1. Trap

Kernel code (assembler or C) that processes a trap is often called a handler; the first handler instructions are usually written in assembler (trampoline.S, rather than C) and are called a vector.

Popular names

• kernel mode: 内核态

• user mode: 用户态

• supervisor: 在内核态工作的具有较高权限的管理者身份

• user: 与计算机进行交互的普通用户身份 🧑🏻‍💻 <=> 💻

• stvec: The kernel writes the address of its trap handler here; the RISC-V jumps to the address in stvec to handle a trap. (指向了内核中处理 trap 的指令的起始地址)

• sepc: When a trap occurs, RISC-V saves the program counter here (since the pc is then overwritten with the value in stvec).

• sret: The sret (return from trap) instruction copies sepc to the pc. The kernel can write sepc to control where sret goes.

• scause: RISC-V puts a number here that describes the reason for the trap. (exception, system call, device interrupt)

• sscratch: (Supervisor mode scratch) Helps trap handler avoid over writing user registers before saving them. (🌟 OS 留了一个 reg 在自己手上, 对用户进程不可见.)

• sstatus: The SIE bit in sstatus controls whether device interrupts are enabled. If the kernel clears SIE (0), the RISC-V will defer device interrupts until the kernel sets SIE (1). The SPP bit indicates whether a trap came from user mode (0) or supervisor mode (1), and controls to what mode sret returns.

  SSTATUS [SPP _ _ SPIE UPIE _ _ SIE UIE]
            ^  ^ ^  ^    ^   ^ ^  ^   ^
            8  7 6  5    4   3 2  1   0
  

• satp: (Supervisor mode address translation and protection) Tells trampoline the user/supervisor page table to switch to. (包含了指向 page table 的物理内存地址)

• uservec: Saves user registers in the trapframe, an assembly function.

• TRAPFRAME (0x3fffffe000, 保存进程寄存器现场的内存) and TRAMPOLINE ($stvec = 0x3ffffff00, Read-only, 跳板)

  

• memlayout.h

  // User memory layout
  // Address zero first:
  //   text
  //   original data and bss
  //   fixed-size stack
  //   expandable heap
  //   ...
  //   TRAPFRAME (p->trapframe, used by the trampoline)
  //   TRAMPOLINE (the same page as in the `kernel`)
  
  // Map the trampoline page to the highest address,
  // in both user and kernel space.
  #define TRAMPOLINE (MAXVA - PGSIZE)
  
  // TRAPFRAME is the next page of TRAMPOLINE
  #define TRAPFRAME (TRAMPOLINE - PGSIZE)
  

• PTE_U: flag determines whether the user mode can use current page table. PTE_U = 0: Supervisor, 1: User

• proc.h

  struct trapframe & 32 regs

  Good practice