`tart run --nested` to enable nested virtualization when available (#938)

Only works for Linux VMs under Sequoia hosts.

Fixes #933
Fixes #701

Sources/tart/Commands/Run.swift

@@ -156,6 +156,9 @@ struct Run: AsyncParsableCommand {
   """, valueName: "[name:]path[:options]"))
   var dir: [String] = []
 
+  @Flag(help: ArgumentHelp("Enable nested virtualization if possible"))
+  var nested: Bool = false
+
   @Option(help: ArgumentHelp("""
   Use bridged networking instead of the default shared (NAT) networking \n(e.g. --net-bridged=en0 or --net-bridged=\"Wi-Fi\")
   """, discussion: """
@@ -221,6 +224,14 @@ struct Run: AsyncParsableCommand {
       throw ValidationError("--captures-system-keys can only be used with the default VM view")
     }
 
+    if nested {
+      if #unavailable(macOS 15) {
+        throw ValidationError("Nested virtualization is supported on hosts starting with macOS 15 (Sequia), and later.")
+      } else if !VZGenericPlatformConfiguration.isNestedVirtualizationSupported {
+        throw ValidationError("Nested virtualization is available for Mac with the M3 chip, and later.")
+      }
+    }
+
     let localStorage = VMStorageLocal()
     let vmDir = try localStorage.open(name)
     if try vmDir.state() == .Suspended {
@@ -294,6 +305,7 @@ struct Run: AsyncParsableCommand {
       directorySharingDevices: directoryShares() + rosettaDirectoryShare(),
       serialPorts: serialPorts,
       suspendable: suspendable,
+      nested: nested,
       audio: !noAudio,
       clipboard: !noClipboard,
       sync: VZDiskImageSynchronizationMode(diskOptions.syncModeRaw)

Sources/tart/Platform/Darwin.swift

@@ -58,7 +58,11 @@ struct UnsupportedHostOSError: Error, CustomStringConvertible {
       VZMacOSBootLoader()
     }
 
-    func platform(nvramURL: URL) throws -> VZPlatformConfiguration {
+    func platform(nvramURL: URL, needsNestedVirtualization: Bool) throws -> VZPlatformConfiguration {
+      if needsNestedVirtualization {
+        throw RuntimeError.VMConfigurationError("macOS virtual machines do not support nested virtualization")
+      }
+
       let result = VZMacPlatformConfiguration()
 
       result.machineIdentifier = ecid

Sources/tart/Platform/Linux.swift

@@ -14,8 +14,12 @@ struct Linux: Platform {
     return result
   }
 
-  func platform(nvramURL: URL) throws -> VZPlatformConfiguration {
-    VZGenericPlatformConfiguration()
+  func platform(nvramURL: URL, needsNestedVirtualization: Bool) throws -> VZPlatformConfiguration {
+    let config = VZGenericPlatformConfiguration()
+    if #available(macOS 15, *) {
+      config.isNestedVirtualizationEnabled = needsNestedVirtualization
+    }
+    return config
   }
 
   func graphicsDevice(vmConfig: VMConfig) -> VZGraphicsDeviceConfiguration {

Sources/tart/Platform/Platform.swift

@@ -3,7 +3,7 @@ import Virtualization
 protocol Platform: Codable {
   func os() -> OS
   func bootLoader(nvramURL: URL) throws -> VZBootLoader
-  func platform(nvramURL: URL) throws -> VZPlatformConfiguration
+  func platform(nvramURL: URL, needsNestedVirtualization: Bool) throws -> VZPlatformConfiguration
   func graphicsDevice(vmConfig: VMConfig) -> VZGraphicsDeviceConfiguration
   func keyboards() -> [VZKeyboardConfiguration]
   func pointingDevices() -> [VZPointingDeviceConfiguration]

Sources/tart/VM.swift

@@ -47,6 +47,7 @@ class VM: NSObject, VZVirtualMachineDelegate, ObservableObject {
        directorySharingDevices: [VZDirectorySharingDeviceConfiguration] = [],
        serialPorts: [VZSerialPortConfiguration] = [],
        suspendable: Bool = false,
+       nested: Bool = false,
        audio: Bool = true,
        clipboard: Bool = true,
        sync: VZDiskImageSynchronizationMode = .full
@@ -66,6 +67,7 @@ class VM: NSObject, VZVirtualMachineDelegate, ObservableObject {
                                                 directorySharingDevices: directorySharingDevices,
                                                 serialPorts: serialPorts,
                                                 suspendable: suspendable,
+                                                nested: nested,
                                                 audio: audio,
                                                 clipboard: clipboard,
                                                 sync: sync
@@ -295,6 +297,7 @@ class VM: NSObject, VZVirtualMachineDelegate, ObservableObject {
     directorySharingDevices: [VZDirectorySharingDeviceConfiguration],
     serialPorts: [VZSerialPortConfiguration],
     suspendable: Bool = false,
+    nested: Bool = false,
     audio: Bool = true,
     clipboard: Bool = true,
     sync: VZDiskImageSynchronizationMode = .full
@@ -309,7 +312,7 @@ class VM: NSObject, VZVirtualMachineDelegate, ObservableObject {
     configuration.memorySize = vmConfig.memorySize
 
     // Platform
-    configuration.platform = try vmConfig.platform.platform(nvramURL: nvramURL)
+    configuration.platform = try vmConfig.platform.platform(nvramURL: nvramURL, needsNestedVirtualization: nested)
 
     // Display
     configuration.graphicsDevices = [vmConfig.platform.graphicsDevice(vmConfig: vmConfig)]