System Call Table Implementation Plan

Author

longjin <longjin@dragonos.org>

Date: 2025/05/13

Overview

Compared to the original approach of dispatching system calls in a single large match statement, this approach uses a trait-based and system call table-based implementation. The main advantages include:

• Reduced stack memory usage: Avoids a single large function consuming excessive stack space
• Support for parameter printing: Through a unified parameter formatting interface
• Better extensibility: Adding new system calls does not require modifying the dispatch logic

Core Design

Syscall Trait

All system call handler functions must implement the Syscall trait:

 pub trait Syscall: Send + Sync + 'static {
     fn num_args(&self) -> usize;
     fn handle(&self, args: &[usize], from_user: bool) -> Result<usize, SystemError>;
     fn entry_format(&self, args: &[usize]) -> Vec<FormattedSyscallParam>;
 }

• num_args(): Returns the number of arguments required by the system call
• handle(): Executes the actual system call handling
• entry_format(): Formats the parameters for debugging printing

SyscallHandle

The SyscallHandle struct associates a system call number with its handler:

 pub struct SyscallHandle {
     pub nr: usize,  // System call number
     pub inner_handle: &'static dyn Syscall,  // Handler function
     pub name: &'static str,
 }

SyscallTable

The SyscallTable manages all system calls:

• Fixed size of 512 entries
• Initialized at compile time
• Allows quick lookup of the handler function by system call number

Usage

Implementing a System Call

1. Define the implementation of Syscall for the specific system call
2. Register the system call in the system call table
3. Load all registered system calls during system initialization