
              ==============================================
              Release notes for the Genode OS Framework 9.05
              ==============================================

                               Genode Labs


Shortly after including support for the L4ka::Pistachio kernel in the
previous release, the Genode version 9.05 gives a further evidence of
Genode's high portability by making the framework available on top of
the OKL4 kernel. This kernel is a commercial-grade microkernel
developed by [http://ok-labs.com - Open Kernel Labs]. In Section
[Supporting the OKL4 kernel as new base platform], we elaborate on the
new base platform and summarize the experiences made during our porting
work.

The previous Genode release was accompanied by a source-code archive containing
the initial version of Qt4 for Genode. Our approach is to make the Qt4
framework available for building Genode applications running natively on the
microkernel rather than within a virtualization environment. As advertised in
our [https://genode.org/about/road-map - road map], we have now seamlessly
integrated the Qt4 framework into our mainline source tree.  Furthermore, we
have adapted our port to the Qt4 version 4.5.1. Section [Integration of Qt4
into the mainline repository] gives a rough overview of the changes and an
introduction on how to use the Qt4 framework with Genode.

The third major feature of the release is the addition of preliminary USB
support. We have been able to port major parts of Linux' USB infrastructure
to Genode using the DDE Kit introduced in autumn 2008. Section [USB support]
presents an overview about the pursued design and the current state of
implementation.

Section [OKLinux on Genode] outlines our ongoing efforts of running Linux
as a node in Genode's process tree.


Supporting the OKL4 kernel as new base platform
###############################################

The OKL4 kernel developed by Open Kernel Labs started as a fork of the
L4ka::Pistachio kernel. Whereas L4ka::Pistachio remained true to the L4
x.2 specification, OKL4 was subject of major API changes geared towards high
performance on the ARM architecture. OKL4 earned much fame for executing a
user-level variant of Linux (OKLinux) on top the microkernel, which turned out
to be faster than executing Linux natively on the ARM9 architecture. Even
though OKL4 is primary targeted at the ARM architecture, we wanted to go for
the x86 variant because of two reasons. First, there exists the just mentioned
user-level port of Linux for OKL4, which looks like an attractive way to execute
Linux on Genode once Genode runs on OKL4. Second, we think that distributing
Genode in the form of ISO images bootable on plain PC hardware is the best
way to reach the OS community. Therefore, we decided to use OKL4 version 2.1 as
the base for our work. In contrast to later releases, this version supports
both x86 and ARM. The following section reviews the unique features of the
OKL4 kernel from our perspective.


OKL4 viewed from the angle of a Genode developer
================================================

On the kernel-API level, OKL4 has several interesting properties that had been
both welcome and challenging. We want to highlight the following points:

In contrast to prior L4 kernels, OKL4 has *removed wall-clock timeouts* from
the kernel interface. On L4, timeouts were used as arguments for for blocking
IPC operations serving two purposes. First, specifying IPC timeouts allowed the
IPC caller for regaining control over the blocking thread after the specified
time elapsed. This is considered as important in the case that the called
thread misbehaves and never answers the call. However, the problem of choosing
an appropriate timeout was never properly resolved.  When dimensioning the
timeout too small, the called thread may miss the timeout just because it had
no chance to be selected by the scheduler in time. Such timeouts rely on the
presumption that there is low load on the system. On the other hand, when
dimensioning the timeout too high, the system will become sluggish when the
called thread misbehaves. For example, a simple GUI server may want to send
input events to its clients with a timeout to be robust against misbehaving
clients that never wait for events. When choosing a timeout too small, chances
are high that an event will occur at a time when the receiver is handling a
previous event. The timeout would trigger and the event would get lost. When
choosing the timeout too large, say 1 second, any misbehaving client could make
the GUI server freeze for 1 second. Therefore, timeouts for regaining control
over a blocked thread seem to be a bad idea.  So we welcome their absence in
OKL4. The second use of timeouts is their use as user-level time source. On L4,
sleep is typically implemented as a blocking IPC with a timeout set to the
sleep value. For this purpose, a system built on top of OKL4 has to employ a
user level device driver accessing a timer device. In Genode, we already have a
timer service for this purpose. So we won't miss timeouts at all.

Classical L4 kernels provide two variants of *synchronous IPC*. So called long
IPC could copy any amount of memory from the sending to the receiving address
space. This is complicated operation because either communication partner may
specify communication buffers that contain unmapped pages. Hence, page faults
may occur during long-IPC operations. On L4, page faults, in turn, are handled
by the user land.  Not until a user-level pager thread resolves the page fault
by establishing a mapping at the faulting address, the kernel can proceed the
IPC operation. This sounds pretty complicated, and it is. The second IPC
variant is called short IPC. It constrains the transferable payload to CPU
registers. Hence, these IPC operations should only be used for messages with a
payload of a maximum of 2 machine words. Because short IPCs are never touching
user-level memory pages, no page faults can occur.
On OKL4, there is only one IPC operation, which copies payload from the
sender's user-level thread-control block (UTCB) to the receiver's UTCB. An
UTCB is an always-mapped memory region. Hence no page faults can occur during
IPC operations. On Genode, the UTCB size of 256 bytes may become a limitation
when RPC messages get large. For example, session requests may include large
session-argument strings specifying session-constructor arguments. Current
services rely only on a few arguments so the size limitation is not an
apparent problem. But that may change for future services. Furthermore, in
contrast to L4 x.2, OKL4 does not allow for transferring payload other than
plain data. In particular, OKL4 does not support the transfer of memory
mappings via IPC.   Removing memory mappings from the IPC operation is a very
good idea. On Genode, only roottask (core) establishes mappings and shared
memory is implemented as a user-level protocol (data spaces). There is no need
to allow arbitrary processes to establish memory mapping via IPC.

The *boot procedure* of OKL4 largely differs from other L4 kernels.  This is
attributed to Open Kernel Labs' focus on embedded systems, which mostly rely on
single-image boot loading. OKL4 employs a tool (elfweaver) for creating a
bootable image from a bunch of files and an XML configuration file.  Among the
declarations about which processes to be loaded and which policies to enforce,
the configuration file contains platform parameters such as the amount of
physical memory of the machine. This static approach to configure a system is
certainly useful for embedded systems but PC hardware uses to vary a lot. In
this case, evaluating boot-time memory descriptors would be the preferred
solution.

OKL4 introduces kernel support for *user-level synchronization*. Prior L4
kernels facilitated user-level synchronization through a combination of
synchronous IPC operations with either priorities or delayed preemption.
OKL4's mutexes can make the life in the user land much easier. However, we have
not looked into OKL4 mutexes yet.

There does not exist a recursive *map operation* as the source operand of the
map operation is a physical memory descriptor rather than a virtual address in
the mapper's address space. Consequently, this design eliminates the need for
having a recursive unmap operation and thereby, the need to maintain a mapping
data base in the kernel. This is cool because Genode keeps track of the
mappings at the user level anyway (within core). From our perspective, there is
no need to maintain mapping relationships in the kernel.  Removing the mapping
database effectively discards a lot of much-discussed problems about how to
manage the mapping database in a clever way.

There exists *no root memory manager* (sigma0). Because the map operation
takes a physical memory descriptor as argument instead of a virtual address
in the mapper's address space. The mapper does not need to have the mapped
page locally mapped within its own address space. In fact, core (as the only
mapper in a Genode system) does only have very little memory mapped locally.
This design accelerates the boot time because there is no need to map each
physical page in core at startup as performed when running core on the other
L4 kernels.

These differences of OKL4 compared with the microkernels already supported
by Genode posed a number of interesting challenges and opportunities. We have
thoroughly documented the process in
[https://genode.org/documentation/articles/genode-on-okl4 - Bringing Genode to OKL4].


Usage
=====

For using Genode with OKL4, please refer to the following dedicated page:

:[https://genode.org/documentation/platforms/okl4 - Genode on the OKL4 microkernel]:
  Site about building and using Genode with the OKL4 kernel.


Limitations of the current implementation
=========================================

The current implementation is able to execute the complete Genode demonstration
scenario on OKL4. This means, we can build and destroy arbitrary trees of
processes, have all the needed infrastructure in place to execute user-level
device drivers such as VESA and PS/2, perform inter-process communication
via RPC and shared memory, and have all basic framework API functions available.
We regard the current state as the first functional version. However, there are
the following points that need improvement and are subject to our future work.

:Incomplete timer driver:

  On x86, the timer driver should program the PIT to dispatch sleep requests.
  However, the I/O ports of the PIT can only by made available to one party in
  the system (which naturally would be the timer driver). Unfortunately, there
  are some VESA BIOSes around, which try using the PIT directly. The current
  version of our VESA driver does not virtualize these accesses. It rather
  tries to gain direct access to the I/O ports from core. This would not work
  if the timer already uses this device resource. Our plan is to supplement
  our VESA driver with a virtual PIT that uses the timer service as back end.
  Then we can safely use the PIT by the timer driver.

:Signalling framework not yet implemented:

  We have not yet implemented Genode's API for asynchronous notifications
  in the OKL4 version. In fact, the goal of the initial version of the
  OKL4 support was running the default demonstration scenario, which does
  not rely on signals. The second and more technical reason is that we
  consider exploiting OKL4's event mechanism for implementing the signalling
  API but have not finalized the design. The generic implementation as used
  on the other platforms cannot be used on OKL4 because this implementation
  utilizes one helper thread per signal transmitter. Within core, each RM
  session is a potential signal transmitter, which means that we need to
  create a helper thread per process. Unfortunately, by default, OKL4
  limits the number of threads within roottask (core) to only 8 threads,
  which would impose a severe limit on the number of processes we could
  run on OKL4.

:OKL4's kernel mutexes yet to be used:

  We have not yet explored the use of mutexes provided by the OKL4 kernel
  for implementing Genode synchronization APIs but we rather rely on a
  yielding spin lock for now. This has a number of drawbacks such as high
  wake-up latencies in the contention case (depending on the scheduling
  time slice), no support for priorities, and no fairness. Although it
  is a simple and robust solution to start with, we plan to facilitate
  the OKL4 kernel feature with our upcoming work.

:Overly simplistic UTCB allocation:

  Right now, we allocate a fixed amount of 32 UTCBs per address space and
  thereby limit the maximum number of threads per process. In the future,
  this limit should be made configurable.

:Managed dataspaces not yet supported:

  The support of managed dataspaces relies on the signal API, which is
  not yet available for OKL4.

:Message buffers are limited to 256 bytes:

  Because OKL4 performs message-based inter-process communication by
  copying data between the UTCBs of the communicating threads, the
  UTCB size constaints the maximum message size. Therefore, message
  must not exceed 256 bytes. This is not a huge problem for the currently
  available Genode programs but we can imagine session argument-lists
  to become larger in the future.

:Advanced thread functions are incomplete:

  Thread functions such as querying registers of remote threads are not yet
  implemented.


Integration of Qt4 into the mainline repository
###############################################

Qt4 is a tool kit for developing platform-independent applications. It
comprises a complete platform-abstraction layer and a rich GUI tool kit
widely used for commercial and open-source applications. It is particularly
known as the technical foundation of the KDE project. The previous Genode
release was accompanied by a snapshot of our initial port of Qt4 to Genode. For
the current release, we have turned this proof-of-concept implementation into a
properly integrated part of the Genode mainline development. This enables Qt4
applications to be executed natively on the full range of kernels supported by
Genode.


Usage
=====

We complemented Genode's source tree with the new 'qt4' source-code repository,
which contains the Genode-specific parts of the Qt4 framework. The most
portions for the Qt4 framework are used unmodified and thereby have not been
made part of the Genode source tree. Instead, we fetch the original Qt4 source
code from Trolltech's FTP server. This way, our source tree remains tidy and
neat.

For using Qt4 for your Genode applications, you first need to download and
prepare the original Qt4 source codes and build a few Qt4 tools such as the
meta-object compiler and the resource compiler. The makefile found in the
top-level directory of the 'qt4' repository automates this task:

! make prepare

To include the 'qt4' repository into the Genode build process, just add the
'qt4' directory to the 'REPOSITORIES' declaration of the 'etc/build.conf' file
within your build directory. Make sure that the repositories 'demo' and 'libc'
are included as well. The 'qt4' repository comes with a couple of demo applications.
The 'qt_launchpad' is especially interesting because it makes use of both the
Qt4 framework and the Genode framework in one application.


Features and limitations
========================

The Qt4 port comprises Qt's Core library, GUI library, Script library, XML
library, and the UI tools library.

For using Qt4 on the Linux version of Genode, we recommend using the Genode
tool chain rather than your host's tool chain. Qt4 makes use of a lot of libc
functionality, supplied by Genode's 'libc' repository. However, on Linux we
still link against your host's libc. This becomes a problem if your host
compiler's C++ support code references libc functionality that is not part of
Genode's libc. Thereby the linker will silently create references to glibc
symbols, making both libraries collide. So if using Qt4, we recommend using the
Genode tool chain:

:[https://genode.org/download/tool-chain]:
  Information about downloading and using the Genode tool chain


USB support
###########

This release introduces the first fragments of USB support to Genode, taking
the USB human-interface device (HID) class as starting point. With this work,
we follow our approach of reusing unmodified Linux device drivers executed
within a device-driver environment called DDE Linux. In the previous release,
we already utilized this approach for realizing basic networking on Genode.
With this release, we complement DDE Linux with support required by USB
drivers. We are grateful for being able to base our implementation on the
excellent foundation laid by Dirk Vogt. He described his work in
[https://os.inf.tu-dresden.de/papers_ps/beleg-vogt.pdf - USB for the L4 environment].

For USB HID support, we added the Linux USB and input subsystems to the DDE
Linux 2.6 framework. Besides the 'dde_linux26/net.h' API for network drivers
added in Genode 9.02, the current version also includes APIs for input
('dde_linux26/input.h') and USB ('dde_linux26/usb.h'). We intend these
interfaces to mature towards generic driver-library APIs in the future. For
example, BSD-based drivers shall transparently provide the same functionality
as the current Linux drivers, which permits the simple reuse of driver server
implementations.

[image usb_current]

Image [usb_current] illustrates the current implementation of the USB-based
human-interface device (HID) driver. In this monolithic setup, all parts of the
USB stack and the device API are executed within one address space. These parts
are

* Input server glue code
* HID driver and input subsystem
* Core functions for management of USB request buffers (URBs),
  attached devices, and registered drivers
* Host controller drivers for UHCI, OHCI, and EHCI

[image usb_aspired]

We regard this as an intermediate step towards our goal to decompose the USB
stack. Image [usb_aspired] shows our aspired design. In this design, the
USB server and one or more USB gadget drivers run in dedicated address spaces.
The USB server provides two interfaces called USB session interface and USB
device interface. A USB session interface corresponds to a virtual root hub,
from which USB devices can be queried. The client of the USB session interface
is usually an USB gadget driver that uses the USB device interface. Because
this interface is used for transferring the actual payload at a potentially
high bandwidth, it is based on shared memory and signals. The USB server
consists of the following components:

* USB server glue code
* Virtual USB device driver managing all attached devices
* Core functions including hardware hub management
* Host controller drivers

The USB server presents a virtual USB hub to each USB gadget driver. Such
a driver consists of:

* Device interface, e.g., input server glue code
* Gadget driver, e.g., HID driver and input subsystem
* Core functions
* Virtual host controller
* USB client glue code

The HID driver uses the USB session API to monitor ports of its virtual root
hub and submit URBs to attached devices. The session interface facilitates the
signalling framework for event notification and a shared-memory dataspace for
URB transmission.

The 'os' repository already contains the USB session and USB device interfaces.
However, the decomposition is not yet in a functional state.


:Current limitations:

The current monolithic implementation of the USB HID service can already be
used as a replacement of the PS/2 driver. However, both drivers cannot be used
at the same time, yet. To enable the use of both USB HID and PS/2, we plan to
create a further component that merges multiple streams of input events and
passes the result to the GUI server.


OKLinux on Genode
#################

According to our road map, we pursued the goal to run Linux as a node in
Genode's process tree. We explored two approaches:

:Reanimating the Afterburner project conducted by the [http://l4ka.org - L4Ka group]:
  This approach is the result of the L4Ka groups's long-year experience with
  manually supporting L4Linux on top of the L4ka::Pistachio kernel. Because of
  the high costs of maintaining the paravirtualized Linux kernel, a
  semiautomatic paravirtualization technique was created. According to the
  impressive results presented in
  [http://www.l4ka.org/l4ka/publ_2006_levasseur-ua_soft-layering.pdf - Pre-Virtualization: Soft Layering for Virtual Machines],
  this approach is able to drastically reduce maintenance costs while retaining
  good performance. Furthermore, the approach was applied not only to Linux
  running on the L4 kernel but also for using Xen or Linux as underlying
  host operating systems.

:Porting the OKL4-specific version of L4Linux to Genode:
  Open Kernel Labs maintain a custom version of L4Linux that runs on OKL4. This
  version is mostly referred to as OKLinux aka Wombat. Since Genode can now use OKL4
  as base platform, the reuse of OKLinux in combination with Genode has become
  a feasible option.

Both approaches have pros and cons. Whereas Afterburner is a intriguing
approach, this project seems to be stalled. It relies on a rather old tool
chain, and recent Linux features such as thread-local storage support are not
considered, yet. To pick up this solution for Genode will require us to fully
understand the mechanisms and the code. So we consider this as a mid-term
solution. In short term, running OKLinux on Genode is more feasible. We were
already able to create a prototype version of OKLinux running on Genode. This
version starts up the kernel including all Linux kernel threads, mounts the
boot partition supplied as memory image, and starts the init process. The
engineering costs had been rather low. We replaced the Iguana user land
libraries as originally used by Wombat by a Genode-specific reimplementation to
keep our manual adaptions of the Linux kernel code as small as possible.
Our custom reimplementation of the needed Iguana APIs consists of less than
1,000 lines of code (SLOC). The diff for our changes to the OKLinux kernel code
comprises less than 1,000 lines. We plan to make a snapshot of this prototype
publicly available soon.


Operating-system services and libraries
#######################################

Nitpicker GUI server
====================

We optimized the performance of the 'refresh' call, which updates all views of
a session, which display a given buffer portion. The new implementation restricts
the redraw operations to the fragment of each view that displays the specified
buffer portion. The performance improvement becomes most visible when updating
only small parts of a buffer.


USB session interface
=====================

Genode's emerging USB support introduces two new interfaces to the 'os' repository,
which are called USB session and USB device.

An _USB_session_ is a virtual USB bus with just one root hub with 'MAX_PORTS'
downstream ports. The client of such as session submits USB request blocks
(URBs) and is, in turn, informed about port changes on the root hub as well as
request completion. Connected USB devices can be referenced by USB device
capabilities and are associated with one port at the virtual root hub on
server side.

An _USB_device_ references a hardware device connected to a virtual USB bus's
root hub. The device capability enables the client to send USB request
blocks to the hardware device.


Input interface
===============

We updated the key codes of the input interface in response to recent changes
of Linux' 'dev/event' definitions.


VESA driver
===========

Until now, there existed different processes that tried to access the PCI bus
via I/O ports, in particular the VESA framebuffer driver and the PCI bus
driver.

Since core enforces that each I/O port can only be assigned exclusively to one
component in the system, multiple processes that need access to the same I/O
ports cannot run at the same time. For our default demonstration scenario, we
had been able to allow the VESA driver to use the PCI I/O ports because nobody
else needed them. However, our growing base of device drivers relies on the
PCI bus driver. To be able to use the VESA driver together with other drivers,
we virtualized the access to the PCI bus from within the VESA driver.

Our current PCI virtualization code is pretty limited. The VESA driver sees a
virtual PCI bus with only the VGA card attached. For now, we only allow reading
the PCI configuration space of this device, but not writing to it. Apparently,
this simple approach is sufficient to run the VESA BIOS of Qemu. However, other
VESA BIOS implementations may need further access to the PCI device's
configuration space. For example, for querying the size of a PCI resource,
write access to base address registers is required. In such an event, the VESA
driver will print a message about the missing virtualization feature:
! writing data register not supported
If you see such a message, we are very interested to see your log output such
that we can enhance our PCI virtualization code as needed. Please contact us!


Base framework
##############

In the process of bringing Genode to the OKL4 kernel, we have generalized much
of former platform-specific code:

* The initialization of C++ exception handling has now become part of the
  generic 'cxx' library in the 'base' repository. All platforms except
  Linux are using this generic library now.

* The 'server' library used to contain a platform-specific part that
  implemented the 'manage' function of a 'Server_entrypoint'. The
  generalized version of this library is now being used on all platforms
  other than Linux.

* We unified core-internal interfaces and their implementations such as
  'Dataspace_component', 'Cap_session_component', 'Rm_session_component',
  and 'Irq_session_component'. The result has become part of the 'base'
  repository.

* On OKL4, threads need to execute small startup code for querying their
  own thread IDs. Therefore, we have extended the 'Thread_base' interface
  with a platform-specific hook function called '_thread_bootstrap'.

* The types defined in 'base/native_types.h' had been complemented by a
  new 'Native_thread_id' type. This type is exclusively used by core and the
  framework libraries. For using the Genode API, this type is meaningless.

* For the 64bit support, we slightly refined the interfaces of some utility
  template functions in 'util/misc_math.h'. Furthermore, parts of the generic
  marshalling code of the IPC framework needed refinement, but no API changes
  were needed.


Linux-specific changes
######################

Adaptation to 64 bit
====================

Because most Genode developers tend to work with the Linux version of Genode,
supporting 64-bit Linux becomes increasingly important. With the current release,
we start to officially support 64-bit Linux as base platform. This comes
along with the following changes:

* We replaced the 'spec-x86.mk' file with new 'spec-x86_32.mk' and 'spec-x86_64.mk'
  files. The default version of 'base-linux/etc/specs.conf' automatically
  chooses the right spec file according to the output of 'uname -m'. Therefore,
  output of the build processes matches your host architecture. This behaviour
  can be changed by placing a customized 'spec.conf' file in your build directory's
  'etc/' subdirectory.

* We added type definitions for 64-bit-specific fixed-size integers in the form
  of a 64-bit-specific 'fixed_stdint.h' file.

* Because using 64 bit instead of 32 bit changes the payload size of RPC
  messages, we had to adjust several message buffers such as 'Ram_session_client'
  and 'Input::Session_client', and adapted the used stack sizes.

* Towards the goal of completely dissolving Genode's dependency on the Linux' glibc,
  we implemented custom system-call bindings. Apparently, Linux' syscall interface
  differs between 32 bit and 64 bit. For example, the 32-bit version handles
  all socket-related calls via a compound 'socketcall' whereas the 64-bit
  version uses distinct syscalls. Another difference is the handling of the
  'mmap' syscall and different behaviour of 'tkill'. The latter problem was
  resolved by replacing 'tkill' with 'tgkill' and setting the thread-group
  argument of the corresponding PID. Therefore, a 'Native_thread_id' on Linux
  now comprises both the TID and the PID.

* The 'Platform_env' on Linux contains a local implementation of the 'Rm_session'
  interface, which uses 'mmap' to attach dataspaces to the process' address
  space and maintains the region list in the form of a static array. This array
  was dimensioned to 256 entries, which constrained the maximum amount of
  usable memory when allocating a lot of small blocks via Genode's heap. Since
  the heap allocates backing store at the granularity of only 16KB, the worst
  case for reaching this limit was about 4MB. This was OK for our simple test
  applications. But for using Qt4, in particular on 64 bit, this has become a
  serious limitation. For now, we increased the region limit to 4096 and plan
  to replace the static array with a dynamically growing data structure.
  Furthermore, we made the heap granularity depend on the actual machine-word
  size. Therefore, the heap allocates its backing store in 32KB blocks when
  running on 64 bit.


Debugging hooks
===============

On Linux, we use gdb for debugging Genode. This is feasible as long as the
targeted process is running. However, during low-level debugging, we had the
recurring problem of a thread dying shortly after starting up. So we added a hook
for halting a thread at the startup in order to be able to attach gdb to the
thread before it dies. This simple hook lets the thread wait for a key press by
directly calling the 'read' syscall. We also added a simple debug facility for
printing debug messages bypassing Genode's LOG service by calling the 'write'
syscall directly.  Both hooks are now part of the Linux version of the 'env'
library (see 'base-linux/src/base/env/debug.cc'). Note that these hooks are not
part of the Genode API. There exists no header file.

