210 lines
7.5 KiB
ReStructuredText
210 lines
7.5 KiB
ReStructuredText
=============
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Syscall Layer
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=============
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This page discusses supports a syscall layer from communication between a
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monolithic, kernel-mode NuttX kernel and a separately built, user-mode
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application set.
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With most MCUs, NuttX is built as a flat, single executable image
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containing the NuttX RTOS along with all application code. The RTOS code
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and the application run in the same address space and at the same kernel-
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mode privileges. In order to exploit security features of certain
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processors, an alternative build model is also supported: NuttX can
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be built separately as a monolithic, kernel-mode module and the applications
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can be added as a separately built, user-mode module.
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The syscall layer provided in this directory serves as the communication
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layer from the user-mode application into the kernel-mode RTOS. The
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switch from user-mode to kernel-mode is accomplished using software
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interrupts (SWIs). SWIs are implemented differently and named differently
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by different manufacturers but all work essentially the same: A special
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instruction is executed in user-mode that causes a software generated
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interrupt. The software generated interrupt is caught within the kernel
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and handle in kernel-mode.
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Header Files
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------------
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``include/syscall.h``
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~~~~~~~~~~~~~~~~~~~~~
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This header file supports general access to SWI facilities. It is simply
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a wrapper file that includes ``include/sys/syscall.h`` and
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``include/arch/syscall.h``.
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``include/sys/syscall.h``
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~~~~~~~~~~~~~~~~~~~~~~~~~
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The SWIs received by the kernel are distinguish by a code that identifies
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how to process the SWI. This header file defines all such codes understood
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by the NuttX kernel.
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``include/arch/syscall.h`` (or ``arch/<cpu>/include/syscall.h``)
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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This header file is provided by the platform-specific logic and declares
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(or defines) the mechanism for providing software interrupts on this
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platform. The following functions must be declared (or defined) in this
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header file:
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- ``SWI`` with ``SYS_`` call number and one parameter:
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.. code-block:: C
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uintptr_t sys_call0(unsigned int nbr);
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- ``SWI`` with ``SYS_`` call number and one parameter:
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.. code-block:: C
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uintptr_t sys_call1(unsigned int nbr, uintptr_t parm1);
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- ``SWI`` with ``SYS_`` call number and two parameters:
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.. code-block:: C
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uintptr_t sys_call2(unsigned int nbr, uintptr_t parm1, uintptr_t parm2);
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- ``SWI`` with ``SYS_`` call number and three parameters:
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.. code-block:: C
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uintptr_t sys_call3(unsigned int nbr, uintptr_t parm1,
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uintptr_t parm2, uintptr_t parm3);
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- ``SWI`` with ``SYS_`` call number and four parameters:
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.. code-block:: C
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uintptr_t sys_call4(unsigned int nbr, uintptr_t parm1, uintptr_t parm2,
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uintptr_t parm3, uintptr_t parm4);
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- ``SWI`` with ``SYS_`` call number and five parameters:
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.. code-block:: C
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uintptr_t sys_call5(unsigned int nbr, uintptr_t parm1, uintptr_t parm2,
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uintptr_t parm3, uintptr_t parm4, uintptr_t parm5);
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- ``SWI`` with ``SYS_`` call number and six parameters:
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.. code-block:: C
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uintptr_t sys_call6(unsigned int nbr, uintptr_t parm1, uintptr_t parm2,
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uintptr_t parm3, uintptr_t parm4, uintptr_t parm5,
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uintptr_t parm6);
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Syscall Database
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~~~~~~~~~~~~~~~~
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Sycall information is maintained in a database. That "database" is
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implemented as a simple comma-separated-value file, ``syscall.csv``. Most
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spreadsheets programs will accept this format and can be used to maintain
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the syscall database.
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The format of the CSV file for each line is:
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* Field 1: Function name
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* Field 2: The header file that contains the function prototype
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* Field 3: Condition for compilation
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* Field 4: The type of function return value.
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* Field 5 - N+5: The type of each of the N formal parameters of the function
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* Fields N+5 - : If the last parameter is "...", then the following fields
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provide the type and number of of possible optional parameters.
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See note below about variadic functions
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Each type field has a format as follows:
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* type name:
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For all simpler types
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* formal type | actual type:
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For array types where the form of the formal (eg. ``int parm[2]``)
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differs from the type of actual passed parameter (eg. ``int*``).
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This is necessary because you cannot do simple casts to array types.
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* formal type | union member actual type | union member fieldname:
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A similar situation exists for unions. For example, the formal
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parameter type union sigval -- You cannot cast a uintptr_t to
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a union sigval, but you can cast to the type of one of the union
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member types when passing the actual parameter. Similarly, we
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cannot cast a union sigval to a uinptr_t either. Rather, we need
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to cast a specific union member fieldname to ``uintptr_t``.
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Variadic Functions
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------------------
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General variadic functions which may have an arbitrary number of argument
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or arbitrary types cannot be represented as system calls.
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``syslog()`` is a good example. Normally you would work around this by
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using the non- variadic form of the OS interface that accepts a ``va_list``
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as an argument, ``vsyslog()`` in this case.
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There there are many functions that have a variadic form but take only
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one or two arguments optional arguments. There can be handled as system
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calls, but only by treating them as though they had a fixed number of
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arguments.
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These are are handled in ``syscall.csv`` by appending the number and type of
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optional arguments. For example, consider the ``open()`` OS interface. Its
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prototype is:
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.. code-block:: C
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int open(const char *path, int oflag, ...);
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In reality, open may take only a single optional argument of type ``mode_t``
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and is represented in syscall.cvs like this::
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"open","fcntl.h","","int","const char*","int","...","mode_t"
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The existence of the ``mode_t`` tells ``tools/mksyscall`` that there is at most
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one optional parameter and, if present, it is of type ``mode_t``.
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NOTE: This CSV file is used both to support the generate of trap information,
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but also for the generation of symbol tables. See ``Documentation/components/tools/``
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and ``Documentation/components/libs/`` for further information.
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Auto-Generated Files
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--------------------
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Stubs and proxies for the sycalls are automatically generated from this CSV
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database. Here the following definition is used:
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* Proxy - A tiny bit of code that executes in the user space. A proxy
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has exactly the same function prototype as does the "real" function
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for which it proxies. However, it only serves to map the function
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call into a syscall, marshaling all of the system call parameters
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as necessary.
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* Stub - Another tiny bit of code that executes within the NuttX kernel
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that is used to map a software interrupt received by the kernel to
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a kernel function call. The stubs receive the marshaled system
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call data, and perform the actually kernel function call (in
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kernel-mode) on behalf of the proxy function.
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Sub-Directories
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---------------
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* ``stubs`` - Autogenerated stub files are placed in this directory.
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* ``proxies`` - Autogenerated proxy files are placed in this directory.
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mksyscall
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---------
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mksyscall is C program that is used used during the initial NuttX build
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by the logic in the top-level ``syscall/`` directory. Information about the
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stubs and proxies is maintained in a comma separated value (CSV) file
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in the ``syscall/`` directory. The mksyscall program will accept this CSV
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file as input and generate all of the required proxy or stub files as
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output. See ``Documentation/components/tools/`` for additional information.
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