    configMedium = true;
+    username = "vmixsetup";
+    password = "vmixsetup";
+    profilesDirectory = "D:\\Users";
+    dataDisk = { driveLetter = "D"; label = "data"; };
+  } // templateArgs);
+
   # Offline registry templates
   reg = import ./registry args;
 

lib/images/windows/templates/generalize.nix

@@ -42,6 +42,30 @@ in
   # delayOobeRun = true: sysprep only, OOBE + activation on real hardware
   # delayOobeRun = false: sysprep + OOBE + activation in build VM
   delayOobeRun ? false,
+  # configMedium = true: this is a generic sealed base whose per-VM data
+  # (hostname, static IP, timezone, desktop tint) is NOT baked. The target's
+  # first boot reads it off a small removable config CD (see makeConfigMedium)
+  # via baked finder/apply scripts. Implies the OOBE is deferred to the target,
+  # so it is only meaningful together with delayOobeRun = true. Lets one sealed
+  # store path be shared by every VM built from it.
+  configMedium ? false,
+  # Skip sysprep's SID reset (use /oobe without /generalize), so every rebuild
+  # of the layers above the cached base install carries the same machine SID --
+  # and therefore the same account SID. A profile kept on a persistent disk then
+  # matches on file ACLs, its NTUSER.DAT hive, and ProfileList across rebuilds,
+  # with no ownership fixups. As a side effect MountedDevices survives too, so
+  # the data disk keeps its drive letter without a boot-time reassign.
+  #
+  # Correct only for an image that is always this one machine; a fleet that
+  # deploys the same image to many hosts wants the default generalization.
+  keepMachineSid ? false,
+  # Known-Folder redirection: keep the SID-bound profile on C: (so /generalize
+  # can still randomize the SID per machine) but point the user's data folders
+  # at the persistent data disk, so files -- not per-user registry settings --
+  # survive an OS rebuild. e.g. { base = "D:\\UserData"; folders = [ "Desktop"
+  # "Documents" "Downloads" ]; }. Needs dataDisk to provide the volume, and is
+  # mutually exclusive with profilesDirectory.
+  folderRedirect ? null,
 }: let
   # Convert "8e8cd8" hex to "142 140 216" decimal RGB for Windows registry
   hexToRgbStr = hex: let
@@ -93,12 +117,161 @@ in
   # interface arrives DHCP-managed -- assigning an address without turning DHCP
   # off first does not stick, which is how a VM meant to be at a fixed address
   # ended up holding a lease instead.
+  # PowerShell in its own file: it grew a wait loop and a retry, which are no
+  # fun to keep correct inside a cmd one-liner.
+  #
+  # Two things it must survive. DHCP is turned off before the address is set,
+  # so any failure to set it strands the box with no address at all -- which is
+  # exactly what happened after an internal reboot, where a stale ARP entry for
+  # the address from the previous instance tripped duplicate-address detection
+  # and New-NetIPAddress threw. DadTransmits 0 turns that detection off so the
+  # static always binds, and the assignment is retried rather than fatal.
+  # Two shapes. Baked: the address is a build-time literal. configMedium: the
+  # address is read from C:\vmix-config.ps1 (dot-sourced), which the finder
+  # dropped there off the config CD -- so the same sealed script serves every
+  # VM. The wait/retry logic is identical either way.
+  staticIPAssign = if configMedium
+    then { addr = "$VmixIpAddress"; prefix = "$VmixPrefixLength"; gw = "$VmixGateway"; dns = "$VmixDns"; }
+    else { addr = "'${staticIP.address}'"; prefix = toString staticIP.prefixLength; gw = "'${staticIP.gateway}'"; dns = staticDnsList; };
+  staticIPScriptPs1 = pkgs.writeText "vmix-static-ip.ps1" ''
+    ${lib.optionalString configMedium ''
+    if (-not (Test-Path C:\vmix-config.ps1)) { Write-Output 'vmix: no config yet'; exit 0 }
+    . C:\vmix-config.ps1
+    if (-not $VmixIpAddress) { Write-Output 'vmix: no static address in config'; exit 0 }
+    ''}
+    $a = $null
+    for ($n = 0; $n -lt 30; $n++) {
+      $a = Get-NetAdapter -Physical | Where-Object Status -eq 'Up' | Sort-Object ifIndex | Select-Object -First 1
+      if ($a) { break }
+      Start-Sleep -Seconds 2
+    }
+    if (-not $a) { Write-Output 'vmix: no adapter came up'; exit 1 }
+    $i = $a.ifIndex
+    Set-NetIPInterface -InterfaceIndex $i -Dhcp Disabled -DadTransmits 0 -ErrorAction SilentlyContinue
+    Remove-NetIPAddress -InterfaceIndex $i -AddressFamily IPv4 -Confirm:$false -ErrorAction SilentlyContinue
+    Remove-NetRoute -InterfaceIndex $i -AddressFamily IPv4 -Confirm:$false -ErrorAction SilentlyContinue
+    $ok = $false
+    for ($k = 0; $k -lt 5 -and -not $ok; $k++) {
+      try {
+        New-NetIPAddress -InterfaceIndex $i -IPAddress ${staticIPAssign.addr} -PrefixLength ${staticIPAssign.prefix} -DefaultGateway ${staticIPAssign.gw} -ErrorAction Stop | Out-Null
+        $ok = $true
+      } catch {
+        Write-Output ('vmix: assign attempt ' + $k + ' failed: ' + $_.Exception.Message)
+        Start-Sleep -Seconds 2
+        Remove-NetIPAddress -InterfaceIndex $i -AddressFamily IPv4 -Confirm:$false -ErrorAction SilentlyContinue
+      }
+    }
+    if (-not $ok) { Write-Output 'vmix: could not set static address'; exit 1 }
+    Set-DnsClientServerAddress -InterfaceIndex $i -ServerAddresses ${staticIPAssign.dns}
+    New-NetFirewallRule -DisplayName 'ICMPv4 Echo' -Protocol ICMPv4 -IcmpType 8 -Direction Inbound -Action Allow -Profile Any -Enabled True -ErrorAction SilentlyContinue | Out-Null
+    Write-Output ('vmix: set ' + ${staticIPAssign.addr} + ' on ifIndex ' + $i)
+  '';
+
+  # Finder: the config CD's drive letter is unknown, so scan for the marker file
+  # and stage it on C: where the baked scripts expect it. Runs on the target's
+  # first boot (post-oobe), before the per-boot static-IP task needs it.
+  loadConfigScript = pkgs.writeText "vmix-load-config.cmd" ''
+    @echo off
+    for %%D in (E F G H I J K L M N O P Q R S T U V W X Y Z D) do (
+      if exist %%D:\vmix-config.ps1 (
+        copy /y %%D:\vmix-config.ps1 C:\vmix-config.ps1 >nul
+        goto :done
+      )
+    )
+    :done
+  '';
+
+  # Applies the per-VM config that is not an answer-file field: timezone, the
+  # desktop tint (per user, so run under the created account in post-oobe), and
+  # the machine rename. Rename is pending until the post-oobe reboot.
+  applyConfigScript = pkgs.writeText "vmix-apply-config.ps1" ''
+    if (-not (Test-Path C:\vmix-config.ps1)) { exit 0 }
+    . C:\vmix-config.ps1
+    if ($VmixTimeZone) { & tzutil /s "$VmixTimeZone" }
+    if ($VmixBgColor) {
+      $hex = ([string]$VmixBgColor).TrimStart('#')
+      $r = [Convert]::ToInt32($hex.Substring(0,2),16)
+      $g = [Convert]::ToInt32($hex.Substring(2,2),16)
+      $b = [Convert]::ToInt32($hex.Substring(4,2),16)
+      Set-ItemProperty 'HKCU:\Control Panel\Colors'  -Name Background     -Value "$r $g $b"
+      Set-ItemProperty 'HKCU:\Control Panel\Desktop' -Name WallPaper      -Value ""
+      Set-ItemProperty 'HKCU:\Control Panel\Desktop' -Name WallpaperStyle -Value '0'
+    }
+    if ($VmixHostname -and $env:COMPUTERNAME -ne $VmixHostname) {
+      Rename-Computer -NewName $VmixHostname -Force -ErrorAction SilentlyContinue
+    }
+  '';
+
+  # Creates the real per-VM account from the config and hands the machine over
+  # to it. The sealed image bakes only a generic bootstrap account (${username})
+  # -- enough to carry OOBE to a logon so this can run -- and the real account
+  # is made here, on the target, from the CD. Autologon is switched to it and a
+  # one-shot cleanup is armed; the post-oobe reboot then lets the real account
+  # log in and build its own SID-bound profile on D:\Users\<username>, after
+  # which the bootstrap is retired. So the account, like the SID, is per-VM and
+  # nothing about it is shared or baked. Runs as the bootstrap user in post-oobe.
+  createUserScript = pkgs.writeText "vmix-create-user.ps1" ''
+    if (-not (Test-Path C:\vmix-config.ps1)) { exit 0 }
+    . C:\vmix-config.ps1
+    if (-not $VmixUsername) { exit 0 }
+    if ($VmixUsername -ieq '${username}') { exit 0 }
+    & net user $VmixUsername $VmixPassword /add
+    & net localgroup Administrators $VmixUsername /add
+    $w = 'HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon'
+    Set-ItemProperty $w -Name AutoAdminLogon  -Value '1'
+    Set-ItemProperty $w -Name DefaultUserName -Value $VmixUsername
+    Set-ItemProperty $w -Name DefaultPassword -Value $VmixPassword
+    Remove-ItemProperty $w -Name DefaultDomainName -ErrorAction SilentlyContinue
+    # Fires at the real account's first logon (HKLM RunOnce = next user to log
+    # on), i.e. after the reboot below, once the bootstrap is no longer in use.
+    Set-ItemProperty 'HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\RunOnce' `
+      -Name vmixFinalize -Value 'cmd /c C:\vmix-finalize.cmd' -Type String
+  '';
+
+  # Runs once as the real account (RunOnce, after the hand-over reboot), so the
+  # fresh machine SID and the real profile already exist. This is where activation
+  # belongs -- on that fresh SID, not the throwaway bootstrap's. It then finishes
+  # per-user setup, retires the bootstrap, unmounts the config CD for good, and
+  # wipes every vmix artifact off C:\ so the running machine carries no leftover
+  # setup files. Self-deletes last.
+  finalizeScript = pkgs.writeText "vmix-finalize.cmd" ''
+    @echo off
+    :: 1) Activate Windows on the real account's fresh SID (TSforge, offline).
+    cscript //nologo C:\Windows\System32\slmgr.vbs /ipk M7XTQ-FN8P6-TTKYV-9D4CC-J462D
+    cscript //nologo C:\Windows\System32\slmgr.vbs /rilc
+    net stop sppsvc /y 2>nul
+    net start sppsvc
+    ping -n 8 127.0.0.1 >nul
+    if exist C:\MAS_AIO.cmd ( echo. | call C:\MAS_AIO.cmd /Z-Windows )
+    if exist "C:\Program Files\Microsoft Office\root\Office16\WINWORD.EXE" (
+      if exist C:\MAS_AIO.cmd ( echo. | call C:\MAS_AIO.cmd /Ohook )
+    )
+    :: 2) Per-user desktop tint, now that the real account is logged in.
+    powershell -NoProfile -ExecutionPolicy Bypass -File C:\vmix-apply-config.ps1 >nul 2>&1
+    :: 3) Retire the bootstrap account and its profile (idle now).
+    powershell -NoProfile -ExecutionPolicy Bypass -Command "Get-CimInstance Win32_UserProfile | Where-Object { $_.LocalPath -like '*\${username}' } | Remove-CimInstance -ErrorAction SilentlyContinue"
+    net user ${username} /delete >nul 2>&1
+    :: 4) Unmount the config CD and keep it unmounted -- drop its letter and stop
+    ::    the mount manager auto-lettering it (D: keeps its explicit assignment).
+    powershell -NoProfile -ExecutionPolicy Bypass -Command "Get-CimInstance Win32_Volume -Filter 'DriveType=5 AND DriveLetter IS NOT NULL' | ForEach-Object { $_.DriveLetter=$null; [void]($_ | Set-CimInstance) }"
+    > C:\Windows\Temp\vmix-am.txt echo automount disable
+    >> C:\Windows\Temp\vmix-am.txt echo automount scrub
+    diskpart /s C:\Windows\Temp\vmix-am.txt >nul 2>&1
+    del /q C:\Windows\Temp\vmix-am.txt 2>nul
+    :: 5) Wipe every vmix setup artifact from C:\.
+    del /q C:\MAS_AIO.cmd C:\vmix-config.ps1 C:\vmix-data-initialized 2>nul
+    del /q C:\vmix-load-config.cmd C:\vmix-apply-config.ps1 C:\vmix-create-user.ps1 2>nul
+    del /q C:\vmix-init-data-disk.cmd C:\vmix-data-profile.cmd C:\vmix-heal-profile.ps1 2>nul
+    del /q C:\vmix-static-ip.cmd C:\vmix-static-ip.ps1 2>nul
+    del /q C:\oobe-unattend.xml C:\post-oobe.cmd 2>nul
+    :: 6) Self-delete.
+    (goto) 2>nul & del "%~f0"
+  '';
+
+  # Thin launcher, so the scheduled task has a cmd to point at.
   staticIPScript = pkgs.writeText "vmix-static-ip.cmd" ''
     @echo off
-    powershell -NoProfile -ExecutionPolicy Bypass -Command "$a = $null; for ($n = 0; $n -lt 30; $n++) { $a = Get-NetAdapter -Physical | Where-Object Status -eq 'Up' | Sort-Object ifIndex | Select-Object -First 1; if ($a) { break }; Start-Sleep -Seconds 2 }; if (-not $a) { Write-Output 'vmix: no adapter came up'; exit 1 }; $i = $a.ifIndex; Set-NetIPInterface -InterfaceIndex $i -Dhcp Disabled -ErrorAction SilentlyContinue; Remove-NetIPAddress -InterfaceIndex $i -AddressFamily IPv4 -Confirm:$false -ErrorAction SilentlyContinue; Remove-NetRoute -InterfaceIndex $i -AddressFamily IPv4 -Confirm:$false -ErrorAction SilentlyContinue; New-NetIPAddress -InterfaceIndex $i -IPAddress '${staticIP.address}' -PrefixLength ${toString staticIP.prefixLength} -DefaultGateway '${staticIP.gateway}' -ErrorAction Stop | Out-Null; Set-DnsClientServerAddress -InterfaceIndex $i -ServerAddresses ${staticDnsList}; Write-Output ('vmix: set ' + '${staticIP.address}' + ' on ifIndex ' + $i)" > C:\Windows\Temp\vmix-static-ip.log 2>&1
-    :: Answer pings. Windows blocks ICMP by default, which makes a box with a
-    :: fixed address look dead to everything that checks it the obvious way.
-    powershell -NoProfile -Command "New-NetFirewallRule -DisplayName 'ICMPv4 Echo' -Protocol ICMPv4 -IcmpType 8 -Direction Inbound -Action Allow -Profile Any -Enabled True | Out-Null" > nul 2>&1
+    powershell -NoProfile -ExecutionPolicy Bypass -File C:\vmix-static-ip.ps1 > C:\Windows\Temp\vmix-static-ip.log 2>&1
   '';
 
   dataDriveLetter = if dataDisk != null then (dataDisk.driveLetter or "D") else "D";
@@ -112,7 +285,59 @@ in
   # a GPT label, one full-size NTFS partition and the drive letter, while a disk
   # that already holds data keeps it and only has its letter re-asserted. The
   # OS disk is added to QEMU first and so is always disk 0.
-  initDataDiskScript = pkgs.writeText "vmix-init-data-disk.cmd" ''
+  initDataDiskScript = pkgs.writeText "vmix-init-data-disk.cmd" (if configMedium then ''
+    @echo off
+    :: Sealed-image variant. Two extra hazards over the baked path:
+    ::
+    :: 1. The per-VM config rides an optical drive, and on the target's first
+    ::    boot the raw data disk has no volume yet -- so Windows letters the CD
+    ::    as ${dataDriveLetter}:, exactly where the profile volume must go. The
+    ::    plain `if exist ${dataDriveLetter}:\` guard would then see the CD and
+    ::    skip, leaving ProfilesDirectory pointed at read-only media. So a first
+    ::    boot is tracked by a marker, not by the letter, and any occupant of
+    ::    ${dataDriveLetter}: is moved aside before the data disk claims it.
+    :: 2. Left to Shell-Setup's FolderLocations, ProfilesDirectory can be
+    ::    evaluated before this disk exists (unordered within specialize) and
+    ::    silently fall back to C:. Setting it here, in the same step that just
+    ::    created the volume, removes that race.
+    if exist C:\vmix-data-initialized goto :ensure
+
+    :: First boot: park whatever holds ${dataDriveLetter}: (the config CD) on Y:
+    :: so the data disk can take the letter. Harmless if the letter is free.
+    > C:\Windows\Temp\vmix-cd.txt echo select volume ${dataDriveLetter}
+    >> C:\Windows\Temp\vmix-cd.txt echo assign letter=Y noerr
+    diskpart /s C:\Windows\Temp\vmix-cd.txt > nul 2>&1
+
+    :: Lay disk 1 (the host zvol) out from scratch and give it the letter.
+    > C:\Windows\Temp\vmix-dd-init.txt echo select disk 1
+    >> C:\Windows\Temp\vmix-dd-init.txt echo clean
+    >> C:\Windows\Temp\vmix-dd-init.txt echo convert gpt
+    >> C:\Windows\Temp\vmix-dd-init.txt echo create partition primary
+    >> C:\Windows\Temp\vmix-dd-init.txt echo format fs=ntfs quick label="${dataLabel}"
+    >> C:\Windows\Temp\vmix-dd-init.txt echo assign letter=${dataDriveLetter}
+    diskpart /s C:\Windows\Temp\vmix-dd-init.txt
+    echo initialized > C:\vmix-data-initialized
+    goto :ensure
+
+    :ensure
+    :: The letter normally persists via MountedDevices; re-assert if it is gone.
+    if exist ${dataDriveLetter}:\ goto :profiledir
+    > C:\Windows\Temp\vmix-dd-assign.txt echo select disk 1
+    >> C:\Windows\Temp\vmix-dd-assign.txt echo select partition 1
+    >> C:\Windows\Temp\vmix-dd-assign.txt echo assign letter=${dataDriveLetter}
+    diskpart /s C:\Windows\Temp\vmix-dd-assign.txt > nul 2>&1
+
+    :profiledir
+    ${lib.optionalString (profilesDirectory != null) ''
+    :: Point new profiles at the data volume, now that it exists. REG_EXPAND_SZ
+    :: to match Windows' own ProfilesDirectory type.
+    if exist ${dataDriveLetter}:\ (
+      if not exist "${profilesDirectory}" mkdir "${profilesDirectory}"
+      reg add "HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList" /v ProfilesDirectory /t REG_EXPAND_SZ /d "${profilesDirectory}" /f > nul 2>&1
+    )''}
+    del /q C:\Windows\Temp\vmix-cd.txt C:\Windows\Temp\vmix-dd-init.txt C:\Windows\Temp\vmix-dd-assign.txt 2>nul
+    :done
+  '' else ''
     @echo off
     :: diskpart rather than the Storage cmdlets. New-Partition and
     :: Format-Volume need services that are not up yet this early in
@@ -141,6 +366,102 @@ in
     :cleanup
     del /q C:\Windows\Temp\vmix-dd-assign.txt C:\Windows\Temp\vmix-dd-init.txt 2>nul
     :done
+  '');
+
+  # PowerShell in its own file, so neither cmd quoting nor Nix's '' need to be
+  # fought. If the account's real profile got backed up to a .bak key (the
+  # temporary-profile fallback), put it back: drop the temp key, rename .bak to
+  # the live SID, remove the temp directory, and drop a flag so the caller
+  # knows to reboot.
+  # Known-Folder GUIDs for the redirectable user folders.
+  knownFolderGuids = {
+    Desktop   = "{B4BFCC3A-DB2C-424C-B029-7FE99A87C641}";
+    Documents = "{FDD39AD0-238F-46AF-ADB4-6C85480369C7}";
+    Downloads = "{374DE290-123F-4565-9164-39C4925E467B}";
+    Pictures  = "{33E28130-4E1E-4676-835A-98395C3BC476}";
+    Music     = "{4BD8D571-6D19-48D3-BE97-422220080E43}";
+    Videos    = "{18989B1D-99B5-455B-841C-AB7C74E4DDFC}";
+    Favorites = "{1777F761-68AD-4D8A-87BD-30B759FA33DD}";
+  };
+  redirectFolders = if folderRedirect != null
+    then (folderRedirect.folders or [ "Desktop" "Documents" "Downloads" "Pictures" "Music" "Videos" ])
+    else [ ];
+  redirectBase = if folderRedirect != null then (folderRedirect.base or "D:\\UserData") else "D:\\UserData";
+
+  # Per-user, run once per profile via Active Setup: point each known folder at
+  # its directory under the data volume. SHSetKnownFolderPath updates both the
+  # registration and the shell-folder registry; it does not move files, so a
+  # freshly created profile's empty C: folder is simply repointed at the D: one,
+  # which already holds this user's accumulated files after a rebuild.
+  folderRedirectPs1 = pkgs.writeText "vmix-redirect-folders.ps1" (lib.optionalString (folderRedirect != null) ''
+    $sig = @'
+    [DllImport("shell32.dll", CharSet=CharSet.Unicode)]
+    public static extern int SHSetKnownFolderPath(ref System.Guid rfid, uint dwFlags, System.IntPtr hToken, string pszPath);
+    '@
+    $kf = Add-Type -MemberDefinition $sig -Name KFP -Namespace Vmix -PassThru
+    $map = @{
+    ${lib.concatMapStringsSep "\n    " (f: "'${f}' = '${knownFolderGuids.${f}}'") redirectFolders}
+    }
+    foreach ($name in $map.Keys) {
+      $target = Join-Path '${redirectBase}' $name
+      New-Item -ItemType Directory -Force -Path $target -ErrorAction SilentlyContinue | Out-Null
+      $guid = [System.Guid]$map[$name]
+      [void]$kf::SHSetKnownFolderPath([ref]$guid, 0, [System.IntPtr]::Zero, $target)
+    }
+  '');
+
+  # Active Setup fires StubPath once per user at first logon -- including the
+  # fresh profile each generalized rebuild creates -- which is exactly when the
+  # redirection needs re-applying. Backslashes doubled for .reg.
+  activeSetupRegistry = lib.optionalString (folderRedirect != null) ''
+    Windows Registry Editor Version 5.00
+
+    [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Active Setup\Installed Components\{6f3b8c2a-1d4e-4f9a-b8c1-0a1b2c3d4e5f}]
+    @="vmix folder redirection"
+    "StubPath"="powershell.exe -NoProfile -ExecutionPolicy Bypass -WindowStyle Hidden -File C:\\vmix-redirect-folders.ps1"
+    "Version"="1"
+  '';
+
+  healProfileScript = pkgs.writeText "vmix-heal-profile.ps1" (lib.optionalString (profilesDirectory != null) ''
+    $pl = 'HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList'
+    $bak = Get-ChildItem $pl -ErrorAction SilentlyContinue | Where-Object {
+      $_.PSChildName -like '*.bak' -and
+      (Get-ItemProperty $_.PSPath -ErrorAction SilentlyContinue).ProfileImagePath -eq '${profilesDirectory}\${username}'
+    } | Select-Object -First 1
+    if ($bak) {
+      $sid = $bak.PSChildName -replace '\.bak$'
+      Remove-Item (Join-Path $pl $sid) -Recurse -Force -ErrorAction SilentlyContinue
+      Rename-Item $bak.PSPath $sid -ErrorAction SilentlyContinue
+      Remove-Item '${profilesDirectory}\TEMP' -Recurse -Force -ErrorAction SilentlyContinue
+      New-Item -Path C:\Windows\Temp\vmix-profile-healed -ItemType File -Force | Out-Null
+    }
+  '');
+
+  # One onstart / SYSTEM script for whatever the data disk needs before logon:
+  # assign its letter, and then either heal a relocated profile that went
+  # temporary (profilesDirectory) or make the redirected data folders reachable
+  # by whatever account this rebuild created (folderRedirect). Both cannot apply
+  # at once -- a profile is either wholly on D: or only its data folders are.
+  bootDataProfileScript = pkgs.writeText "vmix-data-profile.cmd" ''
+    @echo off
+    ${lib.optionalString (dataDisk != null) "call C:\\vmix-init-data-disk.cmd"}
+    ${lib.optionalString (profilesDirectory != null) ''
+    powershell -NoProfile -ExecutionPolicy Bypass -File C:\vmix-heal-profile.ps1 > C:\Windows\Temp\vmix-data-profile.log 2>&1
+    if exist C:\Windows\Temp\vmix-profile-healed (
+      del /q C:\Windows\Temp\vmix-profile-healed
+      shutdown /r /t 5 /c "vmix: repaired relocated profile, restarting"
+    )
+    ''}
+    ${lib.optionalString (folderRedirect != null) ''
+    :: The redirected folders live under a per-user account whose SID changes on
+    :: every generalized rebuild, so grant the well-known Users group -- which
+    :: any account joins and which is SID-stable across machines -- inheritable
+    :: full control, and let the per-user redirect (Active Setup) point the known
+    :: folders here. Runs as SYSTEM, before any logon.
+    if not exist "${redirectBase}" mkdir "${redirectBase}"
+    ${lib.concatMapStringsSep "\n    " (f: ''if not exist "${redirectBase}\${f}" mkdir "${redirectBase}\${f}"'') redirectFolders}
+    icacls "${redirectBase}" /grant *S-1-5-32-545:(OI)(CI)F /t > C:\Windows\Temp\vmix-redirect-prep.log 2>&1
+    ''}
   '';
 
   folderLocationsXml = lib.optionalString (profilesDirectory != null) ''
@@ -150,6 +471,17 @@ in
             <ProfilesDirectory>${profilesDirectory}</ProfilesDirectory>
           </FolderLocations>'';
 
+  # ProfilesDirectory as an offline .reg merge, for the keepMachineSid path
+  # where the specialize pass (and its FolderLocations) does not run. virt-win-reg
+  # applies this before the Audit Mode boot, so it is in place when OOBE creates
+  # the account. Backslashes are doubled for .reg syntax.
+  profileListRegistry = lib.optionalString (keepMachineSid && profilesDirectory != null) ''
+    Windows Registry Editor Version 5.00
+
+    [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList]
+    "ProfilesDirectory"="${builtins.replaceStrings [''\''] [''\\''] profilesDirectory}"
+  '';
+
   dataDiskXml = lib.optionalString (dataDisk != null) ''
         <!-- Runs during specialize, before the first profile is created -->
         <component name="Microsoft-Windows-Deployment" processorArchitecture="amd64"
@@ -166,6 +498,13 @@ in
   # Post-OOBE script: runs as the created user via FirstLogonCommands.
   postOobeScript = pkgs.writeText "post-oobe.cmd" ''
     @echo off
+    ${lib.optionalString configMedium ''
+    :: Stage the per-VM config off the removable CD, then apply the parts that
+    :: are not answer-file fields (timezone, desktop tint, machine rename). The
+    :: static address is left to the per-boot task registered below.
+    call C:\vmix-load-config.cmd
+    powershell -NoProfile -ExecutionPolicy Bypass -File C:\vmix-apply-config.ps1 > C:\Windows\Temp\vmix-apply-config.log 2>&1
+    ''}
     ${lib.optionalString (!autoLogon) ''
     reg delete "HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon" /v AutoAdminLogon /f 2>nul
     reg delete "HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon" /v DefaultUserName /f 2>nul
@@ -202,6 +541,7 @@ in
     powershell -Command "Get-AppxPackage *MicrosoftEdgeDevToolsClient* | Remove-AppxPackage -ErrorAction SilentlyContinue"
 
 
+    ${lib.optionalString (!configMedium) ''
     :: Re-install product key and licenses to restore activation IDs after sysprep
     cscript //nologo C:\Windows\System32\slmgr.vbs /ipk M7XTQ-FN8P6-TTKYV-9D4CC-J462D
     cscript //nologo C:\Windows\System32\slmgr.vbs /rilc
@@ -220,6 +560,9 @@ in
       )
     )
     del /q C:\MAS_AIO.cmd 2>nul
+    ''}
+    :: configMedium: activation is deferred to the boot-2 finalize, so it runs
+    :: on the real account's fresh SID. MAS_AIO.cmd is kept for it until then.
 
     ${lib.optionalString enableRDP ''
     :: Enable RDP
@@ -233,9 +576,19 @@ in
     reg add "HKLM\SYSTEM\CurrentControlSet\Services\SessionEnv" /v Start /t REG_DWORD /d 2 /f
     reg add "HKLM\SYSTEM\CurrentControlSet\Services\UmRdpService" /v Start /t REG_DWORD /d 2 /f
     reg add "HKLM\SYSTEM\CurrentControlSet\Services\TermService" /v Start /t REG_DWORD /d 2 /f
+    :: A VM reached over RDP must never suspend itself off the network. The
+    :: default Balanced plan sleeps after 15 min idle; switch to High
+    :: Performance and zero every idle timeout, and turn hibernate off.
+    powercfg /setactive SCHEME_MIN
+    powercfg /change standby-timeout-ac 0
+    powercfg /change standby-timeout-dc 0
+    powercfg /change hibernate-timeout-ac 0
+    powercfg /change hibernate-timeout-dc 0
+    powercfg /change monitor-timeout-ac 0
+    powercfg /hibernate off
     ''}
 
-    ${lib.optionalString (staticIP != null) ''
+    ${lib.optionalString (staticIP != null && !configMedium) ''
     :: This VM's only NIC sits on a macvtap bridged to the host's LAN, so its
     :: address is a LAN address that nothing hands out -- the guest asserts it.
     :: Registered to run at every boot rather than applied here. OOBE runs in
@@ -247,6 +600,23 @@ in
     schtasks /create /tn "vmix-static-ip" /tr "C:\vmix-static-ip.cmd" /sc onstart /ru SYSTEM /rl HIGHEST /f > nul 2>&1
     ''}
 
+    ${lib.optionalString configMedium ''
+    :: configMedium (sealed images) run OOBE on the real target NIC, so the
+    :: address is set here once and left in the persistent store -- it survives
+    :: reboots on its own, no per-boot task and no script left on disk. Runs on
+    :: this bootstrap boot so the real account is already reachable on boot 2.
+    powershell -NoProfile -ExecutionPolicy Bypass -File C:\vmix-static-ip.ps1 > C:\Windows\Temp\vmix-static-ip.log 2>&1
+    ''}
+
+    ${lib.optionalString (dataDisk != null && !configMedium) ''
+    :: Ensures D: is assigned on every boot -- the image ships without a
+    :: persisted letter for the data disk -- and heals a profile that went
+    :: temporary before D: was ready. Onstart / SYSTEM, like the address task.
+    :: configMedium does not need this: the letter persists via MountedDevices
+    :: in the overlay after the first boot, so there is nothing to re-assert.
+    schtasks /create /tn "vmix-data-profile" /tr "C:\vmix-data-profile.cmd" /sc onstart /ru SYSTEM /rl HIGHEST /f > nul 2>&1
+    ''}
+
     ${lib.optionalString (writeFilter != null) ''
     :: Install the feature now, but defer configuring it: uwfmgr does not exist
     :: until this has been through a reboot, and the swapfile belongs on the
@@ -255,12 +625,31 @@ in
     reg add "HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\RunOnce" /v "vmixUwf" /t REG_SZ /d "C:\vmix-uwf-config.cmd" /f
     ''}
 
+
+    ${lib.optionalString keepMachineSid ''
+    :: /oobe without /generalize leaves the system set to re-run windeploy (OOBE)
+    :: on every boot -- shutting down here in a FirstLogonCommand cuts OOBE off
+    :: before it resets that state itself. Clear it, or the target boots into a
+    :: Setup with no unattend left to consume and hangs on a black screen.
+    reg add "HKLM\SYSTEM\Setup" /v SetupType /t REG_DWORD /d 0 /f
+    reg delete "HKLM\SYSTEM\Setup" /v OOBEInProgress /f 2>nul
+    reg delete "HKLM\SYSTEM\Setup" /v CmdLine /f 2>nul
+    ''}
+    ${lib.optionalString configMedium ''
+    :: Runs last, as the generic bootstrap account: create the real per-VM
+    :: account from the config, switch autologon to it and arm the cleanup. The
+    :: reboot below then logs the real account in for the first time, building
+    :: its SID-bound profile on D:\Users\<username>.
+    powershell -NoProfile -ExecutionPolicy Bypass -File C:\vmix-create-user.ps1 > C:\Windows\Temp\vmix-create-user.log 2>&1
+    ''}
     :: Clean up
     del /q C:\oobe-unattend.xml 2>nul
     del /q C:\vmix-audit-script.cmd 2>nul
     del /q C:\vmix-audit-wrapper.cmd 2>nul
 
-    ${if delayOobeRun then "" else "shutdown /s /t 5 /c \"vmix generalize complete\""}
+    ${if configMedium then "shutdown /r /t 5 /c \"vmix: applying per-VM config\""
+      else if delayOobeRun then ""
+      else "shutdown /s /t 5 /c \"vmix generalize complete\""}
     del /q C:\post-oobe.cmd 2>nul
   '';
 
@@ -346,20 +735,46 @@ ${folderLocationsXml}
     </unattend>
   '';
 in {
-  name = if delayOobeRun then "generalize-delay-oobe" else "generalize";
+  name = if configMedium then "seal" else if delayOobeRun then "generalize-delay-oobe" else "generalize";
   inherit nicModel;
+  # With keepMachineSid the specialize pass never runs (see the sysprep line),
+  # so the profile relocation cannot ride the unattend there. It is written to
+  # the registry offline instead, before the build's OOBE creates the profile,
+  # so the account still lands on the data volume. Empty otherwise.
+  windowsRegistry = profileListRegistry + activeSetupRegistry;
   # The blank disk is attached for the Audit Mode boot itself, so the disk-init
   # command and the profile relocation both happen under OOBE in the build VM.
   # That is what makes delayOobeRun unnecessary: nothing is left to do on real
   # hardware. The written disk comes back as this derivation's `data` output.
-  extraDisk = if dataDisk != null then { size = dataDisk.size or "100G"; } else null;
+  #
+  # Under delayOobeRun there is no build-VM OOBE to relocate into, and the real
+  # data volume is the host's zvol attached at deploy time -- so building an
+  # empty throwaway disk here would be pure waste. Gated off: the target's
+  # specialize formats the real disk (dataDiskXml) and OOBE creates the profile
+  # on it. This is what lets a sealed image ship without a `data` output.
+  extraDisk = if (dataDisk != null && !delayOobeRun) then { size = dataDisk.size or "100G"; } else null;
   uploads = [
     { source = oobeXml; dest = "/oobe-unattend.xml"; }
     { source = postOobeScript; dest = "/post-oobe.cmd"; }
     { source = masScript; dest = "/MAS_AIO.cmd"; }
-  ] ++ lib.optional (dataDisk != null) { source = initDataDiskScript; dest = "/vmix-init-data-disk.cmd"; }
+  ] ++ lib.optionals (dataDisk != null) (
+      [ { source = initDataDiskScript; dest = "/vmix-init-data-disk.cmd"; }
+        { source = bootDataProfileScript; dest = "/vmix-data-profile.cmd"; }
+      ]
+      ++ lib.optional (profilesDirectory != null) { source = healProfileScript; dest = "/vmix-heal-profile.ps1"; }
+      ++ lib.optional (folderRedirect != null) { source = folderRedirectPs1; dest = "/vmix-redirect-folders.ps1"; }
+    )
     ++ lib.optional (writeFilter != null) { source = uwfConfigScript; dest = "/vmix-uwf-config.cmd"; }
-    ++ lib.optional (staticIP != null) { source = staticIPScript; dest = "/vmix-static-ip.cmd"; };
+    ++ lib.optionals configMedium [
+      { source = loadConfigScript; dest = "/vmix-load-config.cmd"; }
+      { source = applyConfigScript; dest = "/vmix-apply-config.ps1"; }
+      { source = createUserScript; dest = "/vmix-create-user.ps1"; }
+      { source = finalizeScript; dest = "/vmix-finalize.cmd"; }
+    ]
+    ++ lib.optionals (staticIP != null || configMedium) [
+      { source = staticIPScript; dest = "/vmix-static-ip.cmd"; }
+      { source = staticIPScriptPs1; dest = "/vmix-static-ip.ps1"; }
+    ];
   # delayOobeRun: sysprep + shutdown — OOBE runs on real hardware
   # generalize: sysprep + reboot into OOBE in the same QEMU session
   auditScript = ''
@@ -368,7 +783,7 @@ in {
     del /q C:\Windows\Panther\unattend.xml 2>nul
     del /q C:\Windows\Panther\Unattend\unattend.xml 2>nul
     del /q C:\Windows\System32\Sysprep\Panther\unattend.xml 2>nul
-    ${lib.optionalString (dataDisk != null) ''
+    ${lib.optionalString (dataDisk != null && !delayOobeRun) ''
     :: Lay the data disk out here, in Audit Mode, rather than leaving it to the
     :: specialize pass alone. Component order within a pass is not guaranteed,
     :: and FolderLocations is applied by Shell-Setup while the disk is prepared
@@ -377,9 +792,13 @@ in {
     :: fully booted OS with the disk already attached, so this always works.
     :: The specialize copy stays as a letter re-assertion after generalize
     :: clears MountedDevices.
+    ::
+    :: Only when there is a build disk to lay out. Under delayOobeRun (sealed
+    :: images) the disk is the host's zvol, present only on the target, so this
+    :: is left to the target's specialize pass alone.
     call C:\vmix-init-data-disk.cmd
     ''}
-    C:\Windows\System32\Sysprep\sysprep.exe /generalize /oobe ${if delayOobeRun then "/shutdown" else "/reboot"} /quiet /unattend:C:\oobe-unattend.xml
+    C:\Windows\System32\Sysprep\sysprep.exe ${lib.optionalString (!keepMachineSid) "/generalize "}/oobe ${if delayOobeRun then "/shutdown" else "/reboot"} /quiet /unattend:C:\oobe-unattend.xml
   '';
 }
 

nixos/vms/config.nix

@@ -125,13 +125,34 @@ let
         if [ ! -f "$PERSIST_PATH" ]; then
           echo "Seeding persistent disk from store image..."
           mkdir -p "$(dirname "$PERSIST_PATH")"
-          cp --no-preserve=mode "${toString storeImage}" "$PERSIST_PATH"
+          ${if vmCfg.disks.os.persistMode == "backing"
+            then ''qemu-img create -f qcow2 -F qcow2 -b "${toString storeImage}" "$PERSIST_PATH"''
+            else ''cp --no-preserve=mode "${toString storeImage}" "$PERSIST_PATH"''}
           chmod 600 "$PERSIST_PATH"
         fi
       '';
 
       persistExecStartPre = lib.optional (hasOsDisk && vmCfg.disks.os.persist) seedPersistentDiskScript;
 
+      # A GC root pinning the OS overlay's ACTUAL backing store path, so
+      # nix-collect-garbage cannot delete the store image the disk reads through.
+      gcrootLink = "/nix/var/nix/gcroots/vmix-${vmCfg.name}-osbacking";
+      gcrootScript = pkgs.writeShellScript "${vmCfg.name}-gcroot-vmix" ''
+        # Read the live overlay's backing (not the config's current image, which
+        # drifts to a new store path after a rebuild while the overlay keeps
+        # backing the old one). Pinning the top of the chain transitively keeps
+        # the whole chain -- qcow2 backing_file paths are registered nix refs.
+        BACK=""
+        if [ -f "${vmCfg.disks.os.persistPath}" ]; then
+          BACK=$(qemu-img info "${vmCfg.disks.os.persistPath}" 2>/dev/null | awk '/^backing file:/ {print $3; exit}')
+        fi
+        [ -z "$BACK" ] && BACK="${toString storeImage}"
+        if [ -n "$BACK" ]; then
+          mkdir -p /nix/var/nix/gcroots
+          ln -sfn "$BACK" "${gcrootLink}"
+        fi
+      '';
+
       # QEMU expects single-letter boot codes (e.g. c,d,n), while vmix uses readable names.
       bootOrderQemu =
       let
@@ -212,14 +233,18 @@ let
           -machine type=${vmCfg.pc.type}${machineIrqchipArg} \
           ${optionalString vmCfg.bios.efi "-bios ${pkgs.OVMF.fd}/FV/OVMF.fd"} \
           ${optionalString vmCfg.bios.tpm "-chardev socket,id=chrtpm,path=/tmp/mytpm-sock -tpmdev emulator,id=tpm0,chardev=chrtpm -device tpm-tis,tpmdev=tpm0"} \
-          ${# Windows: localtime RTC, USB tablet for mouse, disable S3/S4 sleep
+          ${# Windows: localtime RTC, USB tablet for mouse, disable S3/S4 sleep.
+            # The q35 power/LPC bridge is ICH9-LPC -- ICH9-LMB was a typo qemu
+            # rejects ("invalid class name"), so the sleep states stayed offered
+            # and an idle guest could suspend itself right off the network.
             optionalString isWindows ''
           -rtc base=localtime,clock=host \
           -device qemu-xhci -device usb-tablet \
-          -global ICH9-LMB.disable_s3=1 -global ICH9-LMB.disable_s4=1 \
+          -global ICH9-LPC.disable_s3=1 -global ICH9-LPC.disable_s4=1 \
           ''} \
           ${optionalString hasOsDisk "-drive file=${osDiskPath},format=qcow2,if=virtio${optionalString (vmCfg.disks.os.persist == false) ",snapshot=on"}"} \
           ${optionalString (vmCfg.disks.iso.file != null) "-drive file=${toString vmCfg.disks.iso.file},media=cdrom,readonly=on"} \
+          ${optionalString (vmCfg.disks.config.file != null) "-drive file=${toString vmCfg.disks.config.file},media=cdrom,readonly=on"} \
           ${concatMapStrings (diskCfg: ''
             -drive file=${toString diskCfg.file},format=${diskCfg.format},if=${vmCfg.disks.bus} \
           '') (attrValues vmCfg.disks.add)} \
@@ -254,7 +279,8 @@ let
     "vm.vmix@${vmCfg.name}" = rec {
       bindsTo = [ "net.vmix@${spaceName}.target" ] ++ lib.optional (allMacvtaps != []) "macvtaps.vm.vmix@${vmCfg.name}.service";
       unitConfig.JoinsNamespaceOf = "ns.net.vmix@${spaceName}.service";
-      after = bindsTo;
+      after = bindsTo ++ lib.optional (hasOsDisk && vmCfg.disks.os.persist) "vm.vmix-gcroot@${vmCfg.name}.service";
+      wants = lib.optional (hasOsDisk && vmCfg.disks.os.persist) "vm.vmix-gcroot@${vmCfg.name}.service";
       path = with pkgs; [ iproute2 qemu gawk coreutils ];
       serviceConfig = {
         ExecStartPre = persistExecStartPre ++ [ createTapsforLansScript ];
@@ -284,6 +310,18 @@ let
         ExecStop = deleteMacvTapsScript;
       };
     };
+  }
+  // lib.optionalAttrs (cfg.enable && hasOsDisk && vmCfg.disks.os.persist) {
+    "vm.vmix-gcroot@${vmCfg.name}" = {
+      before = [ "vm.vmix@${vmCfg.name}.service" ];
+      wantedBy = [ "multi-user.target" ];
+      path = with pkgs; [ qemu coreutils ];
+      serviceConfig = {
+        Type = "oneshot";
+        RemainAfterExit = true;
+        ExecStart = gcrootScript;
+      };
+    };
   };
 
   vmServices = concatMapAttrs (spaceName: namespaceCfg: (concatMapAttrs (mkServices4aVMInNamespace spaceName) namespaceCfg.vms)) vmixCfg.namespaces;

nixos/vms/submoduleOptions.nix

@@ -173,11 +173,32 @@ with lib;
       default = "";
       description = "Mutable path for the persistent OS disk (e.g. /storage/vms/myvm/os.qcow2). Required when persist = true.";
     };
+    disks.os.persistMode = mkOption {
+      type = types.enum [ "copy" "backing" ];
+      default = "copy";
+      description = ''
+        How the persistent OS disk is seeded from the store image (persist = true).
+        copy: a full cp of the store image; the mutable disk holds everything.
+        backing: a thin qcow2 overlay backing onto the shared store image, so many
+        VMs share one base and each holds only its own deltas. The store image (and
+        its backing chain) must then survive GC -- vmix pins it via a per-VM gcroot.
+      '';
+    };
     disks.iso.file = mkOption {
       type = types.nullOr (types.either types.path types.str);
       description = "Path to the ISO file. Can be a Nix store path or a string path to a local file.";
       default = null;
     };
+    disks.config.file = mkOption {
+      type = types.nullOr (types.either types.path types.str);
+      default = null;
+      description = ''
+        A small read-only config medium attached as a second CD-ROM. For Windows
+        sealed images (templates.seal) this is the per-VM ISO from
+        vmixLib.windows.makeConfigMedium, carrying hostname/static-IP/etc. that
+        the image's baked first-boot scripts consume. Null to attach nothing.
+      '';
+    };
     disks.add = mkOption {
       default = {};
       type = types.attrsOf (types.submodule {