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===============================================
Release notes for the Genode OS Framework 10.02
===============================================
Genode Labs
After the release of the feature-packed version 9.11, we turned our attention
to improving the platform support of the framework. The current release 10.02
bears fruit of these efforts on several levels.
First, we are proud to announce the support for two new base platforms, namely
the NOVA hypervisor and the Codezero microkernel. These new kernels complement
the already supported base platforms Linux, L4/Fiasco, L4ka::Pistachio, and
OKL4. So why do we address so many different kernels instead of focusing our
efforts to one selected platform? Our observation is that different applications
pose different requirements on the kernel. Most kernels have a specific profile
with regard to security, hardware support, complexity, scheduling, resource
management, and licensing that may make them fit well for one application area
but not perfectly suited for a different use case. There is no single perfect
kernel and there doesn't need to be one. By using Genode, applications
developed for one kernel can be ported to all the other supported platforms with
a simple recompile. We believe that making Genode available on a new kernel is
beneficial for the kernel developers, application developers, and users alike.
For kernel developers, Genode brings additional workloads to stress-test their
kernel, and it extends the application area of the kernel. Application
developers can address several kernel platforms at once instead of tying their
programs to one particular platform. Finally, users and system integrators can
pick their kernel of choice for the problem at hand. Broadening the platform
support for Genode helps to make the framework more relevant.
Second, we introduced a new way for managing real-time priorities, which fits
perfectly with the recursive system structure of Genode. This clears the way to
multi-media and other real-time workloads that we target with our upcoming
work. We implemented the concept for the L4ka::Pistachio and OKL4 platforms.
With real-time priorities on OKL4, it is possible to run multiple instances of
the OKLinux kernel at the same time, each instance at a different priority.
Third, we vastly improved the existing framework, extended the ARM architecture
support to cover dynamic loading and the C runtime, introduced a new
thread-context management, added a plugin-concept to our C runtime, and
improved several device drivers.
Even though platform support is the main focus of this release, we introduced a
number of new features, in particular the initial port of the Python 2.6 script
interpreter.
NOVA hypervisor as new base platform
####################################
When we started the development of Genode in 2006 at the OS Group of the
Technische Universität Dresden, it was originally designated to be the user
land of a next-generation and to-be-developed new kernel called NOVA. Because
the kernel was not ready at that time, we had to rely on intermediate solutions
as kernel platform such as L4/Fiasco and Linux during development. These
circumstances led us to the extremely portable design that Genode has today and
motivated us to make Genode available on the whole family of L4 microkernels.
In December 2009, the day we waited for a long time had come. The first version
of NOVA was publicly released:
:Official website of the NOVA hypervisor:
[http://hypervisor.org]
Besides the novel and modern kernel interface, NOVA has a list of features that
sets it apart from most other microkernels, in particular support for
virtualization hardware, multi-processor support, and capability-based
security.
Why bringing Genode to NOVA?
============================
NOVA is an acronym for NOVA OS Virtualization Architecture. It stands for a
radically new approach of combining full x86 virtualization with microkernel
design principles. Because NOVA is a microkernelized hypervisor, the term
microhypervisor was coined. In its current form, it successfully addresses
three main challenges. First, how to consolidate a microkernel system-call API
with a hypercall API in such a way that the API remains orthogonal? The answer
to this question lies in NOVA's unique IPC interface. Second, how to implement
a virtual machine monitor outside the hypervisor without spoiling
performance? The Vancouver virtual machine monitor that runs on top NOVA proves
that a decomposition at this system level is not only feasible but can yield
high performance. Third, being a modern microkernel, NOVA set out to pursue a
capability-based security model, which is a challenge on its own.
Up to now, the NOVA developers were most concerned about optimizing and
evaluating NOVA for the execution of virtual machines, not so much about
running a fine-grained decomposed multi-server operating system. This is where
Genode comes into play. With our port of Genode to NOVA, we contribute the
workload to evaluate NOVA's kernel API against this use case. We are happy to
report that the results so far are overly positive.
At this point, we want to thank the main developers of NOVA Udo Steinberg and
Bernhard Kauer for making their exceptional work and documentation publicly
available, and for being so responsive to our questions. We also greatly
enjoyed the technical discussions we had and look forward to the future
evolution of NOVA.
Challenges
==========
From all currently supported base platforms of Genode, the port to NOVA was the
most venturesome effort. It is the first platform with kernel support for
capabilities and local names. That means no process except the kernel has
global knowledge. This raises a number of questions that seem extremely hard
to solve at the first sight. For example: There are no global IDs for threads
and other kernel objects. So how to address the destination for an IPC message?
Or another example: A thread does not know its own identity per se and there is
no system call similar to 'getpid' or 'l4_myself', not even a way to get a
pointer to a thread's own user-level thread-control block (UTCB). The UTCB,
however, is needed to invoke system calls. So how can a thread obtain its UTCB
in order to use system calls? The answers to these questions must be provided by
user-level concepts. Fortunately, Genode was designed for a capability kernel
right from the beginning so that we already had solutions to most of these
questions. In the following, we give a brief summary of the specifics of Genode
on NOVA:
* We maintain our own system-call bindings for NOVA ('base-nova/include/nova/')
derived from the NOVA specification. We put the bindings under MIT license
to encourage their use outside of Genode.
* Core runs directly as roottask on the NOVA hypervisor. On startup, core
maps the complete I/O port range to itself and implements debug output via
comport 0.
* Because NOVA does not allow rootask to have a BSS segment, we need a slightly
modified linker script for core (see 'src/platform/roottask.ld').
All other Genode programs use Genode's generic linker script.
* The Genode 'Capability' type consists of a portal selector expressing the
destination of a capability invocation and a global object ID expressing
the identity of the object when the capability is specified as an invocation
argument. In the latter case, the global ID is needed because of a limitation
of the current system-call interface. In the future, we are going to entirely
remove the global ID.
* Thread-local data such as the UTCB pointer is provided by the new thread
context management introduced with the Genode release 10.02. It enables
each thread to determine its thread-local data using the current stack
pointer.
* NOVA provides threads without time called local execution contexts (EC).
Local ECs are intended as server-side RPC handlers. The processing time
needed to perform RPC requests is provided by the client during the RPC call.
This way, RPC semantics becomes very similar to function call semantics with
regard to the accounting of CPU time. Genode already distinguishes normal
threads (with CPU time) and server-side RPC handlers ('Server_activation')
and, therefore, can fully utilize this elegant mechanism without changing the
Genode API.
* On NOVA, there are no IPC send or IPC receive operations. Hence, this part
of Genode's IPC framework cannot be implemented on NOVA. However, the
corresponding classes 'Ipc_istream' and 'Ipc_ostream' are never used directly
but only as building blocks for the actually used 'Ipc_client' and
'Ipc_server' classes. Compared with the other Genode base platforms, Genode's
API for synchronous IPC communication maps more directly onto the NOVA
system-call interface.
* The Lock implementation utilizes NOVA's semaphore as a utility to let a
thread block in the attempt to get a contended lock. In contrast to the
intuitive way of using one kernel semaphore for each user lock, we use only
one kernel semaphore per thread and the peer-to-peer wake-up mechanism we
introduced in the release 9.08. This has two advantages: First, a lock does
not consume a kernel resource, and second, the full semantics of the Genode
lock including the 'cancel-blocking' semantics are preserved.
* NOVA does not support server-side out-of-order processing of RPC requests.
This is particularly problematic in three cases: Page-fault handling, signal
delivery, and the timer service.
A page-fault handler can receive a page fault request only if the previous
page fault has been answered. However, if there is no answer for a
page-fault, the page-fault handler has to decide whether to reply with a
dummy answer (in this case, the faulter will immediately raise the same page
fault again) or block until the page-fault can be resolved. But in the latter
case, the page-fault handler cannot handle any other page faults. This is
unfeasible if there is only one page-fault handler in the system. Therefore,
we instantiate one pager per user thread. This way, we can block and unblock
individual threads when faulting.
Another classical use case for out-of-order RPC processing is signal
delivery. Each process has a signal-receiver thread that blocks at core's
signal service using an RPC call. This way, core can selectively deliver
signals by replying to one of these in-flight RPCs with a zero-timeout
response (preserving the fire-and-forget signal semantics). On NOVA however,
a server cannot have multiple RPCs in flight. Hence, we use a NOVA semaphore
shared between core and the signal-receiver thread to wakeup the
signal-receiver on the occurrence of a signal. Because a semaphore-up
operation does not carry payload, the signal has to perform a non-blocking
RPC call to core to pick up the details about the signal. Thanks to Genode's
RPC framework, the use of the NOVA semaphore is hidden in NOVA-specific stub
code for the signal interface and remains completely transparent at API
level.
For the timer service, we currently use one thread per client to avoid the need
for out-of-order RPC processing.
* Because NOVA provides no time source, we use the x86 PIT as user-level time
source, similar as on OKL4.
* On the current version of NOVA, kernel capabilities are delegated using IPC.
Genode supports this scheme by being able to marshal 'Capability' objects as
RPC message payload. In contrast to all other Genode base platforms where
the 'Capability' object is just plain data, the NOVA version must marshal
'Capability' objects such that the kernel translates the sender-local name to
the receiver-local name. This special treatment is achieved by overloading
the marshalling and unmarshalling operators of Genode's RPC framework. The
transfer of capabilities is completely transparent at API level and no
modification of existing RPC stub code was needed.
How to explore Genode on NOVA?
==============================
The Genode release 10.02 supports the NOVA pre-release version 0.1. You can
download the archive here:
:Download NOVA version 0.1:
[http://os.inf.tu-dresden.de/~us15/nova/nova-hypervisor-0.1.tar.bz2]
For building NOVA, please refer to the 'README' file contained in the archive.
Normally, a simple 'make' in the 'build/' subdirectory is all you need to
get a freshly baked 'hypervisor' binary.
The NOVA platform support for Genode resides in the 'base-nova/' repository.
To create a build directory prepared for compiling Genode for NOVA, you can use
the 'create_builddir' tool. From the top-level Genode directory, issue the
following command:
! ./tool/builddir/create_builddir nova_x86 GENODE_DIR=. BUILD_DIR=<dir>
This tool will create a fresh build directory at the location specified
as 'BUILD_DIR'. Provided that you have installed the
[http://genode.org/download/tool-chain - Genode tool chain], you can now build
Genode by using 'make' from within the new build directory.
Note that in contrast to most other kernels, the Genode build process does not
need to know about the source code of the kernel. This is because Genode
maintains its own system-call bindings for this kernel. The bindings reside in
'base-nova/include/nova/'.
NOVA supports multi-boot boot loaders such as GRUB, Pulsar, or gPXE. For
example, a GRUB configuration entry for booting the Genode demo scenario
with NOVA looks as follows, whereas 'genode/' is a symbolic link to the
'bin/' subdirectory of the Genode build directory and the 'config' file
is a copy of 'os/config/demo'.
! title Genode demo scenario
! kernel /hypervisor noapic
! module /genode/core
! module /genode/init
! module /config/demo/config
! module /genode/timer
! module /genode/ps2_drv
! module /genode/pci_drv
! module /genode/vesa_drv
! module /genode/launchpad
! module /genode/nitpicker
! module /genode/liquid_fb
! module /genode/nitlog
! module /genode/testnit
! module /genode/scout
Please note the 'noapic' argument for the NOVA hypervisor. This argument
enables the use of ordinary PIC IRQ numbers, as relied on by our current
PIT-based timer driver.
Limitations
===========
The current NOVA version of Genode is able to run the complete Genode demo
scenario including several device drivers (PIT, PS/2, VESA, PCI) and the GUI.
At version 0.1, however, NOVA is not yet complete and misses some features
needed to make Genode fully functional. The current limitations are:
* No real-time priority support: NOVA supports priority-based scheduling
but, in the current version, it allows each thread to create scheduling
contexts with arbitrary scheduling parameters. This makes it impossible
to enforce priority assignment from a central point as facilitated with
Genode's priority concept.
* No multi-processor support: NOVA supports multi-processor CPUs through
binding each execution context (ECs) to a particular CPU. Because everyone
can create ECs, every process could use multiple CPUs. However, Genode's API
devises a more restrictive way of allocating and assigning resources. In
short, physical resource usage should be arbitrated by core and the creation
of physical ECs should be performed by core only. However, Remote EC creation
is not yet supported by NOVA. Even though, multiple CPU can be used with
Genode on NOVA right now by using NOVA system calls directly, there is no
support at the Genode API level.
* Missing revoke syscall: NOVA is not be able to revoke memory mappings or
destroy kernel objects such as ECs and protection domains. In practice, this
means that programs and complete Genode subsystems can be started but not
killed. Because virtual addresses cannot be reused, code that relies on
'unmap' will produce errors. This is the case for the dynamic loader or
programs that destroy threads at runtime.
Please note that these issues are known and worked on by the NOVA developers.
So we expect Genode to become more complete on NOVA soon.
Codezero kernel as new base platform
####################################
Codezero is a microkernel primarily targeted to ARM-based embedded systems.
It is developed as an open-source project by a British company called B-Labs.
:B-Labs website:
[http://b-labs.com]
The Codezero kernel was first made publicly available in summer 2009. The
latest version, documentation, and community resources are available at the
project website:
:Codezero project website:
[http://l4dev.org]
As highlighted by the name of the project website, the design of the kernel is
closely related to the family of L4 microkernels. In short, the kernel provides
a minimalistic set of functionality for managing address spaces, threads, and
communication between threads, but leaves complicated policy and device access
to user-level components.
To put Codezero in relation to other L4 kernels, here is a quick summary on the
most important design aspects as implemented with the version 0.2, and how
our port of Genode relates to them:
* In the line of the original L4 interface, the kernel uses global name spaces
for kernel objects such as threads and address spaces.
* For the interaction between a user thread and the kernel, the concept of
user-level thread-control blocks (UTCB) is used. A UTCB is a small
thread-specific region in the thread's virtual address space, which is
always mapped. The access to the UTCB can never raise a page fault,
which makes it perfect for the kernel to access system-call arguments,
in particular IPC payload copied from/to user threads. In contrast to other
L4 kernels, the location of UTCBs within the virtual address space is managed
by the user land.
On Genode, core keeps track of the UTCB locations for all user threads.
This way, the physical backing store for the UTCB can be properly accounted
to the corresponding protection domain.
* The kernel provides three kinds of synchronous inter-process communication
(IPC): Short IPC carries payload in CPU registers only. Full IPC copies
message payload via the UTCBs of the communicating parties. Extended IPC
transfers a variable-sized message from/to arbitrary locations of the
sender/receiver address spaces. During an extended IPC, page faults may
occur.
Genode solely relies on extended IPC, leaving the other IPC mechanisms to
future optimizations.
* The scheduling of threads is based on hard priorities. Threads with the
same priority are executed in a round-robin fashion. The kernel supports
time-slice-based preemption.
Genode does not support Codezero priorities yet.
* The original L4 interface leaves the question on how to manage and account
kernel resources such as the memory used for page tables unanswered.
Codezero makes the accounting of such resources explicit, enables the
user-land to manage them in a responsible way, and prevent kernel-resource
denial-of-service problems.
* In contrast to the original L4.v2 and L4.x0 interfaces, the kernel provides
no time source in the form of IPC timeouts to the user land. A time source
must be provided by a user-space timer driver. Genode employs such a timer
services on all platforms so that it is not constricted by this limitation.
In several ways, Codezero goes beyond the known L4 interfaces. The most
noticeable addition is the support of so-called containers. A container is
similar to a virtual machine. It is an execution environment that holds a set
of physical resources such as RAM and devices. The number of containers and the
physical resources assigned to them are static and have to be defined at build
time. The code executed inside a container can roughly be classified by two
categories. First, there are static programs that require strong isolation from the
rest of the system but no classical operating-system infrastructure, for
example special-purpose telecommunication stacks or cryptographic functionality
of an embedded device. Second, there are kernel-like workloads, which use the L4
interface to substructure the container into address spaces, for example a
paravirtualized Linux kernel that uses Codezero address spaces to protect Linux
processes. Genode runs inside a container and facilitates Codezero's L4
interface to implement its multi-server architecture.
The second major addition is the use of a quite interesting flavor of a
capability concept to manage the authorization of processes to access system
resources and system calls. In contrast to most current approaches, Codezero
does not attempt to localize the naming of physical objects such as
address-space IDs and thread ID. So a capability is not referred to via a local
name but a global name. However, for delegating authorization throughout the
system, the capability approach is employed. A process that possesses a capability
to an object can deal with the object. It can further delegate this access
right to another party (to which it holds a capability). In a way, this
approach keeps the kernel interface true to the original L4 interface but
provides a much stronger concept for access control. However, it is important
to point out that the problem of ambient authority is not (yet) addressed by
this concept. If a capability is not used directly but specified as an argument
to a remote service, this argument is passed as a plain value not
protected by the kernel. Because the identity of the referenced object can be
faked by the client, the server has to check the plausibility of the argument.
For the server, however, this check is difficult/impossible because it has no
way to know whether the client actually possesses the capability it is talking
about.
The current port of Genode to Codezero does not make use of the capability
concept for fine-grained communication control, yet. As with the other L4
kernels, each object is identified by a unique ID allocated by a core service.
There is no mechanism in place to prevent faked object IDs.
:Thanks:
We want to thank the main developer of Codezero Bahadir Balban for his great
responsiveness to our feature requests and questions. Without his help, the
port would have taken much more effort. We hope that our framework will be of
value to the Codezero community.
Using Genode with Codezero
==========================
The port of Genode is known to work with the devel branch of Codezero version
0.2 as of 2010-02-19.
To download the Codezero source code from the official source-code repository,
you can use the following commands:
!git clone git://git.l4dev.org/codezero.git
!git checkout -b devel --track origin/devel
In addition to downloading the source code, you will need to apply the small
patch 'base-codezero/lcd.patch' to the Codezero kernel to enable the device
support for the LCD display. Go to the 'codezero.git/' directory and issue:
!patch -p1 < <genode-dir>/base-codezero/lcd.patch
For a quick start with Codezero, please follow the "Getting Started with the
Codezero Development" guide, in particular the installation of the tool chain:
:Getting started with Codezero:
[http://www.l4dev.org/getting_started]
The following steps guide you through building and starting Genode on Codezero
using the Versatilepb platform as emulated by Qemu.
# Create a Genode build directory for the Codezero/Versatilepb platform.
Go to the Genode directory and use the following command where '<build-dir>'
is the designated location of the new Genode build directory and
'<codezero-src-dir>' is the 'codezero.git/' directory with the Codezero
source tree, both specified as absolute directories.
! ./tool/builddir/create_builddir codezero_versatilepb \
! GENODE_DIR=. \
! BUILD_DIR=<genode-build-dir> \
! L4_DIR=<codezero-src-dir>
With the build directory created, Genode targets can immediately be
compiled for Codezero. For a quick test, go to the new build directory and
issue:
! make init
In addition to being a Genode build directory, the directory is already
prepared to be used as Codezero container. In particular, it holds a
'SConstruct' file that will be called by the Codezero build system. In this
file, you will find the list of Genode targets to be automatically built when
executing the Codezero build process. Depending on your work flow, you may
need to adapt this file.
# To import the Genode container into the Codezero configuration system,
go to the 'codezero.git/' directory and use the following command:
! ./scripts/baremetal/baremetal_add_container.py \
! -a -i Genode -s <genode-build-dir>
# Now, we can add and configure a new instance of this container via the
Codezero configuration system:
! ./configure.py
Using the interactive configuration tool, select to use a single container
and set up the following values for this bare-metal container, choose a
sensible 'Container Name' (e.g., 'genode0') and select the 'Genode' entry in
the 'Baremetal Project' menu.
:Default pager parameters:
! 0x40000 Pager LMA
! 0x100000 Pager VMA
These values are important because they are currently hard-wired in the
linker script used by Genode. If you need to adopt these values, make
sure to also update the Genode linker script located at
'base-codezero/src/platform/genode.ld'.
:Physical Memory Regions:
! 1 Number of Physical Regions
! 0x40000 Physical Region 0 Start Address
! 0x4000000 Physical Region 0 End Address
We only use 64MB of memory. The physical memory between 0 and 0x40000 is
used by the kernel.
:Virtual Memory Regions:
! 1 Number of Virtual Regions
! 0x0 Virtual Region 0 Start Address
! 0x50000000 Virtual Region 0 End Address
It is important to choose the end address such that the virtual memory
covers the thread context area. The context area is defined at
'base/include/base/thread.h'.
:Container Devices (Capabilities):
Enable the LCD display in the 'CLCD Menu'.
The configuration system will copy the Genode container template to
'codezero.git/conts/genode0'. Hence, if you need to adjust the container's
'SConscript' file, you need to edit 'codezero.git/conts/genode.0/SConscript'.
The original Genode build directory is only used as template when creating
a new Codezero container but it will never be looked at by the Codezero build
system.
# After completing the configuration, it is time to build both Codezero and
Genode. Thanks to the 'SConscript' file in the Genode container, the Genode
build process is executed automatically:
! ./build.py
You will find the end result of the build process at
! ./build/final.elf
# Now you can try out Genode on Qemu:
! qemu-system-arm -s -kernel build/final.elf \
! -serial stdio -m 128 -M versatilepb &
The default configuration starts the nitpicker GUI server and the launchpad
application. The versatilepb platform driver is quite limited. It does
support the LCD display as emulated by Qemu but no user input, yet.
Limitations
===========
At the current stage, the Genode version for Codezero is primarily geared
towards the developers of Codezero as a workload to stress their kernel. It
still has a number of limitations that would affect the real-world use:
* Because the only platform supported out of the box by the official Codezero
source tree is the ARM-based Versatilebp board, Genode is currently tied to
this hardware platform. When Codezero moves beyond this particular platform,
we will add a modular concept for platform support packages to Genode.
* The current timer driver at 'os/src/drivers/timer/codezero/' is a dummy
driver that just yields the CPU time instead of blocking. It is not
suitable as time source.
* The versatilepb platform driver at 'os/src/drivers/platform/versatilepb/'
does only support the LCD display as provided by Qemu but it was not tested on
real hardware. Because Codezero does not yet allow the assignment of the
Versatilepb PS/2 controller to a container, the current user-input driver is
just a dummy.
* The lock implementation is based on a simple spinlock using an atomic
compare-exchange operation, which is implemented via Codezero's kernel mutex.
The lock works and is safe but it has a number of drawbacks with regard to
fairness, efficiency, and its interaction with scheduling.
* Core's IRQ service is not yet implemented because the IRQ-handling interface
of Codezero is still in flux.
* Because we compile Genode with the same tool chain (Codesourcery ARM tool
chain) as used for Codezero, there are still subtle differences in the
linker scripts, making Genode's dynamic linker not yet functional on
Codezero.
* Even though Codezero provides priority-based scheduling, Genode does not
allow assigning priorities to Codezero processes, yet.
* Currently, all Genode boot modules are linked as binary data against core,
which is then loaded as single image into a container. For this reason, core
must be build after all binaries. This solution is far from being convenient
because changing the list of boot modules requires changes in core's
'platform.cc' and 'target.mk' file.
New thread-context management
#############################
With the current release, we introduced a new stack management concept that is
now consistently used on all Genode base platforms. Because the new concept
does not only cover the stack allocation but also other thread-specific context
information, we speak of thread-context management. The stack of a Genode
thread used to be a member of the 'Thread' object with its size specified as
template argument. This stack-allocation scheme was chosen because it was easy
to implement on all base platforms and is straight-forward to use. But there
are two problems with this approach.
First, the implementation of thread-local storage (TLS) is either platform
dependent or costly. There are kernels with support for TLS, mostly by the
means of a special register that holds a pointer to a thread-local data
structure (e.g., the UTCB pointer). But using such a facility implicates
platform-specific code on Genode's side. For kernels with no TLS support, we
introduced a unified TLS concept that registers stacks alongside with
thread-local data at a thread registry. To access the TLS of a thread, this
thread registry can be queried with the current stack pointer of a caller.
This query, however, is costly because it traverses a data structure. Up to
now, we accepted these costs because native Genode code did not use TLS. TLS
was only needed for code ported from the Linux kernel. However, with NOVA,
there is now a kernel that requires the user land to provide a fast TLS
mechanism to look up the current thread's UTCB in order to perform system
calls. On this kernel, a fast TLS mechanism is important.
The second disadvantage of the original stack allocation scheme is critical
to all base platforms: Stack overflows could not be detected. For each stack,
the developer had to specify a stack size. A good estimation for this value
is hard, in particular when calling functions of library code with unknown
stack usage patterns. If chosen too small, the stack could overflow, corrupting
the data surrounding the 'Thread' object. Such errors are extremely cumbersome
to detect. If chosen too large, memory gets wasted.
For storing thread-specific data (called thread context) such as the stack and
thread-local data, we have now introduced a dedicated portion of the virtual address
space. This portion is called thread-context area. Within the thread-context
area, each thread has a fixed-sized slot, a thread context. The layout of each
thread context looks as follows
[image img/thread_context]
; lower address
; ...
; ============================ <- aligned at 'CONTEXT_VIRTUAL_SIZE'
;
; empty
;
; ----------------------------
;
; stack
; (top) <- initial stack pointer
; ---------------------------- <- address of 'Context' object
; additional context members
; ----------------------------
; UTCB
; ============================ <- aligned at 'CONTEXT_VIRTUAL_SIZE'
; ...
; higher address
On some platforms, a user-level thread-control block (UTCB) area contains
data shared between the user-level thread and the kernel. It is typically
used for transferring IPC message payload or for system-call arguments.
The additional context members are a reference to the corresponding
'Thread_base' object and the name of the thread.
The thread context is a virtual memory area, initially not backed by real
memory. When a new thread is created, an empty thread context gets assigned
to the new thread and populated with memory pages for the stack and the
additional context members. Note that this memory is allocated from the RAM
session of the process environment and gets not accounted for when using the
'sizeof()' operand on a 'Thread_base' object.
This way, stack overflows are immediately detected because the corresponding
thread produces a page fault within the thread-context area. Data corruption
can never occur.
We implemented this concept for all base platforms and thereby made the
stack-overflow protection and the fast TLS feature available to all platforms.
On L4ka::Pistachio, OKL4, L4/Fiasco, Codezero, and NOVA, the thread-context
area is implemented as a managed dataspace. This ensures that the unused
virtual memory of the sparsely populated thread-context area is never selected
for attaching regular dataspaces into the process' address space. On Linux, the
thread-context area is implemented via a fixed offset added to the local
address for the 'mmap' system call. So on this platform, there is no protection
in place to prevent regular dataspaces from being attached within the
thread-context area.
Please note that in contrast to the original 'Thread' object, which contained
the stack, the new version does not account for the memory consumed by the
stack when using the 'sizeof()' operator. This has to be considered for
multi-threaded servers that want to account client-specific threads to the
memory donated by the corresponding client.
Real-time priorities
####################
There are two application areas generally regarded as predestined for
microkernels, high security and real time. Whereas the development of Genode
was primarily focused on the former application area so far, we observe growing
interest in using the framework for soft real-time applications, in particular
multi-media workload. Most of Genode's supported base platforms already provide
some way of real-time scheduling support, hard priorities with round-robin
scheduling of threads with the same priority being the most widely used
scheduling scheme. What has been missing until now was a way to access these
facilities through Genode's API or configuration interfaces. We deferred the
introduction for such interfaces for a very good reason: It is hard to get
right. Even though priority-based scheduling is generally well understood, the
combination with dynamic workload where differently prioritized processes are
started and deleted at runtime and interact with each other is extremely hard
to manage. At least, this had been our experience with building complex
scenarios with the Dresden real-time operating system (DROPS). Combined with
optimizations such as time-slice donating IPC calls, the behaviour of complex
scenarios tended to become indeterministic and hardly possible to capture.
Genode imposes an additional requirement onto all its interfaces. They have to
support the recursive structure of the system. Only if any subsystem of
processes is consistent on its own, it is possible to replicate it at an arbitrary
location within Genode's process tree. Assigning global priorities to single
processes, however, would break this condition. For example, non-related
subsystems could interfere with each other if both used the same range of
priorities for priority-based synchronization within the respective subsystem.
If executed alone, each of those subsystems would run perfectly but integrated
into one setup, they would interfere with each other, yielding unpredictable
results. We have now found a way to manage real-time priorities such that the
recursive nature Genode is not only preserved but actually put to good use.
Harmonic priority-range subdivision
===================================
We call Genode's priority management concept harmonic priority-range
subdivision. Priorities are not assigned to activities as global values but
they can be virtualized at each node in Genode's process tree. At startup time,
core assigns the right to use the complete range of priorities to the init
process. Init is free to assign those priorities to any of the CPU sessions it
creates at core, in particular to the CPU sessions it creates on behalf its
children and their grandchildren. Init, however, neither knows nor is it
interested in the structure of its child subsystems. It only wants to make sure
that one subsystem is prioritized over another. For this reason, it uses the
most significant bits of the priority range to express its policy but leaves
the lesser significant bits to be defined by the respective subsystems. For
example, if init wants to enforce that one subsystem has a higher priority than
all others, it would need to distinguish two priorities. For each CPU-session
request originating from one of its clients, it would diminish the supplied
priority argument by shifting the argument by one bit to the right and
replacing the most significant bit with its own policy. Effectively, init
divides its own range of priorities into two subranges. Both subranges, in
turn, can be managed the same way by the respective child. The concept works
recursively.
Implementation
==============
The implementation consists of two parts. First, there is the actual management
implemented as part of the parent protocol. For each CPU session request,
the parent evaluates the priority argument and supplements its own policy.
At this management level, a logical priority range of 0...2^16 is used to pass
the policy arguments from child to parent. A lower value represents a higher
priority. The second part is the platform-specific code in core that translates
priority arguments into kernel priorities and assigns them to physical
threads. Because the typical resolution for priority values is lower than 2^16,
this quantization can lead to the loss of the lower-significant priority bits.
In this case, differently prioritized CPU sessions can end up using the same
physical priority. For this reason, we recommend to not use priorities for
synchronization purposes.
Usage
=====
The assignment of priorities to subsystems is done via two additional tags in
init's 'config' file. The '<priolevels>' tag specifies how many priority levels
are distinguished by the init instance. The value must be a power of two. Each
'<start>' node can contain an optional '<priority>' declaration, which holds a
value between -priolevels + 1 and 0. This way, priorities can only be lowered,
never alleviated above init's priority. If no '<priority>' tag is specified,
the default value of 0 (init's own priority) is used. For an example, here is a
'config' file starting several nested instances of the init process using
different priority subranges.
! <config>
! <!--
! divides priority range 1..128 into
! 65..128 (prio 0)
! 1..64 (prio -1)
! -->
! <priolevels>2</priolevels>
! <start>
! <filename>init</filename>
! <priority>0</priority>
! <ram_quota>5M</ram_quota>
! <config>
! <!--
! divides priority range 65..128 into
! 113..128 (prio 0)
! 97..112 (prio -1)
! 81..96 (prio -2)
! 65..80 (prio -3)
! -->
! <priolevels>4</priolevels>
! <start>
! <filename>init</filename>
! <!-- results in platform priority 112 -->
! <priority>-1</priority>
! <ram_quota>512K</ram_quota>
! </start>
! <start>
! <filename>init</filename>
! <!-- results in platform priority 96 -->
! <priority>-2</priority>
! <ram_quota>2M</ram_quota>
! <config>
! <start>
! <filename>init</filename>
! <ram_quota>768K</ram_quota>
! </start>
! </config>
! </start>
! </config>
! </start>
! <start>
! <filename>init</filename>
! <!-- results in platform priority 64 -->
! <priority>-1</priority>
! <ram_quota>6M</ram_quota>
! <config></config>
! </start>
! </config>
On kernels that support priorities and where priority 128 is used as priority
limit (this is the case for OKL4 and Pistachio), this configuration should
result in the following assignments of physical priorities to process-tree
nodes:
[image img/priorities]
The red marker shows the resulting priority of the corresponding process.
; 128 : core
; 128 : core->init
; 128 : core->init->init
; 112 : core->init->init->init
; 98 : core->init->init->init.2
; 98 : core->init->init->init.2->init
; 64 : core->init->init.2
With Genode 10.02, we implemented the described concept for the OKL4 and
L4ka::Pistachio base platforms first. On both platforms, a priority range of 0
to 128 is used.
On L4/Fiasco, we were not yet able to apply this concept because on this
kernel, the used lock implementation is based on a yielding spinlock.
If a thread at a high priority would attempt to acquire a contended lock,
it would infinitely yield the CPU to itself, letting all other threads in
the system starve. In order to make real-time priorities usable on L4/Fiasco
we would need to change the lock first.
Base framework
##############
Read-only dataspaces
====================
Until now, we have not handled ROM dataspaces any different from RAM dataspaces
in core except for their predefined content. With the Genode workload becoming
more complex, ROM files tend to get shared between different processes and need
protection. Now, dataspaces of ROM modules are always mapped read-only.
Enabled the use of super pages by default
=========================================
Since release 9.08, we support super pages as an experimental feature. Now,
this feature is enabled by default on L4/Fiasco, L4ka::Pistachio, and NOVA.
Enabled managed dataspaces by default
=====================================
We originally introduced managed dataspaces with the release 8.11. However,
because we had no pressing use cases, it remained a experimental feature
until now. The new thread-context management introduced with this release
prompted us to promote managed dataspaces to become a regular feature.
Originally there was one problem holding us back from this decision, which
was the handling of cyclic references between nested dataspaces. However,
we do now simply limit the number of nesting levels to a fixed value.
Streamlined server framework
============================
We removed the 'add_activation()' functionality from the server and pager
libraries because on all platforms server activations and entry points have
a one-to-one relationship. This API was originally intended to support
platforms that are able to trigger one of many worker threads via a single
entry point. This was envisioned by an early design of NOVA. However, no
kernel (including NOVA) supports such a feature as of today.
Furthermore, we added a dedicated 'Pager_capability' type. On most
platforms, a pager is simply a thread. So using a 'Thread_capability' as type
for the 'Pager_capability' was sufficient. On NOVA, however, a pager is not
necessarily a thread. So we need to reflect this difference in the types.
PD session interface
====================
To support capability kernels with support for local names, it is not
sufficient to provide the parent capability to a new child by passing a plain
data argument to the new child during ELF loading anymore. We also need to tell
the kernel about the delegated right of the child to talk to its parent. This is
achieved using the new 'assign_parent' function of the PD session interface.
This function allows the creator of a new process to register the parent
capability.
Singleton services
==================
There are services, in particular device drivers, that support only one session
at a time. This characteristic was not easy to express in the framework.
Consequently, such services tended to handle the case of a second session
request inconsistently. We have now enhanced the 'Root_component' template with
a policy parameter to 'Root_component' that allows the specification of a
session-creation policy. The most important policy is whether a service can
have a single or multiple clients.
[http://genode.org/documentation/api/static_content/code/base/include/root/component - See the improved template...]
Out-of-order RPC replies
========================
In the previous release, we introduced a transitional API for supporting
out-of-order RPC replies. This API is currently used by the timer and
signal services but is declared deprecated. The original implementation
used a blocking send operation to deliver replies, which is not desired
and can cause infinite blocking times in the presence of misbehaving clients.
Therefore, we changed the implementation to send explicit replies with no
timeout. Thanks to Frank Kaiser for pointing out this issue.
Operating-system services and libraries
#######################################
Python scripting
================
We have ported a minimal Python 2.6.4 interpreter to Genode. The port is
provided with the 'libports' repository. It is based on the official
Python code available from the website:
:Python website:
[http://www.python.org]
To fetch the upstream Python source code, call 'make prepare' from within the
'libports' directory. To include Python in your build process, add 'libports'
to your 'build.conf' file.
A test program for the script interpreter is provided at
'libports/src/test/python'. When building this test program, a shared library
'python.lib.so' will be generated. A sample Genode configuration
('config_sample') file that starts a Python script can be found within this
directory. If you are not using Linux as a Genode base platform, do not forget
to add 'python.lib.so' to your boot module list.
We regard this initial port as the first step to make a complete Python
runtime. At the current stage, there is support for 'Rom_session' Python
scripts to serve basic scripting needs, currently geared towards automated
testing. Modules and standard modules are not yet supported.
Plugin-interface for the C library
==================================
The recent addition of the lwIP stack to Genode stimulated our need to make the
C runtime extensible by providing multiple back ends, lwIP being one of them.
Therefore, we introduced a libc-internal plugin interface, which is able to
dispatch libc calls to one of potentially many plugins. The plugin interface
covers the most used file operations and a few selected networking functions.
By default, if no plugin is used, those functions point to dummy
implementations. If however, a plugin is linked against a libc-using program,
calls to 'open' or 'socket' are directed to the registered plugins, resulting
in plugin-specific file handles. File operations on such a file handle are then
dispatched by the corresponding plugin.
The first functional plugin is the support for lwIP. This makes it possible to
compile BSD-socket based network code to use lwIP on Genode. Just add the
following declaration in your 'target.mk':
! LIBS += libc libc_lwip lwip
The 'libc' library is the generic C runtime, 'lwip' is the raw lwIP stack, and
'libc_lwip' is the lwip plugin for the C runtime - the glue between 'lwip' and
'libc'. The initialization of lwip is not yet part of the 'lwip' plugin.
:Limitations:
We expand the libc-plugin interface on a per case basis. Please refer to
'libc/include/libc-plugin/plugin.h' to obtain the list of currently supported
functions. Please note that 'select' is not yet supported.
ARM architecture support for the C library
==========================================
We enhanced our port of the FreeBSD libc with support for the ARM
architecture. In the ARM version, the following files are excluded:
:libm: 'e_acosl.c', 'e_asinl.c', 'e_atan2l.c', 'e_hypotl.c', 's_atanl.c',
's_cosl.c', 's_frexpl.c', 's_nextafterl.c', 's_nexttoward.c',
's_rintl.c', s_scalbnl.c', 's_sinl.c', 's_tanl.c', 's_fmal.c',
:libc-gen: 'setjmp.S'
Atomic operation on ARM are not supported. Although these operations are
defined in 'machine/atomic.h', their original FreeBSD implementations are
not functional because we do not emulate the required FreeBSD environment
(see: 'sysarch.h'). However, these functions are not a regular part of
the libc anyway and are not referenced from any other libc code.
Light-weight IP stack
=====================
After introducing LwIP support with our last release, we stabilized the port
and combined it with our libc implementation. Moreover, we upgraded the lwIP
library to the latest stable version 1.3.2. For convenience reasons, we
added initialization code, setting up the LwIP stack, the NIC session back end,
and optionally DHCP.
The example programs 'http_srv' and 'loopback' within the 'libports' repository
show how to use the LwIP stack directly or as a plugin with the libc. The
first one makes direct use of the LwIP library and demonstrates how to deal
with the new initialization routine, to setup the session to the NIC driver
and to request an IP address via DHCP. The second example doesn't use the
socket interface of the LwIP library directly but uses the libc variant instead.
It doesn't initialize the NIC session back end but uses the loopback
device provided by the LwIP library itself.
Device-driver environment kit
=============================
The basis for Genode's legacy driver emulation environment was granted some
maintenance. DDE kit now utilizes the thread registry and is able to adopt
alien threads. This unimpressive feature permits the execution of driver code
directly from server activations, i.e., adds support for single-threaded
drivers.
Dynamic linker
==============
We added dynamic linking support for OKL4 on the ARM architecture.
Because of the tool chain used on this platform, we had to revisit our
linker scripts (one warning is left because of 'gc-sections') and removed the
dependency on gcc builtin functions (with the exception of 'alloca').
To ease debugging on Linux, we revised the handling of registrations of
libraries and dynamic binaries, and thereby, made gdb debugging of
dynamically linked programs possible.
Furthermore, we prepared the future support for the 'dl' API ('dlopen', 'dlsym'
etc.) calls by enabling the linker to register exported linker symbols at startup.
This is achieved by emulating '.hash', '.dynsym', and '.dynstr' sections within
the linker object.
Misc
====
* Prevent running over the XML data on sub node identification. This
change fixes a problem with parsing the 'config' file on OKL4.
* C Runtime: Disable definition of 'pthread_cancel' symbol because it
collides with a weak implementation provided (and relied on) by the C++
support library.
Device drivers
##############
PIT timer driver
================
We use the x86 Programmable Interval Timer (PIT) on kernels that provide no
time source via their kernel APIs, i.e., OKL4 and NOVA.
Up to now, the accuracy of the timer implementation was not a big concern
because we wanted to satisfy the use cases of blocking for a short amount
of time (ca. 10ms) as needed by many periodic processes such as interactive
GUI applications, DDE device drivers, and the OKLinux timer loop. Achieving
exact wake-up times with a user-level timing service that get preemptively
scheduled alongside an unknown number of other threads is impossible anyway.
However, with the introduction of real-time priorities in the current release,
real-time workload and the accuracy of the timer driver becomes important.
For this reasons we improved the timer implementation.
* Corrected programming of one-shot timer IRQs. In the function for assigning
the next timeout, the specified argument was not taken over to the
corresponding member variable. This way, the timer implementation was not
operating in one-shot mode but it periodically triggered at a high rate.
This change should take off load from the CPU.
* Replaced counter-latch command with read-back in PIT timer. We use the PIT
status byte to detect counter wrap arounds and adjust our tick count
accordingly. This fixes problems with long single timeouts.
Thanks to Frank Kaiser for investigating these timer-accuracy issues and
providing us with suggestions to fix them.
NIC driver for Linux
====================
Genode provides the NIC session interface and a DDE Linux 2.6 based
driver for AMD PCnet32 devices since release 9.11. The NIC driver adds
networking support for all Genode base platforms except Linux. With the current
release we filled that gap with the TAP-based 'nic_drv'. The driver
itself accesses '/dev/net/tun' for 'tap0' and needs no super-user
privileges. Therefore, the device has to be configured prior to
running Genode like the following.
! sudo tunctl -u $$USER -t tap0
! sudo ip link set tap0 up
! sudo ip address add 10.0.0.1/24 brd + dev tap0
Give it a try with the
[http://genode.org/documentation/release-notes/9.11#section-17 - lwIP example scenario].
Please note that lwIP is configured for DHCP and does not assign a
static IP configuration to its end of the wire. Hence, you should run
a DHCP server on tap0, e.g.
! sudo /usr/sbin/dhcpd3 -d -f -cf /tmp/dhcpd.conf \
! -pf /tmp/dhcpd.pid -lf /tmp/dhcpd.lease tap0
An example 'dhcpd.conf' may look like
! subnet 10.0.0.0 netmask 255.255.255.0 {
! range 10.0.0.16 10.0.0.31;
! }
The DHCP server's log will show you that the driver fakes an ethernet
NIC with the MAC address 02-00-00-00-00-01.
VESA driver
===========
Our VESA driver used to set a default resolution of 1024x768 at 16 bit color
depth, which could be changed by specifying session arguments. However, most
of the time, clients are able to adapt itself to the framebuffer resolution and
do not want to implement the policy of defining the screen mode. Now we made the
VESA driver configurable, taking the burden of choosing a screen mode from the
client. A client can still request a particular resolution but for the common
case, it is policy free.
If no configuration is supplied, the driver sets up a resolution of 1024x768 at
16 bit color depth. This behaviour can be overridden by supplying the following
arguments via Genode's config mechanism:
! <config>
! <!-- initial screen width -->
! <width>1024</width>
!
! <!-- initial screen height -->
! <height>768</height>
!
! <!-- initial color depth (bits per pixel) -->
! <depth>16</depth>
! </config>
Note that only VESA modes but no arbitrary combination of values are supported.
To find out which graphics modes exist on your platform, you might use the
'vbeprobe' command of the GRUB boot loader. Also, the driver will print a list
of supported modes if the specified values are invalid.
Paravirtualized Linux refinements
#################################
The para-virtualized Linux port introduced in Genode Release 9.11 has been
refined, especially the block driver providing a root file system for Linux
has been completely reworked. Also the configuration facilities changed a bit.
Moreover, few problems that occurred when using multiple Linux instances, or
when using one instance under heavy load have been fixed. At this point, we
like to thank Sven Fülster for providing information and a fix for a bug
triggered by a race condition.
:Repository structure:
We rearranged the structure of the 'oklinux' repository. The downloaded
archive and the original OKLinux code are now stored under 'download'
and respectively 'contrib' analog to the 'libports' repository
structure.
:Rom-file block driver:
The block driver using a ramdisk as backing store as contained in the original
OKLinux port has been replaced by a new implementation that uses a dataspace
provided by the ROM session interface to provide a read-only block driver.
The read-only block driver can be used together with UnionFS (stackable
file system) or the Cowloop driver (copy-on-write block device) for Linux to
obtain a writeable root-file system, like it is done in many Linux Live-CDs.
To use the new rom-file block driver you first need to specify what file to use
as block device. This can be done by adding a 'rom_file' section in the XML
configuration of your Linux instance:
! <config>
! <rom_file>rootfs.img</rom_file>
! </config>
Of course, you need to add this file to your list of boot modules.
The block device is named 'gda' within the Linux Kernel (e.g., take
a look at '/proc/partitions'). When using it as root-file system, you
might specify the following in your configuration:
! <config>
! <commandline>root=/dev/gda1</commandline>
! <rom_file>rootfs.img</rom_file>
! </config>
Assuming the rom-file contains a valid partition table and the root file system
is located in the first partition.
Distribution changes
####################
Starting with the release 10.02, we will no longer distribute our slightly
customized version of the L4/Fiasco kernel together with the official Genode
distribution but instead will provide this kernel as a separate archive. Our
original intention with packaging L4/Fiasco with Genode was to give newcomers
a convenient way to start working with Genode on a real microkernel without the
need to download the whole TUDOS source tree where the main-line development of
L4/Fiasco is hosted. In the meanwhile, the number of supported base platforms
greatly increased to 6 different kernels. There are now plenty of opportunities
to get started with real microkernels so that the special case of hosting
L4/Fiasco with Genode is no longer justified. We want to leave it up to you to
pick the kernel that suits your needs best, and provide assistance via our wiki
and mailing list.
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