60 lines
2.4 KiB
Plaintext
60 lines
2.4 KiB
Plaintext
#
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# For a description of the syntax of this configuration file,
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# see the file kconfig-language.txt in the NuttX tools repository.
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#
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menuconfig LIB_SYSCALL
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bool "System call support"
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default n
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---help---
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Build in support for "system calls". System calls are used to
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implement a call gate mechanism that can be be used to call from
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user code into the kernel. This is only useful for user code that
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lies outside of the kernel such as when the BUILD_PROTECTED or
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BUILD_KERNEL builds are selected.
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This permits calls from user-mode code into kernel mode; the call
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gate will change the mode of operation from user to supervisor mode,
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then call into the OS code on behalf of the user-mode application.
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If if there are no privilege issues preventing the call, system
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calls may also be of value because it can eliminate the need for
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symbol tables when linking external modules to the NuttX base code.
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The selection will build libsyscall. External modules can then link
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with libsyscall when they are built and they can call into the OS
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with no knowledge of the actual address in the OS. In this case,
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they call into a proxy that is link with the external code; that
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proxy then marshals the call parameter and invokes the system call
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to accomplish the interface.
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if LIB_SYSCALL
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config SYS_RESERVED
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int "Number of reserved system calls"
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default 0
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---help---
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Kernel system calls may share the same software trapping mechanism
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as other functions used by architecture port. Those software traps
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must be reserved for use exclusively by the architecture. These
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value specifies the number of reserved software traps used by the
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architecture; number of the kernel system calls will begin with this
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number.
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config SYS_NNEST
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int "Number of nested system calls"
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default 2
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---help---
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This is architecture dependent. Most architectures allocate
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resources to manage a fixed, maximum number of nested system calls.
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A nested system call occurs in the following scenario: (1) A non-
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privileged user thread executes a system call, (2) part of the
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system call processing cause a call back into the user space code,
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and (3) the user space code performs another system call.
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In the current design, this can happen only under one condition:
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When the kernel calls back into user space in order to allocate user
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space memory. So it is expected that the maximum nesting level will
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be only 2.
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endif # LIB_SYSCALL
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