This base platform is no longer maintained.
For supporting the Microblaze CPU in the future, we might consider
integrating support for this architecture into base-hw. Currently
though, there does not seem to be any demand for it.
This patch simplifies the way of how Genode's base libraries are
organized. Originally, the base API was implemented in the form of many
small libraries such as 'thread', 'env', 'server', etc. Most of them
used to consist of only a small number of files. Because those libraries
are incorporated in any build, the checking of their inter-dependencies
made the build process more verbose than desired. Also, the number of
libraries and their roles (core only, non-core only, shared by both core
and non-core) were not easy to capture.
Hereby, the base libraries have been reduced to the following few
libraries:
- startup.mk contains the startup code for normal Genode processes.
On some platform, core is able to use the library as well.
- base-common.mk contains the parts of the base library that are
identical by core and non-core processes.
- base.mk contains the complete base API implementation for non-core
processes
Consequently, the 'LIBS' declaration in 'target.mk' files becomes
simpler as well. In the most simple case, only the 'base' library must
be mentioned.
Fixes#18
The distinction between 'ipc.h' and 'ipc_generic.h' is no more. The only
use case for platform-specific extensions of the IPC support was the
marshalling of capabilities. However, this case is accommodated by a
function interface ('_marshal_capability', '_unmarshal_capability'). By
moving the implementation of these functions from the headers into the
respective ipc libraries, we can abandon the platform-specific 'ipc.h'
headers.
Some shared libraries of the host system contain search paths for finding
other needed shared libraries. These paths get evaluated only by a native
linker. To find all needed shared libraries, with this patch, the host
linker is used to link hybrid applications.
Fixes#645.
reverts 68156918ee
"base: apply thread.cc fix of foc to base"
Depending on the context area a fixed location is calculated where the
memory for the stack is attached to. If the context area is released before the
detach call, the very same context area can be reused and memory for the new
stack is attached for a new thread. The detach of the old thread would then
revoke the mapping for the new thread which will cause a un-handled page fault.
Issue #549
Prior this patch the startup lock was not released if the call of
'_associate()' failed. In this condition, the caller of the constructor
was infinitely blocked.
During a ram_session->free call in 'core' the lock in core_env.h is taken.
Then in the ram_session::_free_ds implementation the dissolve function for the
dataspace is called. base-nova tries to make sure that the ds is not
accessible anymore by any kind of parallel incoming IPC by performing a
cleanup IPC. Unfortunately the dataspace_session implementation uses the very
same allocator in 'core' and may require to obtain the same lock as taken in
ram_session->free. This leads to a spurious deadlock on base-nova.
The actual free_ds implementation is mostly thread safe, since all used objects
inside there are already locked. The only missing piece is the _payload
variable. By changing the _payload variable in a atomic fashion there is no
need to lock the whole ram_session->free call which avoids deadlocks on
base-nova.
Fixes#549
If page faults are handled concurrently (as for base-nova) the traverse lookup
call in rm_session_component must be thread safe, which it isn't.
If the faulting area is backed by nested dataspaces which are managed by
various rm_sessions then a race happens under following circumstances
(triggered occasionally by the bomb test).
The traverse lookup may return a pointer to a rm_session of a nested dataspace.
If the rm_session is in parallel subject to destruction it happened that faults
got enqueued to the faulters list of the deleted rm_session and internally to
a list of the current rm_session of the Rm_client.
During destruction of the faulting Rm_client the associated rm_session will
be dissolved from the Rm_client, which leads to dereferencing the
dangling pointer of the already destructed rm_session.
On base-nova the memory of the rm_session object get unmapped eventually, so
that the de-referencing of the dangling pointer caused page faults in core.
The memory on other kernels inside core never get unmapped so that the
bug doesn't trigger visible faults.
The patch replace the keeping of a rm_session pointer by keeping a
capability instead. The rm_session object must be looked up now explicitly in
the Object_pool implementation, which implements proper reference counting on
the rm_session object.
Issue #549
First make the clients inaccessible and dissolve them from the entrypoint. If
this isn't the first step the clients may be obtained again between
the unlock and lock steps in the destructor.
Additionally the clients may be removed in between the unlock and call
sequence, which renders such client pointers dangling and causes spurious page
faults. Keep instead a lock as long as possible and when it is required to
release a lock, then the pointer to the objects must be revalidated.
Replace the dissolve function with a remove_client implementation as suggested
by #13, which avoids that the cpu_session may call dissolve with a dangling
pointer of a already removed rm_client object. Instead the pager must be
released explicitly.
Related to issue #549
Related to issue #394
Related to issue #13
By now, the memcmp implementation of Genode's basic string utilities just
returned whether two memory blocks are equal or differ. It gave no hint which
block is greater, or lesser than the other one. This isn't the behaviour
anticipated by implementations that rely on the C standard memcmp, e.g. GCC's
libsupc++, or the nic_bridge's AVL tree implementation.
With this patch, the 'Signal_receiver::dissolve()' function does not return
as long as the signal context to be dissolved is still referenced by one
or more 'Signal' objects. This is supposed to delay the destruction of the
signal context while it is still in use.
Fixes#594.
Remove signal context object from signal source component list (_signal_queue)
before destruction, otherwise we get a dangling pointer.
On native hardware for base-nova, the signal source thread triggered page
faults in the Signal_source_component::wait_for_signal() method when the signal
context got freed up in Signal_session_component::free_context but was still
enqueued in Signal_source_component::_signal_queue.
Fixes#600
Several users of the signal API used custom convenience classes to
invoke signal-handling functions on the reception of incoming signals.
The 'Signal_dispatcher' pattern turned out to be particularly useful. To
avoid the duplication of this code across the code base, this patch
adds the interface to 'base/signal.h'.
Furthermore, the patch changes the 'Signal::num()' return type from int
to unsigned because negative numbers are meaningless here.
Fixes#511
Add functionality to lookup an object and lock it. Additional the case is
handled that a object may be already in-destruction and the lookup will deny
returning the object.
The object_pool generalize the lookup and lock functionality of the rpc_server
and serve as base for following up patches to fix dangling pointer issues.
When releasing a lock we must take care that all state is written back to
memory and is not cached in registers. The volatile flag of the lock variable
only means to the compiler that this value must be written immediately.
Other values changed before may be kept by the compiler in registers, which we
don't want here.
Additionally the compiler is free to reorder the code in order to optimize.
That means the code we intend to be executed inside the critical section can
get be reordered and can be executed after we reset the lock variable in the
unlock implementation. The volatile statement of the lock variable doesn't
prevent reordering of instructions which are independent.
By adding a explicit memory barrier, we force the compiler to generate code
that writes back all the register content to memory/cache (and avoid a
bunch of hard to find bugs ...)
The CPU session interfaces comes with the ability to install an
exception handler per thread. This patch enhances the feature with the
provision of a default signal handler that is used if no thread-specific
handler is installed. The default signal handler can be set by
specifying an invalid thread capability and a valid signal context
capability.
Furthermore, this patch relaxes the requirement of the order of the
calls of 'exception_handler' and 'set_pager'. Originally, the exception
handler could be installed not before setting a pager. Now, we remember
the installed exception handler in the 'Cpu_thread' and propagate to to
the platform thread at a later time.
It happens that ram_session and rm_session itself are invoking alloc
respectively free on the very same sliced heap inside core.
Lock only the sliced_heap list implementation and let the session locking to
the session implementation of rm_session and ram_session.
The ram_session and rm_session must take care to proper lock since inside
both implementations already the session handling thread and the service thread
are running parallel.
With commit 1389b63050 in thread.cc for base-foc
a bug was fixed, where the memory of the context got freed up before running
the de-constructor.
Apply the fix also to base and base-mb.
For base-nova thread creation related exception can be thrown, since the
Pager_objects are threads. Catch the exception and re-throw the
expected/documented exception in rm_session.
This commit avoids that core dies with an unhandled exception if a thread
couldn't be created (e.g. because the limit has been reached).
Sanity check that the context area has been attached. Otherwise the code
later tries to access the context area and core dies with a unhandled page
fault.
The Linux version of core used a part of the BSS to simulate access to
physical memory. All dataspaces would refer to a portion of 'some_mem'.
So every time when core would access the dataspace content, it would
access its local BSS. For all processes outside of core, dataspaces were
represented as files. This patch removes the distinction between core
and non-core processes. Now, core uses the same 'Rm_session_mmap'
implementation as regular processes. This way, the 'some_mem' could be
abandoned. We still use BSS variable for allocating core-local meta
data through.
This patch reflects eventual allocation errors in a more specific way to
the caller of 'alloc_aligned', in particular out-of-metadata and
out-of-memory are considered as different conditions.
Related to issue #526.
This patch introduces clean synchronization between the entrypoint
thread and the caller of the 'Rpc_entrypoint' destructor. The most
important change is the handling of the 'Ipc_server' destruction. This
object is in the local scope of the server's entry function. However,
since the server loop used to be an infinite loop, there was hardly any
chance to destruct the object in a clean way. Hence, the
'Rpc_entrypoint' destructor used to explicitly call '~Ipc_server'.
Unfortunately, this approach led to problems because there are indeed
rare cases where the server thread leaves the scope of the entry
function, namely uncaught exceptions. In such a case, the destructor
would have been called twice.
With the new protocol, we make sure to leave the scope of the entry
function and thereby destroy the 'Ipc_server' object as expected. This
is achieved by propagating the exit condition through a local RPC call
to the entrypoint. This way, the blocking state of the entrypoint
becomes unblocked. Furthermore, '~Rpc_entrypoint' makes use of the new
'join' function to wait for the completion of the server thread.
There is no obvious reason for having two different SPEC variables, definitions,
and pathes for the Pandaboard platform. It even lead to problems regarding the
omap4 framebuffer driver (look at issue #505 and #506).
On Linux, we want to attach additional attributes to processes, i.e.,
the chroot location, the designated UID, and GID. Instead of polluting
the generic code with such Linux-specific platform details, I introduced
the new 'Native_pd_args' type, which can be customized for each
platform. The platform-dependent policy of init is factored out in the
new 'pd_args' library.
The new 'base-linux/run/lx_pd_args.run' script can be used to validate
the propagation of those attributes into core.
Note that this patch does not add the interpretation of the new UID and
PID attributes by core. This will be subject of a follow-up patch.
Related to #510.
Using the new 'join()' function, the caller can explicitly block for the
completion of the thread's 'entry()' function. The test case for this
feature can be found at 'os/src/test/thread_join'. For hybrid
Linux/Genode programs, the 'Thread_base::join()' does not map directly
to 'pthread_join'. The latter function gets already called by the
destructor of 'Thread_base'. According to the documentation, subsequent
calls of 'pthread_join' for one thread may result in undefined behaviour.
So we use a 'Genode::Lock' on this platform, which is in line with the
other platforms.
Related to #194, #501
The IPC-server object exists solely on the stack of the entrypoint
thread and, therefore, would never be destructed as the thread is just
killed. Now, the object is explicitly destructed in the entrypoint
destructor. An alternative solution could instruct the entrypoint thread
the terminate, which would automatically cleanup its stack.
The object pool is assumed to be empty on destruction of the entrypoint.
If not, we warn and at least dissolve all RPC objects.
You cannot check an unsigned size_t variable for underflow, so I
changed the code to first check if an underflow would occur before
performing the subtraction.
Fixes#489.
'Core_tlb' ensures that core never throws pagefaults,
in contrast to its base 'Tlb' that is planned to use displacement
in the future.
'Core_tlb' enables the application of differenet memory attributes
in core, according to the board specific partitioning of the physical
address space. This way it enables caching in core.
Implement 'Signal_receiver::pending()'.
Provide display-subsystem MMIO.
Avoid method ambiguousness in 'Irq_context' in
'dde_linux/src/drivers/usb/signal/irq.cc'
(it derives from two list element classes when using 'base_hw').
Enables demo scenario with 'hw_panda_a2'.
Fix bug regarding idle thread in thread scheduling in
'base-hw/src/core/kernel.cc'.
Fix regarding signal submit in signal framework in
'base-hw/src/core/kernel.cc'.
Implies support for the ARMv6 architecture through 'base-hw'.
Get rid of 'base/include/drivers' expect of 'base/include/drivers/uart'.
Merge with the support for trustzone on VEA9X4 that came from
Stefan Kalkowski.
Leave board drivers in 'base/include/platform'.
Rework structure of the other drivers that were moved to
'base_hw/src/core' and those that came with the trustzone support.
Beautify further stuff in 'base_hw'.
Test 'nested_init' with 'hw_imx31' (hardware) and 'hw_panda_a2' (hardware),
'demo' and 'signal' with 'hw_pbxa9' (qemu) and 'hw_vea9x4'
(hardware, no trustzone), and 'vmm' with 'hw_vea9x4'
(hardware, with trustzone).
This patch introduces principal support for extending session interfaces
with specialized functionality in a clean way. For example, an 'Uart'
interface may implement the 'Terminal' interface but also offers
additional functions for setting the baud rate. A service that
implements the 'Uart' service will then automatically announce both the
'Uart' and 'Terminal' services.
Since the recent move of the process creation into core, the original chroot trampoline
mechanism implemented in 'os/src/app/chroot' does not work anymore. A
process could simply escape the chroot environment by spawning a new
process via core's PD service. Therefore, this patch moves the chroot
support into core. So the chroot policy becomes mandatory part of the
process creation. For each process created by core, core checks for
'root' argument of the PD session. If a path is present, core takes the
precautions needed to execute the new process in the specified chroot
environment.
This conceptual change implies minor changes with respect to the Genode
API and the configuration of the init process. The API changes are the
enhancement of the 'Genode::Child' and 'Genode::Process' constructors to
take the root path as argument. Init supports the specification of a
chroot per process by specifying the new 'root' attribute to the
'<start>' node of the process. In line with these changes, the
'Loader::Session::start' function has been enhanced with the additional
(optional) root argument.
On Linux, we use the session label for naming the corresponding Linux
process. When looking up the processes via 'ps', the Genode process
hierarchy becomes immediately visible.
This patch alleviates the need for any non-core process to create Unix
domain sockets locally. All sockets used for RPC communication are
created by core and subsequently passed to the other processes via RPC
or the parent interface. The immediate benefit is that no process other
than core needs to access the 'rpath' directory in order to communicate.
However, access to 'rpath' is still needed for accessing dataspaces.
Core creates one socket pair per thread on demand on the first call of
the 'Linux_cpu_session::server_sd()' or 'Linux_cpu_session::client_sd()'
functions. 'Linux_cpu_session' is a Linux-specific extension to the CPU
session interface. In addition to the socket accessors, the extension
provides a mechanism to register the PID/TID of a thread. Those
information were formerly propagated into core along with the thread
name as argument to 'create_thread()'.
Because core creates socket pairs for entrypoints, it needs to know all
threads that are potential entrypoints. For lx_hybrid programs, we
hadn't had propagated any thread information into core, yet. Hence, this
patch also contains the code for registering threads of hybrid
applications at core.
The bash-builtin 'pwd' command uses the 'st_dev' and 'st_ino' members of
the 'stat' struct to compare the path from the 'PWD' environment variable
with the path returned by 'getcwd()'. These members don't get set
correctly in Noux and therefore the 'pwd' command sometimes returns wrong
results when building Genode in Noux. With this patch the 'CURDIR' make
variable gets used instead of calling 'pwd'.
Fixes#454.
With this patch an error message gets printed in
'Allocator_avl_base::free()' if the given address is not the start address
of the allocated block.
Fixes#459.
* Introduces Schedule_context
* Use fast-interrupts or normal interrupts
* Add mode-transition between secure/non-secure world
* Limit system resources for Genode apps due to non-secure world
This commit implements the newly introduced Vm session interface to be used
on top of TrustZone capable Armv7 CPUs. Therefore a new Schedule_context is
introduced in the kernel. Threads and Vms are both Schedule_contexts used
by the scheduler. In contrast to a thread a vm uses a different assembler
mode switch to the non-secure, virtual world, as well as another exception
is used, when the non-secure world is left. For both worlds to co-exist
the interrupt-controller needs to be configured, so that the secure (Genode)
world uses fast-interrupts only, and the non-secure world only legacy
interrupts.
The only TrustZone capable platform the base-hw kernel works on top of
is the CoreTile Express 9x4 for the Versatile Express motherboard. For a
virtual machine working properly on top some platform resources must be
reserved. Therefore there exist two flavours of this platform now, one with
the 'trustzone' spec-variable enabled, and one without. If 'trustzone' is
specified most platform resources (DDR-RAM, and most IRQs) are reserved
for the Vm and not available to the secure Genode world.
Driver definitions which are used by kernel/core in base-hw, and also by other
drivers (e.g. from the os repository) have to reside in the generic
base-repository, for instance some uart drivers. All drivers which are
interesting for one of the sites only (sp804 for timer driver, or
cortex_a9 cpu driver for base-hw) should reside in the respective repos.
Factorize cpu context out of Cortex A9 specific definitions. Moreover, there
is already a Cpu_state object containing all common ARM registers. We use
this as a base for the cpu context switching done by the base-hw kernel.
The Cpu_state class get extended by a cpu-exception field, that stores the kind
of exception raised when the corresponding context got interrupted. This
information is used not only by the base-hw kernel, but also by the TrustZone
VMM that is build currently.
The 'Cpu_state' in 'base/include/base/cpu_state.h' is not needed anymore.
Moreover, it's inconsistent with the architecture-specific definitions
of 'Cpu_state' that all reside in <cpu/cpu_state.h>.
By now all services in core where created, and registered in the generic
main routine. Although there exists already a x86-specific service (I/O ports)
there was no possibility to announce core-services for certain platforms only.
This commit introduces a hook function in the 'Platform' class, that enables
registration of platform-specific services. Moreover, the io-port service
is offered on x86 platforms only now.
By naming all board declaration (previously in base/include/drivers/board) the
same way, and putting them in platform-specific include-pathes, we save additional
declaration redirection in the base-hw kernel, and in driver definitions.
The 'delete (void *)' operator gets referenced by compiler generated code,
so it must be publicly defined in the 'cxx' library. These compiler
generated calls seem to get executed only subsequently to explicit
'delete (void *)' calls in application code, which are not supported by
the 'cxx' library, so the 'delete (void *)' implementation in the 'cxx'
library does not have to do anything. Applications should use the
'delete (void *)' implementation of the 'stdcxx' library instead. To make
this possible, the 'delete (void *)' implementation in the 'cxx' library
must be 'weak'.
Fixes#419.
Added generic 'Irq_proxy' class to core includes (ported from base-okl4). It can
be used to implement shared IRQ support for various base platforms. It will
generate one thread per IRQ and unblocks waiting clients (IRQ sessions) upon
interrupt receipt.
Issue #390
GCC warns about uninitialized local variables in cases where no
initialization is needed, in particular in the overloads of the
'Capability::call()' function. Prior this patch, we dealt with those
warnings by using an (unreliable) GCC pragma or by disabling the
particular warning altogether (which is a bad idea). This patch removes
the superfluous warnings by telling the compiler that the variable in
question is volatile.
Formerly, GENODE_RELEASE just undef'd PDBG() which concealed bugs in
places PDBG was used, e.g., do to API changes. Unfortunately,
desparately disabling GENODE_RELEASE during bug hunt sometimes
introduced new errors. Now, PDBG is just a branch not taken but seen by
the compiler, which is able to produce warnings/errors when the API is
changed.
Fixes#378.
The 'build.mk' file checks if the tool chain to be used supports the
'-static' and '-fno-stack-protector' flags, but this check always fails
for the current Genode tool chain because it cannot create executable
files without explicitly specifying the 'crt0' and library files to be
linked, which the check doesn't.
This patch removes the compiler check.
Fixes#358.
The exception initialization and handling in gcc_eh allocates early (_main)
memory before executing main. In Vancouver the virtual
region from [0,VM size) must be reserved. Vancouver fails if the memory
allocated by the exception handling and the static objects was allocated
inside the [0, VM size) area.
To circumvent the situation allocate the first memory pieces for the heap
from the bss.
The first metadata-block, which is inherent part of the allocator object
itself has a fixed size which isn't suitable enough for some 64bit platforms
(e.g. core's RAM-allocator on a 64bit platform with lots of different regions).
This commit let the block size be address-width aware.
Every 64-bit binary is effectively 4M too big currently [0]. The GNU linker ld
aligns the text section of the binary to the maximum page size. On i386
the default section alignment is fixed typically to 4K.
Avoid wasting mainly 4M on x86_64 by telling the linker the max page size to
be 4K.
[0] http://sourceware.org/ml/binutils/2009-04/msg00099.html
This patch introduces the functions 'affinity' and 'num_cpus' to the CPU
session interface. The interface extension will allow the assignment of
individual threads to CPUs. At this point, it is just a stub with no
actual platform support.
This patch adds libstdc++ to libports. With the previous version of the
stdcxx library, the build system used the C++ standard library that
comes with the compiler. This mechanism was prone to inconsistencies of
types defined in the header files used at compile time of the tool chain
and the types provided by our libc. By building the C++ standard library
as part of the Genode build process, such inconsistencies cannot happen
anymore.
Note that the patch changes the meaning of the 'stdcxx' library for
users that happened to rely on 'stdcxx' for hybrid Linux/Genode
applications. For such uses, the original mechanism is still available,
in the renamed form of 'toolchain_stdcxx'.
Let the Fiasco.OC base platform succeed the cap_integrity run-script meaning
that it is not feasible anymore to fake a capability by using a valid one
together with a guessed local_name.
On both 32-bit and 64-bit platforms, 'uint64_t' can be defined as
'unsigned long long', which is the type expected by the %llx
format-string specifier. By unifying the type definitions, we resolve
warnings about unmatching type specifiers. This patch also removes
redundant words from the typedefs.
Avoid the use of deprecated 'MASK' enum in CPU register 'Asid'.
Enable the use of the 'K' bit in MMU translations.
Treat any try to modify existing valid entries in section- and
pagetables when doing 'insert_translation' as error.
Beautify concerned files.
For 64-bit registers we cannot compute MASK and SHIFT values via enums
because enum values are always of type int. But we can use static member
functions instead. Furthermore, the patch fixes the type trait for
64-bit registers. (apparently, this access width was never used so far)
The code intended to stop after the first failed attempt,
however the bool variable to control this was reseted
inside the loop and so it endless loops.
Fixes#51
Rm_client is derived from Pager_object. If the Pager_object is also
derived from Thread_base (which is the case for NOVA) then the
Rm_client object must be destructed without holding the rm_session_object
lock. The native platform specific Thread_base implementation has to take
care that all in-flight page handling requests are finished before
destruction. On NOVA it is done by doing an IPC to the pager thread.
(performed in Pager_object::dissolve() in base-nova). The
called thread than executes its operation until end which also requires
in some cases to take the rm_session_object lock.
Since _client_slab insertion/deletion also must be performed
synchronized but can't be protected by the rm_session_object lock
because of the described dead_lock situation, we have
to use a synchronized allocator object to perform insertion and
deletion of Rm_clients.
If nobody is blocked in a semaphore, nothing can be dequeued. If
the semaphore is used for signalling, there can be somebody in the queue,
but not necessarily.
Without this patch the compilation failed with:
/usr/bin/ld: main.o: relocation R_X86_64_32S against
`vtable for Genode::Dataspace' can not be used when making a shared object;
recompile with -fPIC
main.o: could not read symbols: Bad value
collect2: ld returned 1 exit status
make[6]: *** [init] Error 1
For this patch the use of the hardening tool chain must be indicated
using the "hardening_tool_chain" SPECS entry within the file
<build>/etc/specs.conf
Fixes#79
Use git to get recent kernels from github. Adjust NOVA patch to compile
with recent github version. Patch and use makefile of NOVA microkernel
to avoid duplicated (and outdated) makefile in Genode
Furthermore, this patch adds support for using NOVA on x86_64. The
generic part of the syscall bindings has been moved to
'base-nova/include/nova/syscall-generic.h'. The 32/64-bit specific
parts are located at 'base-nova/include/32bit/nova/syscalls.h' and
'base-nova/include/64bit/nova/syscalls.h' respectively.
On x86_64, the run environment boots qemu using the Pulsar boot loader
because GRUB legacy does not support booting 64bit ELF executables.
In addition to the NOVA-specific changes in base-nova, this patch
rectifies compile-time warnings or build errors in the 'ports' and
'libports' repositories that are related to NOVA x86_64 (i.e., Vancouver
builds for 32bit only and needed an adaptation to NOVAs changed
bindings)
Fixes#233, fixes#234
This patch extends the RAM session interface with the ability to
allocate DMA buffers. The client specifies the type of RAM dataspace to
allocate via the new 'cached' argument of the 'Ram_session::alloc()'
function. By default, 'cached' is true, which correponds to the common
case and the original behavior. When setting 'cached' to 'false', core
takes the precautions needed to register the memory as uncached in the
page table of each process that has the dataspace attached.
Currently, the support for allocating DMA buffers is implemented for
Fiasco.OC only. On x86 platforms, it is generally not needed. But on
platforms with more relaxed cache coherence (such as ARM), user-level
device drivers should always use uncacheable memory for DMA transactions.
When creating a 'Child' object with an already active entrypoint,
session requests may arrive as soon as the '_process' is created. We
have to make sure that at least all parts of the 'Child' object needed
for serving 'session' requests are constructed. This is particularly
important for the '_policy' member.
