課程影片連結：

這篇文章目的在於整理這個影片的重點，主要有以下的重點

  • 如何用 GDB 追蹤 user program
  • RISC-V 的 Calling Convention
    • RISC-V 的 register (Caller and Callee saved)
    • RISC-V 的 Stack Frame

影片中的範例程式並沒有附在 lab 中，要自己新增：

新增影片中的範例程式
  • kernel/defs.h
// demos.c
void            demo1(void);
void            demo2(void);
void            demo3(void);
void            demo4(void);
void            demo5(void);
void            demo6(void);
void            demo7(void);

// asmdemo.S
int             sum_to(int);
int             sum_then_double(int);
  • kernel/demos.c
#include "types.h"
#include "riscv.h"
#include "defs.h"
#include "date.h"
#include "param.h"
#include "memlayout.h"
#include "spinlock.h"
#include "proc.h"
#include "sysinfo.h"

void demo1()
{
	printf("Result: %d\n", sum_to(5));
}

void demo2()
{
	printf("Tesult: %d\n", sum_ten_double(5));
}

void _demo3(char a, char b, char c, char d, char e, char f, char g, char h, char i, char j)
{
	printf("%d, %d, %d, %d, %d, %d, %d, %d, %d, %d\n", a, b, c, d, e, f, g, h, i, j);
}

void demo3()
{
	_demo3('a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j');
}

int dummymain(int argc, char *argv[])
{
	int i = 0;
	for (; i < argc; i++)
	{
		printf("Argument %d: %s\n", i, argv[i]);
	}
	return 0;
}

void demo4()
{
	char *args[] = {"foo", "bar", "baz"};
	int result = dummymain(sizeof(args)/sizeof(args[0]), args);
	if (result < 0)
		panic("Demo 4");
}

int _dummy(int n)
{
	int div = 5;
	int sum = 0;
	for (int i=0; i < n; i++)
	{
		if (i % div == 0) sum += i;
	}
	return sum;
}

void demo5()
{
	printf("Demo\n");
	int top = 50;
	int result = _dummy(top);
	printf("Result: %d\n", result);
}

void demo6()
{
	int sum = 0;
	for (int i = 1; i < 10; i++)
	{
		sum += 3 *i;
		printf("%d\n", sum_to(i));
	}
	printf("Sum: %d\n", sum);
}

struct Person {
	int id;
	int age;
	// char *name;
};

void printPerson(struct Person *p)
{
	printf("Person %d (%d)\n", p->id, p->age);
	// printf("Name:%s: %d (%d)\n", p->name, p->id, p->age);
}

void demo7()
{
	// Structs
	struct Person p;
	p.id = 1215;
	p.age = 22;
	// p.name = "Nick";
	printPerson(&p);
}
  • kernel/asmdemo.S
.section .text
.global sum_to
/* 
  int sum_to() {
    int acc = 0;
    for (int i = 0; i <= n; i++) {
      acc += i;
    }
    return acc;
  }
*/

sum_to:
    mv t0, a0     # t0 <- a0
    li a0, 0      # a0 <- 0
  loop:
    add a0, a0, t0  # a0 <- a0 + t0
    addi t0, t0, -1 # t0 <- t0 - 1
    bnez t0, loop   # if t0 != 0: pc <- loop
    ret

.global sum_then_double
sum_then_double:
    addi sp, sp, -16
    sd ra, 0(sp)
    call sum_to
    li t0, 2
    mul a0, a0, t0
    ld ra, 0(sp)
    addi sp, sp, 16
    ret
  • Makefile
OBJS = \
  $K/entry.o \
  $K/kalloc.o \
  $K/string.o \
  ...
  $K/plic.o \
  $K/virtio_disk.o \
  $K/demos.o \
  $K/asmdemo.o

新增 system call demo()

  • user/usys.pl 加上 entry(demo);

  • user/user.h 加上 int demo(int);

  • kernel/syscall.h 加上 #define SYS_demo 22

  • kernel/syscall.c 加上

    • extern uint64 sys_demo(void);
    • [SYS_demo] sys_demo,

  • kernel/sysproc.c: 實做 sys_demo

uint64
sys_demo(void)
{
  int n;

  argint(0, &n);
  if (1 <= n && n <= 7) {
    switch(n) {
    case 1:
      demo1();
      break;
    case 2:
      demo2();
      break;
    case 3:
      demo3();
      break;
    case 4:
      demo4();
      break;
    case 5:
      demo5();
      break;
    case 6:
      demo6();
      break;
    case 7:
      demo7();
      break;
    }
  } else {
    return -1;
  }
  return 0;
}
  • user/demo.c: 使用 system call demo()
#include "kernel/types.h"
#include "kernel/stat.h"
#include "user/user.h"

int
main(int argc, char *argv[])
{
  int n;

  if (argc != 2) {
    fprintf(2, "usage: demo <1 ~ 7>\n");
    exit(1);
  }

  n = argv[1][0] - '0';
  if (1 <= n && n <= 7) {
    demo(n);
  } else {
    fprintf(2, "usage: demo <1 ~ 7>\n");
    exit(1);
  }
  exit(0);
}

從 C language 到 Assembly, 以及用 GDB 追蹤

.section .text
.global sum_to
/* 
  int sum_to() {
    int acc = 0;
    for (int i = 0; i <= n; i++) {
      acc += i;
    }
    return acc;
  }
*/

sum_to:
    mv t0, a0     # t0 <- a0
    li a0, 0      # a0 <- 0
  loop:
    add a0, a0, t0  # a0 <- a0 + t0
    addi t0, t0, -1 # t0 <- t0 - 1
    bnez t0, loop   # if t0 != 0: pc <- loop
    ret

.global sum_then_double
sum_then_double:
    addi sp, sp, -16
    sd ra, 0(sp)
    call sum_to
    li t0, 2
    mul a0, a0, t0
    ld ra, 0(sp)
    addi sp, sp, 16
    ret

b sum_to

c

si

Caller Saved 與 Callee Saved Registers

registers.png

Stack Frames

                   .
                   .
      +->          .
      |   +-----------------+   |
      |   | return address  |   |
      |   |   previous fp ------+
      |   | saved registers |
      |   | local variables |
      |   |       ...       | <-+
      |   +-----------------+   |
      |   | return address  |   |
      +------ previous fp   |   |
          | saved registers |   |
          | local variables |   |
      +-> |       ...       |   |
      |   +-----------------+   |
      |   | return address  |   |
      |   |   previous fp ------+
      |   | saved registers |
      |   | local variables |
      |   |       ...       | <-+
      |   +-----------------+   |
      |   | return address  |   |
      +------ previous fp   |   |
          | saved registers |   |
          | local variables |   |
  $fp --> |       ...       |   |
          +-----------------+   |
          | return address  |   |
          |   previous fp ------+
          | saved registers |
  $sp --> | local variables |
          +-----------------+

如果打破 Convention 會發生什麼事情

好用的 GDB 指令

Break Point

break at user/demo.c

(gdb) add-symbol-file user/_demo

(gdb) b user/demo.c:10

執行
next # 執行一行 c code

s # 往下追蹤 function call

step # 往下追蹤 function call

si # 執行一個 instruction

Layout
layout asm

layout src

layout split # c and assembly

layout reg

focus reg

focus asm

focus src

Print
info registers

i reg

附錄：GDB Cheat List

  • delete: delete all the breakpoints
p *argv@argc