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Remove outdated sections from doc/architecture.txt

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Fixes #71
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Norman Feske 2014-03-14 18:45:52 +01:00
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@ -787,251 +787,3 @@ contains its RM session and RAM session.
; supplying the parent capability to the new process
Framework infrastructure
########################
Apart from the very fundamental mechanisms implemented by core, all
higher-level services have to be implemented as part of the process tree on top
of core.
There are a number of frameworks at hand that provide convenient interfaces to
be used by such components.
In this section, we outline the most important frameworks.
Communication
=============
The basic mode of operation of our RPC framework is based on C++ streams.
It uses four different stream classes: 'Ipc_ostream' for sending messages,
'Ipc_istream' for receiving messages, 'Ipc_client' for performing RPC calls,
and 'Ipc_server' for dispatching RPC calls. In the following, we use
illustrative examples.
:Sending a message:
!Ipc_ostream sender(dst, &snd_buf);
!sender << a << b << IPC_SEND;
The object 'sender' is an output stream that is initialized with a
communication endpoint ('dst') and a message buffer ('snd_buf').
For sending the message, we sequentially insert both arguments into the stream
to transform the arguments to a message and finally invoke the IPC mechanism of
the kernel by inserting the special object 'IPC_SEND'.
:Receiving a message:
!int a, b;
!Ipc_istream receiver(&rcv_buf);
!receiver >> IPC_WAIT >> a >> b;
For creating the 'receiver' input stream object, we specify a receive message
buffer as argument that can hold one incoming message.
By extracting the special object 'IPC_WAIT' from the receiver, we block for
a new message to be stored into 'rcv_buf'.
After returning from the blocking receive operation, we use the extraction
operator to _unmarshal_ the message argument by argument.
:Performing a RPC call:
!Ipc_client client(dst, &snd_buf, &rcv_buf);
!int result;
!client << OPCODE_FUNC1 << 1 << 2
! << IPC_CALL >> result;
The first argument is a constant that references one among
many server functions.
It is followed by the actual server-function arguments.
All arguments are marshalled into the 'snd_buf'.
When inserting the special object 'IPC_CALL' into the 'client'
stream, the client blocks for the result of the RPC.
After receiving the result message in 'rcv_buf', the RPC
results can be sequentially unmarshalled via the extraction
operator. Note that 'rcv_buf' and 'snd_buf' may use the
same backing store as both buffers are used interleaved.
:Dispatching a RPC call:
!Ipc_server server(&snd_buf, &rcv_buf);
!while (1) {
! int opcode;
! server >> IPC_REPLY_WAIT >> opcode;
! switch (opcode) {
! case OPCODE_FUNC1:
! {
! int a, b, ret;
! server >> a >> b;
! server << func1(a, b);
! break;
! }
! ..
! }
!}
The special object 'IPC_REPLY_WAIT' replies to the request of the previous
server-loop iteration with the message stored in 'snd_buf' (ignored for the
first iteration) and then waits for an incoming RPC request to be received
in 'rcv_buf'.
By convention, the first message argument contains the opcode to identify
the server function to handle the request.
After extracting the opcode from the 'server' stream, we branch into
a server-function-specific wrapper that reads the function arguments, calls the
actual server function, and inserts the function result into the 'server' stream.
The result message is to be delivered at the beginning of the next server-loop
iteration.
The two-stage argument-message parsing (the opcode to select the server function,
reading the server-function arguments) is simply done by subsequent extraction
operations.
Server framework
================
[image server_framework]
Relationships between the classes of the server object framework
Each component that makes local objects remotely accessible to other components
has to provide means to dispatch RPC requests that refer to different objects.
This procedure highly depends on the mechanisms provided by the
underlying kernel.
The primary motivation of the server framework is to hide actual kernel
paradigms for communication, control flow, and the implementation of
local names (capabilities) behind a generic interface.
The server framework unifies the control flow of RPC dispatching and the mapping
between capabilities and local objects using the classes depicted in Figure [server_framework].
:'Object_pool': is an associative array that maps capabilities from/to local objects.
Because capabilities are protected kernel objects, the object pool's functionality
is supported by the kernel.
*Note:* _On L4v2 and Linux, capabilities are not protected by the kernel but are_
_implemented as unique IDs. On these base platforms, the object pool performs_
_the simple mapping of such unique IDs to object pointers in the local_
_address space._
:'Server_object': is an object-pool entry that contains a dispatch function.
To make a local object type available to remote components, the local
object type must inherit from 'Server_object' and provide the implementation
of the dispatch function as described in Section [Communication].
:'Server_entrypoint': is an object pool that acts as a logical communication entrypoint.
It can manage any number of server objects. When registering a server object to be
managed by a server entrypoint ('manage' method), a capability for this object
gets created. This capability can be communicated to other processes,
which can then use the server object's RPC interface.
:'Server_activation': is the stack (or thread) to be used for handling RPC requests
of an entrypoint.
*Note:* _On L4v2 and Linux, exactly one server activation must be attached to_
_a server entrypoint. This implicates that RPC requests are handled in a_
_strictly serialized manner and one blocking server function delays all_
_other pending RPC requests referring the same server entrypoint. Concurrent handling_
_of RPC requests should be realized with multiple (completely independent)_
_server entrypoints._
Process environment
===================
As described in Section [Interfaces and Mechanisms], a newly created process
can only communicate to its immediate parent via its parent capability.
This parent capability gets created "magically" dependent on the actual
platform.
| For example, on the L4v2 platform, the parent writes the information about
| the parent capability to a defined position of the new process' address space
| after decoding the ELF image. On the Linux platform, the parent
| uses environment variables to communicate the parent capability to the
| child.
Before entering the 'main' function of the new process, the process' startup
code 'crt0' is executed and initializes the _environment_ framework.
The environment contains RPC communication stubs for communicating with the
parent and the process' RM session, CPU session, PD session, and RAM
session.
Furthermore, the environment contains a heap that uses the process' RAM
session as backing store.
The environment can be used from the actual program by dereferencing the pointer
returned by the global function:
! Env *env();
Child management
================
The class 'Child' provides a generic and extensible abstraction to unify the
creation of child processes, serve parent-interface requests, and to perform the
book keeping of open sessions.
Different access-control and resource-trading policies can be realized by
inheriting from this class and supplementing suitable parent-interface server
functions.
A child process can be created by instantiating a 'Child' object:
!Child(const char *name,
! Dataspace_capability elf_ds_cap,
! Ram_session_capability ram_session_cap,
! Cpu_session_capability cpu_session_cap,
! Cap_session *cap_session,
! char *args[])
*NOTE:* _The 'name' parameter is only used for debugging._
_The 'args' parameter is not yet supported._
;The 'Child' serves the parent interface for the new process by a distinct thread.
Heap partitioning
=================
In Section [Goals and Challenges] where we introduced the different types of
components composing our system, we highlighted _resource multiplexers_ as
being critical for maintaining the isolation and independence of applications from
each other.
If a flawed resource multiplexer serves multiple clients at a time, information
may leak from one client to another (corrupting isolation) or different clients
may interfere in sharing limited physical resources (corrupting independence).
One particular limited resource that is typically shared among all
clients is the heap of the server.
If the server performs heap allocations on behalf of one client, this resource
may exhaust and renders the service unavailable to all other clients (denial of
service).
The resource-trading concept as presented in Section [Quota] enables clients to
donate memory quota to a server during the use of a session.
If the server's parent closes the session on request of the client, the donated
resources must be released by the server.
In order to comply with the request to avoid intervention by its parent, the
server must store the state of each session on dedicated dataspaces that can be
released independently from other sessions.
Instead of using one heap to hold anonymous memory allocations, the server
creates a _heap partition_ for each client and performs client-specific
allocations exclusively on the corresponding heap partition.
There exist two different classes to assist developers in partitioning the heap:
:'Heap': is an allocator that allocates chunks of memory as dataspaces from a
RAM session. Each chunk may hold multiple allocations. This kind of heap
corresponds loosely to a classical heap and can be used to allocate a high
number of small memory objects. The used backing store gets released on the
destruction of the heap.
:'Sliced_heap': is an allocator that uses a dedicated dataspace for each
allocation. Therefore, each allocated block can be released independently
from all other allocations.
The 'Sliced_heap' must be used to obtain the actual session objects and store
them in independent dataspaces.
Dynamic memory allocations during the lifetime of a session must be performed by
a 'Heap' as member of the session object.
When closing a session, the session object including the heap partition gets
destroyed and all backing-store dataspaces can be released without interfering
other clients.
Limitations and Outlook
#######################
In its current incarnation, the design is subject to a number of limitations.
As a prime example for managing resources, we focused our work on physical
memory and ignored other prominent resource types such as processing time, bus
bandwidth, and network bandwidth.
We intend to apply the same methodology that we developed for physical memory to
other resource types analogously in later design revisions.
We do not cover features such as virtual-memory or transparent
copy-on-write support, which we regard as non-essential at the current stage.
At this point, we also do not provide specifics about the device-driver
infrastructure and legacy-software containers.
Note that the presented design does not fundamentally prevent the support
of these features.