Boot Image

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The Maxine Project: VM Boot Image

Starting an instance of the Maxine VM requires the use of a previously constructed boot image, a near-executable binary VM image that includes an initially populated heap, together with compiled code for a dedicated target platform. For a description of the VM startup sequence, see "Understanding Maxine's bootstrapping".


Boot image contents

The boot image is a binary file that contains a near-executable memory image of the running VM in the format of the target platform for which the image was generated. The target platform is not required to be the same as the platform on which the boot image generation is run (i.e. hosted).

The utility class BootImage is responsible for writing the boot image file in the format of the target platform. This data must be read by native C code during VM startup, so there is necessary agreement between the layout as expressed in the Java class BootImage and in the native startup code.

Click to see the boot image layout summarized as a pseudo-C struct

This diagram is excerpted from the Javadoc comments for class BootImage

     struct BootImage {
          struct Header header;            // see declaration of image_Header in Native/substrate/image.h
          struct StringInfo string_info;   // see declaration of image_StringInfo in Native/substrate/image.h
          byte[header.relocationDataSize] relocation_data;
          byte[] pad;                      // padding such that next field will be aligned on an OS page-size address
          byte[header.heapSize];           // header.heapSize is a multiple of page-size
          byte[header.codeSize];           // header.codeSize is a multiple of page-size
          struct Trailer trailer;          // see declaration of image_Trailer in Native/substrate/image.h
     }

The boot image contains the following groups of information:

  • header: a sequence of integer parameters that identify the binary, describe the target platform, and include specific data about the remaining contents of the image.
  • string info: a sequence of strings in native format that describe the build, name the classes that provide detailed platform description, and name the packages that are bound to the schemes in the build.
  • relocation data: a bit field that identifies address locations in the image that need to be relocated.
  • boot heap: binary image of a heap in target platform format, pre-populated with the objects that represent class and other runtime data needed by the VM until it can begin loading classes on its own.
  • boot code: binary image of method compilations in target platform format, pre-populated with the compilations needed by the VM until it can begin compiling methods on its own.
  • trailer: a subset of the integer parameters that also appear in the header, repeated for verification.

Inspecting the boot image

The Boot Image Inspector view in the Maxine Inspector displays summary information about the specific boot image being viewed: identifying parameters, configuration classes for various schemes, memory region information about the boot heap and boot code, and other specific references into the heap and compiled code.

Click to see the Inspector's summary of a boot image

Boot image generation

Maxine's boot image generator creates a boot image according to a specification (see com.sun.max.vm.VMConfiguration) that both specifies which implementations of particular Schema should be included and describes the platform on which the image is intended to run. Further configuration is supported by the class com.sun.max.config.BootImagePackage, whose operation is mentioned in the following sections but specified more completely in the Javadoc package documentation for package com.sun.max.config.

Class loading and initialization

Any class that might be written into the boot image must first be loaded by the boot image generator. Loading can happen in two way. First any class that is used by the classes that comprise the boot image generator is loaded by the normal host class loading mechanisms, that is using the system class loader. Note that the boot image generator uses many of the same classes as the VM. Second, the generator explicitly loads classes that might be written into the boot image by scanning the class path and searching for sub-packages of com.sun.max.config that contains a class named Package that is a subclass of com.sun.max.config.BootImagePackage. The Package classes act as roots for the set of packages to be included in the boot image. BootImagePackage provides a mechanism for including packages that are outside the com.sun.max namespace. See below for details. This second set of classes is actually loaded by a special class loader, com.sun.max.vm.hosted.HostedVMClassLoader,
that performs additional actions, such as creating the Maxine representation of classes used at runtime. HostedVMClassLoader delegates to the system or boot classloader, as appropriate, so a class is only loaded and initialized once, even if it was initially loaded implicitly by the boot image generator.

As noted, Loaded classes are initialized, which means that static class variables in the boot image have values assigned during image generation. Mostly this is perfectly fine, for example, an instance of a collection class. The problem arises with classes that capture values that are host-specific and inappropriate for the target VM environment. Typically this is any value related to the external environment, which includes the operating system and host virtual machine. An example of the latter would be any object that contains a VM-specific native value, for example a soft reference. The boot image generator takes care of all the known cases in the JDK classes, and this is handled in the class com.sun.max.vm.hosted.JDKInterceptor. Extension classes can register a class to have the class initialization re-run at run-time. Except for such classes, no code is generated in the boot image for static initialization.

Class and method selection

Intuitively, the boot image is intended to contain all the classes and methods that are needed to bootstrap the VM to the point where it can dynamically load further classes from the file system and compile methods for execution. In order to minimize the size of the boot image, only those classes should be included. In practice this set is not easy to define precisely and depends, amongst other things, on the details of the JDK implementation. In particular, it can vary from release to release of the JDK.

The Maxine boot image generator determines the content of the boot image by starting from so-called entry points plus a subset of the standard platform classes that are known to be necessary for the bootstrap. An entry point is a method that:

  • can be called from the external environment (e.g. those annotated with @VM_ENTRY_POINT),
  • is called by pre-compiled/generated code such as stubs,
  • is a reflection invocation stub for methods called during class loading or compilation,
  • is manually selected to satisfy bootstrap requirements (and avoid infinite recursion at runtime), or
  • is manually selected to improve startup time at runtime

These entry points form the basis of the analysis used to discover the methods needed to bootstrap the VM. An example is the run method in com.sun.max.vm.MaxineVM, which is the entry point from the native C code that starts the VM bootstrap. Another mechanism for specifying an entry point is the com.sun.max.vm.runtime.CriticalMethod class.

Extending the boot image

Maxine supports both static and dynamic extension of the VM boot image. Static extension adds extra classes to the boot image generation process. Dynamic extension
is supported using a similar mechanism to that for Java agents,

Static Extension

Static extension provides the opportunity to customize the set of classes to be included in an image. For example, if you need to extend the VM in a way that requires some of the extension classes to execute before the VM is ready to load new classes, then those additional classes must be included in the boot image.

As an example consider the Maxine Virtual Edition, which replaces the normal operating system code layer with a Java implementation that sits directly on the Xen hypervisor. In this case, the file systems are implemented in Java. Since class loading necessarily requires a file system, the file system classes must be included in the boot image in order to be able to bootstrap the extended VM.

It is possible to augment the boot image with much larger set of classes, in the limit, every class needed by an application. Such an image would be self contained and require no dynamic class loading at run-time. In certain cases this might be desirable; however, there are issues regarding class initialization that may need to be addressed.

Class re-initialization

As noted above, static class variables in the boot image will have values assigned during boot image generation unless handled specially by the com.sun.max.vm.hosted.JDKInterceptor class. It is your responsibility to check any extension classes you are adding.

Specifying additional classes or packages

The simplest way to do this for a one-time image build is to suffix the class names or packages to the max image command:

% mx help image
mx image [options] classes|packages...

build a boot image

    Run the BootImageGenerator to build a Maxine boot image. The classes
    and packages specified on the command line will be included in the
    boot image in addition to those found by the Package.java mechanism.
    Package names are differentiated from class names by being prefixed
    with '^'.
...

For example, to add all the classes in the package acme.demo as well as the single class foo.Bar:

% mx image ^acme.demo foo.Bar

Note that packages are not processed recursively, that is, nested packages are not included unless explicitly specified. Note also that the above command assumes that the classes are located within one of the Maxine directories. If this is not the case, their location must be specified with the --cp-sfx option. In addition, methods in the extension classes will only be compiled if they meet the conditions outlined above in "Class and Method selection". Finally, protection issues may require your classes to be defined in a Maxine package if your extension classes need package-private access to any Maxine classes.

For more permanent additions to the boot image, or to meet access requirements, the easiest approach is to put your classes in in a sub-package of com.sun.max in one of the existing Maxine projects, and leverage the automatic loading mechanisms described in the earlier section "Class loading and initialization". While this approach might be acceptable for short-term prototyping, it is inappropriate in the long term unless the code is blessed as a standard VM component.

If you place your classes in a sub-package of com.sun.max, whether in an existing project or not, there is one caveat:

  • If your classes are not in an existing Maxine directory, be sure to use the -cp-sfx option to add the directories to end of the the classpath. If you add them at the front (with -cp-pfx), you may well break the mechanism by which the hosted native library is found.

The recommended approach for extending Maxine is to place the extension packages in a separate project. To satisfy the discovery mechanism of the boot image generator, all extension packages must be rooted in a sub-package of com.sun.max.config, for example, com.sun.max.config.acme. However, it is possible to redirect to packages outside the com.sun.max namespace. An extension package to be included in Maxine should contain a Package class that extends BootImagePackage. BootImagePackage provides four constructors:

  • BootImagePackage(): the default constructor is used to include just those classes in the package containing the Package class.
  • BootImagePackage(boolean): this constructor, with a value of true is used to include the classes in the package containing the Package class and all sub-packages, recursively. In particular, there is no need for Package classes in the sub-packages.
  • BootImagePackage(String, boolean): this redirection constructor includes the classes in the package passed as argument. Sub-packages are only included if the boolean argument is true.
  • BootImagePackage(String...): this constructor serves two purposes. First to include (redirected) packages outside the com.sun.max namespace and, second, to restrict the set of classes that are loaded from a given package. The interpretation of the argument is based on the following patterns:
    • a.b.c.*: include all classes from the package a.b.c.
    • a.b.c.**: include all class from the package a.b.c and its sub-packages, recursively.
    • a.b.c.D: include the class a.b.c.D.

Note that, by design, there is no way to exclude a class from a package, other than by explicitly enumerating all the classes except the one to exclude.

It is legal for a package to be referenced multiple times by different Package constructors. For example, two separate constructors might specify different classes to be included. The final specification is the result of merging the specifications.

Note that the packages generated using BootImagePackage are only candidates for inclusion in the image. A package is only actually included if the method BootImagePackage.isPartOfMaxineVM() for the Package instance returns true. This allows additional controls, such as system properties, to fine tune the
inclusion. For recursive and redirected packages, Package instances are created automatically by cloning the Package that initiated the inclusion, then modifying the package name. Therefore any overridden methods, such as isPartOfMaxineVM(), in the including Package will be in effect in for all the included packages. However, any actual Package classes that do exists in included sub-packages are instantiated and replace the clone.

Configuring the extensions classes

As noted above, adding an extension class does not necessarily cause its methods to be compiled in the boot image. Static fields may also need to be reset if they contain host-specific values and it may also be necessary to re-run the static initializer to create the illusion that class was actually loaded at run-time by the VM. The class com.sun.max.vm.hosted.Extensions provided several methods to achieve these goals:

  • resetField(String className, String fieldName): reset a static field back to its default value. Note that presently there is no check on the value of either of these arguments, and bad values will be silently ignored.
  • registerClassForReInit(String className: run the static initializer on VM startup.
  • registerVMEntryPoint(String className, String methodName: register a method as a root for inclusion in the boot image.

Dynamic Extension

A dynamic (VM) extension is packaged in a jar file and loaded by the VM class loader at runtime, just before the main class is loaded, in a very similar way to that for Java agents. The essential difference is that Java agents are considered application extensions and loaded by the system class loader, whereas a VM extension is loaded by the VM classloader. This allows the VM extension classes to reference the VM classes in the boot image and use the extended features of Maxine, e.g., Maxine annotations and systems programming support.

A VM extension jar file must contain a manifest that defines a VMExtension-Class attribute that specifies the class that is the entry point to the extension, e.g.:

Manifest-Version: 1.0
VMExtension-Class: com.oracle.max.vm.ext.acme.AcmeExtension

This class must define a method with the following signature:

public static void onLoad(String args);

The VM is fully functional at the time that this method is called but, as for Java agents, the method must return for the startup sequence to proceed. If the extension cannot be resolved (for example, because the extension class cannot be loaded, or because the extension class does not have a conformant onLoad method), the VM will abort. If an onLoad method throws an uncaught exception, the VM will abort.

To load the extension at runtime use the -vmextension:jarpath[=args] option, vis:

mx vm -vmextension:acme_vmextension.jar=args ...

Multiple extensions may specified and they are processed in the order they appear on the command line. The jar file is appended to the class path used by the VM classloader. If reference needs to be made to additional classes outside the jar file, these may be specified using the optional manifest attribute VM-Class-Path which has a similar specification to Boot-Class-Path for Java agents.

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