Beyond Address Spaces - Type-Safe JX Operating System

Beyond Address Spaces - Type-Safe JX Operating System
Beyond Address Spaces Flexibility, Performance, Protection, and Resource Management in the
Type-Safe JX Operating System
Michael Golm, Jürgen Kleinöder, Frank Bellosa
University of Erlangen-Nürnberg
Dept. of Computer Science 4 (Distributed Systems and Operating Systems)
Martensstr. 1, 91058 Erlangen, Germany
Early type-safe operating systems were hampered by
poor performance. Contrary to these experiences we show
that an operating system that is founded on an object-oriented, type-safe intermediate code can compete with MMUbased microkernels concerning performance while widening the realm of possibilities.
Moving from hardware-based protection to softwarebased protection offers new options for operating system
quality, flexibility, and versatility that are superior to traditional process models based on MMU protection. However,
using a type-safe language—such as Java—alone, is not
sufficient to achieve an improvement. While other Java
operating systems adopted a traditional process concept, JX
implements fine-grained protection boundaries. The JX System architecture consists of a set of Java components executing on the JX core that is responsible for system initialization, CPU context switching and low-level domain management. The Java code is organized in components which
are loaded into domains, verified, and translated to native
JX runs on commodity PC hardware, supports network
communication, a frame grabber device, and contains an
Ext2-compatible file system. Without extensive optimization
this file system already reaches a throughput of 50% of
1 Introduction
For several years there has been an ongoing discussion in
the OS community whether software-based protection is a
promising approach [3]. We want to support the arguments
for software-based protection with the experience we
gained while building the JX operating system.
While MMU-based protection is commonly used in
today’s operating systems it has some deficiencies [10], [3].
From the point of functionality it neither meets the actual
requirements of fine grained protection (page size is too
coarse), nor offers it appropriate abstractions for access control (page tags are not capabilities).
These deficiencies justify the exploration of alternative
protection mechanisms. Java popularized a protection
mechanism that is based on a combination of type-safe
intermediate code and load-time program verification.
Several other research groups have been building Javabased operating systems: Sun’s JavaOS [14], which was
later replaced by “JavaOS for Business” [18], JN [16], JKernel [11], KaffeOS [2], and Joust [9]. But they are either
limited by a monolithic structure or are built upon a full-featured OS and JVM. Furthermore, no performance figures
for OS related functionality are published. KaffeOS and JKernel are two projects that try to overcome the monolithic
structure by intruducing a process concept which is similar
to the domain concept of JX. But their research is mainly
concerned with introducing the traditional process concept
and a red line [6] between user level and kernel into their
Java operating system. While a red line between trusted and
untrusted code is indeed important, we must free our mind
from the MMU-enforced architecture of traditional operating systems. The aim of our research is a customizable and
flexible [4] open OS architecture with fine-grained protection boundaries. Depending on functionality and deployment of a system there are different levels of trust and protection. An embedded real-time system needs a different red
line than a single-user desktop system or a multi-user server
system or an active network node OS [5]. In our architecture
it is possible to draw red lines when and where they are
While other Java operating systems require a microkernel, or even a full operating system including a JVM, JX
runs on the bare hardware with only a minimal statically
linked core (< 100kB). The remaining operating system
functionality, including device drivers, is provided by Java
components that are verified, compiled to native code, and
optimized at load time.
The paper is structured as follows: In section 2 we
describe the architecture of the JX system. The problems
that appear when untrusted modules directly access hardware are discussed in section 3. Section 4 gives examples of
the performance of IPC and file system access.
2 JX System Architecture
The JX system consists of a small core, written in C and
assembler, which is less than 100 kilobytes in size. The
majority of the system is written in Java and running in separate protection domains. The core runs without any protection and therefore must be trusted. It contains functionality
that can not be provided at the Java level (system initialization after boot up, saving and restoring CPU state, low-level
domain management, monitoring).
Thread Control Blocks
Domain A
Domain B
C Code
Thread Control Blocks
Domain Zero
The Java code is organized in components (Sec. 2.2)
which are loaded into domains (Sec. 2.1), verified (Sec.
2.4), and translated to native code (Sec. 2.5). A domain can
communicate with another domain by using portals (Sec.
The protection of the architecture is solely based upon
the JX core, the code verifier, the code translator, and hardware-dependent components (Sec. 3). These elements are
the trusted computing base [7] of our architecture.
A domain is the unit of protection, resource management, and typing.
Protection. Components in one domain trust each other.
One of our aims is code reusability between different system configurations. A component should be able to run in a
separate domain, but also together (co-located) with other
components in one domain. This leads to several problems:
•The parameter passing semantics must be by-copy in interdomain calls, but may be by-reference in the co-located
case. This is an open problem.
•During a portal call a component must check the validity
of the parameters because the caller could be in a different
domain and is not trusted. When caller and callee are colocated (intra-domain call), the checks change their motivation—they are no longer done for security reasons but
for robustness reasons. We currently parametrize the component whether a safety check should be performed or not.
Resource Management. JX domains have their own heap
and own memory area for stacks, code, etc. If a domain
needs memory, a domain-specific policy decides whether
this request is allowed and how it may be satisfied, i.e.,
where the memory comes from. Objects are not shared
between domains, but it is possible to share memory. Other
Java systems use shared objects with the consequence of
complicated and not interdependent garbage collection,
problems during domain termination, and quality-of-service crosstalk [13] between garbage collectors.
Typing. A domain has its own type space, that initially contains exactly one type: java.lang.Object. Types (classes and
interfaces) and code (classes) can then be loaded into the
domain. Our type-space approach differs from the Java type
spaces [12] as we do not use the class loader as type-space
separator but tie type separation to resource management
and protection. By this means a SecurityManager becomes
redundant and protection boundaries are automatically
The C and assembler code of the JX core are encapsulated by a special domain, called DomainZero. All other
domains contain only Java code. We do not allow native
Code is generally loaded as a component. JX does not
support loading of single classes. A component is a collection of classes and interfaces. There are four kinds of components:
•Library: A simple collection of reusable classes and interfaces (example: the Java Development Kit).
•Service: A component that implements a specific service,
e. g., a file system or a device driver. A service component
is started after it has been loaded. To start a service means
to execute a static method that is specified in a configuration file that is part of the component.
•Interface: Access to a service in another domain is always
performed using an interface. An interface component
contains all interfaces that are needed to access a service.
An interface component also contains the classes of
parameter objects. A special interface library zero contains
all interfaces to access DomainZero.
•Domain: A domain is started by loading a domain component. An initial thread is created and a static method is executed.
Components can be shared between domains. Sharing
happens at two levels. At a logical level sharing establishes
a window of type compatibility between two domains. At a
lower level, sharing saves memory, because the (machine)
code of the component has to be stored only once. While
component sharing complicates resource accounting and
domain termination, we believe that code sharing is an
essential requirement for every real operating system.
While code can be shared if the domains use the same type
of execution environment (translator, memory layout),
static variables are never shared. In JX this is implemented
by splitting the internal class representation into a domainlocal part, that contains the statics, and a shared part, that
contains code and meta information.
When a component is loaded into a domain, its bytecode
is verified before it is translated into machine code. As in the
normal Java bytecode verifier, the conformance to the Java
rules is checked. Basically this guarantees type safety. Furthermore the verifier performs additional JX-specific checks
regarding interrupt handlers (Sec. 2.6), memory objects
(Sec. 2.7), and schedulers (Sec. 2.9).
A type-safe operating system has the well-known advantages of robustness and ease of debugging. Furthermore, it
is possible to base protection and optimization mechanisms
on the type information. This is extensively employed in JX
by using well-known interfaces (contained in a trusted
library) and restricting the implementability of these interfaces (Sec. 2.6 and 2.7).
IPC, Portals, and Services
Domains communicate solely by using portals. An
object that may be accessed from another domain is called
service. Each service is associated with a service thread.
A portal is a remote reference that represents a service,
which is running in another domain. Portals are capabilities
that can be passed between domains. Portals allow to establish the “principle of least privilege”. A domain gets only
the portals it needs for doing its job.
A portal looks like a normal object reference. The portal
type is an interface that is derived from the interface Portal.
A portal invocation behaves like a normal synchronous
interface method invocation: The calling thread is blocked,
the service thread executes the method, returns the result
and is then again available for new service requests via a
portal. The caller thread is unblocked when the service
method returns. While a service thread is processing a
request, further requests for the same service are blocked.
Domain A
Domain B
An object reference can be passed as parameter of a portal invocation only if the object is a service. In this case a
portal to the service is transferred and the reference counter
of the service is incremented. Other parameters are passed
by value. When a portal is no longer referenced in a domain,
it is removed by the garbage collector and the reference
counter of the associated service is decremented.
A portal/service connection between two domains
requires that these domains have overlapping type spaces,
i.e. the interface component must be logically shared. If the
interface component depends on other components, they
must be shared, too.
Component Verifier
Component Translator
Components are translated from bytecode into machine
code. The translator is a crucial element of JX to get a reasonable performance. The translator is domain-specific, so
it can be customized for a domain to employ applicationspecific translation strategies. The same component may be
translated differently in different domains. As the translator
is a trusted component, this facility has to be used carefully
because it affects the protection of the whole system.
Furthermore the translator is used to “short-circuit” several portal invocations. Special portals that are exported by
DomainZero often do not need the domain context of
DomainZero. Invocations of such portals can be inlined
directly at the call site.
An interrupt is handled by invoking the handleInterrupt
method of a previously installed interrupt handler object.
The method is executed by a dedicated thread while interrupts on the interrupted CPU are disabled. This would be
called the first-level interrupt handler in a traditional operating system. To guarantee that the handler can not block the
system forever, the verifier checks all classes that implement the InterruptHandler interface whether the handleInterrupt method has certain time bounds. To avoid undecidable
problems, only a simple code structure is allowed (linear
code, loops with constant bound and no write access to the
loop variable inside the loop). A handleInterrupt method
usually acknowledges the interrupt at the device and
unblocks a thread that handles the interrupt asynchronously.
Memory Management
Heap and Garbage Collection. The memory of the
objects within a domain is managed by a heap manager with
garbage collector. Currently, the heap manager is part of the
JX core. It cooperates with the translator to obtain information about the object structure and stack structure. So far we
are working with only one heap manager implementation
and one translator implementation, but it is also possible to
build domain-specific heap managers. They can even be
written in Java and run in their own domain. The heap manager is a trusted part of the system.
Memory objects. To handle large amounts of data, Java
uses arrays. Java arrays are useless for operating system
components, because they do not provide access control and
it is not possible to share only a part of an array. JX uses
Memory objects instead. The memory that is represented by
such a Memory object can be accessed via method invocations. These invocations are inlined by inserting the
machine instructions for the memory access instead of the
method invocation. This makes memory access as fast as
array access. A Memory object can represent a part of the
memory of another Memory object and Memory objects can
be shared between domains like portals. Sharing memory
objects between domains and the ability to create subranges
are the fundamental mechanisms for a zero-copy implementation of system components, like the network stack, the file
system, or an NFS server.
Avoiding range checks by object mapping. A memory
range can be mapped to a (virtual) object that implements a
marker interface (an interface without methods that is only
used to mark a class as MappedLittleEndian or MappedBigEndian). The verifier ensures that a class that implements
one of these interfaces is never instantiated by the new bytecode. Instead the objects are created by mapping and the
translator generates code to directly access the memory
range for access to instance variables. This makes the range
check redundant.
Domain Termination
When a domain terminates, all resources must be
released. Further interaction with the domain raises an
All services are removed by stopping the service thread.
A service contains a reference counter, that is incremented
each time a portal to this service is passed to another
domain. It is also incremented when a client domain passes
the portal to another client domain. It is decremented, when
the portal object in a client domain is garbage collected or
when the client domain is terminated. As long as the reference counter is not zero, the service can not be completely
removed when its domain terminates. Until all reference
counters drop to zero, the domain remains in a zombie state.
Interrupt handlers are uninstalled. All threads are
stopped and the memory (heap, stacks) is released.
CPU scheduling in JX is split into two scheduler levels.
The low-level scheduler decides which domain should run
on the CPU. Each CPU has its own low-level scheduler. The
high-level scheduler is domain-specific - each domain has
one high-level scheduler per available CPU. A domain may
not be allowed to use all CPUs. To use a CPU, the domain
must obtain a CPU portal for the specific CPU. The highlevel schedulers are responsible for scheduling the threads
of a domain.
The high-level scheduler may be part of the domain or
may be located in a different domain.
To avoid that one domain monopolizes the CPU, the
computation can be interrupted by a timer interrupt. The
timer interrupt leads to the invocation of the low-level
scheduler. The low-level scheduler first informs the highlevel scheduler of the interrupted domain about the preemption. For this purpose it invokes a method of the high-level
scheduler with interrupts disabled. An upper bound for the
execution time of this method has been verified during the
verification phase. When the method returns, the system
switches back to the low-level scheduler. The low-level
scheduler then decides, which domain to run next. After
ensuring that it will be reactivated with the next (CPU-local)
timer interrupt, the low-level scheduler activates the highlevel scheduler of the selected domain. The high-level
scheduler chooses the next runnable thread. It can switch to
this thread by calling a method at the CPU portal. This
method can only be called by a thread that runs on the corresponding CPU.
3 Device Drivers
Due to the enormous amount of new hardware that
appeared in the last years, operating system code is dominated by device drivers. While it is rather straight forward to
move most operating system parts, such as file systems or
network protocols, out of the trusted kernel, it is very difficult for device drivers.
Developers of commodity hardware do not assume that
their products are directly accessed by untrusted code.
Although the Nemesis project has demonstrated that it is
possible to build user-safe hardware [17], we do not expect
such hardware to become commercially available in the
near future.
Device drivers in JX are programmed in Java and are
installed as service component in a domain. JX aims at only
trusting the hardware manufacturer (and not the driver provider) in assuming that the device behaves exactly according to the device specification. When special functionality
of the hardware allows bypassing the protection mechanisms of JX, the code for controlling this functionality must
also be trusted. This code can not be part of the JX core,
because it is device dependent. One example for such code
is the busmaster DMA initialization, because a device can
be programmed to transfer data to arbitrary main memory
To reduce the amount of critical code, the driver is split
into a (simple) trusted part and a (complex) untrusted part.
To understand the issues related to device drivers, we
have developed drivers for the IDE controller, the 3C905B
network card, and the Bt848 framegrabber chip. The IDE
controller and network card basically use a list of physical
memory addresses for busmaster DMA. The code that
builds and installs these tables is trusted. The Bt848 chip
can execute a program in a special instruction set (RISC
code). This program writes captured scanlines into arbitrary
memory regions. The memory addresses are part of the
RISC program. We currently trust the RISC generator and
thus limit extensibility. To allow an untrusted component to
download RISC code, we would need a verifier for this
instruction set.
All microkernel-based systems, where drivers are moved
into untrusted address spaces run into the same problems,
but they have much weaker means to cope with these problems. Using an MMU does not help, because busmaster
DMA accesses physical RAM without consulting page
tables. JX uses type-safety, special checks of the verifier,
and splitted drivers to address these problems.
4 Performance
IPC. We measured the performance of a portal call. Table 1
compares the IPC round-trip performance of JX with fast
microkernels and other Java operating systems.
L4KA (PIII, sysenter, sysexit) [8]
Fiasco/L4 (PIII 450 MHz)
J-Kernel (LRMI on MS-VM, PPro 200MHz) [11]
Alta/KaffeOS [1]
JX/hosted (Linux 2.2.14, PIII 500MHz)
JX/native (PIII 500MHz)
hosted JX can be attributed to the use of sigprocmask to disable/restore signals.
The IPC cost of J-Kernel does not include thread switching costs, because the J-Kernel uses a “segmented” stack.
IPC without switching threads complicates resource
accounting, garbage collection, termination, and type separation.
File System. We have implemented the ext2fs in Java [19].
We reused the algorithms that are used in Linux-ext2fs.
We used the iozone benchmark to measure the Linux
ext2fs re-read throughput (file size: 4 kB, record length: 4
kB — iozone -r 4 -s 4 -i 0 -i 1). To measure JX re-read
throughput we wrote a Java benchmark, similar to iozone.
The system configuration that we measured works as follows: The virtual file system, the buffer cache, and the ext2
file system run in one domain (FSDomain). The IDE device
driver runs in another domain. The client runs in a third
domain. A service thread in the FSDomain accepts client
requests. The client domain gets a portal to the virtual file
system and calls lookup to get a FileInode portal. FSDomain
uses one thread to asynchronously receive data from the
block device driver. Only the service thread is active in this
benchmark, because all data comes from the buffer cache.
Linux (PIII 500 MHz)
JX (PIII 500MHz)
JX co-located (PIII 500MHz)
Table 2: File system re-read throughput and latency
We now try to estimate the necessary performance
improvement to reach Linux throughput. The latency can be
broken down as shown in table 3.
Table 1: IPC latency (round-trip)
Comparing IPC times for these systems is not easy
because they were measured on different hardware (cache
size, cache bandwidth, memory bandwidth, etc.), and, more
importantly, they have different protection models. IPC is
usually more expensive on a system with better protection.
Currently the IPC path in JX is implemented in C and not
optimized. It may be better compared with the Fiasco implementation of L4 than with L4KA. The emphasis of our work
was on getting the architecture right and enabling performance, but not achieving it. The bad performance of Linux-
(MByte/s) (µsec/4kB)
JX goal
memory copy
file system logic
Table 3: Latency breakdown (in µsec)
Memory copy and IPC are relative constant costs in JX.
The poor performance of the file system logic is not a problem of the JX architecture but of our non-optimizing compiler. With an improvement of factor 4 in Java performance,
we would reach the Linux performance level. Although
safety-related overhead cannot be avoided completely,
recent research in JIT compiler technology has shown that
an optimizing compiler can improve the performance of a
Java program significantly. Performance differences of factor 10 are not unusual between non-optimizing and optimizing Java compilers.
5 Status and future work
The system runs either on standard PC hardware (i486,
Pentium, and embedded PCs with limited memory) or as a
guest system on Linux. The JX Java components also run on
a standard JDK (with an emulation for Memory objects).
When running on the bare hardware, the system can access
IDE disks [19], 3COM 3C905 NICs [15], and Matrox G200
video cards. The network code contains IP, TCP, UDP,
NFS2 client, and SUN RPC. JX also runs on a PIII SMP
We have already implemented a heap manager that runs
in its own domain and manages the heap of another domain.
This heap manager is always called, when the managed
domain tries to create a new object or array. Creating a new
object with the build-in mechanism costs 250 cycles, calling
another domain adds at least 650 cycles. This is not practical
until we further improve IPC performance. There are also
efforts to improve the quality of the machine code generated
by the translator.
The JX architecture supports a broad spectrum of OS
structures — from pure monolithic to a vertical structure
similar to the Nemesis OS [13]. We are going to investigate
the issues that are involved when reusing components
between these diverse operating system configurations.
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