# rv32emu trace code
###### tags: `Project`
by [\<ypaskell\>](https://github.com/ypaskell)
## 目標
完整了解 rv32emu 的 code
## Setup environment
### 1. Add debug flag to Makefile
Add `-g` in Makefile
```Makefile=
include mk/common.mk
include mk/toolchain.mk
OUT ?= build
BIN := $(OUT)/rv32emu
CFLAGS = -std=gnu99 -O2 -Wall -Wextra
CFLAGS += -Wno-unused-label -g #<--- Add CFLAGS -g here
CFLAGS += -include src/common.h
# Set the default stack pointer
CFLAGS += -D DEFAULT_STACK_ADDR=0xFFFFF000
OBJS_EXT :=
# Control and Status Register (CSR)
ENABLE_Zicsr ?= 1
$(call set-feature, Zicsr)
```
### 2. Build debug build
```shell
make all
```
## Understanding elf files `hello.elf`
### Objdump `hello.elf`
Run `riscv32-unknown-elf-objdump -dhs build/hello.elf | less`
* `-d`: Display assembler contents of executable sections
* `-h`: Display the contents of the section headers
* `-s`: Display the full contents of all sections requested
```Text=
$ riscv32-unknown-elf-objdump -dhs build/hello.elf
build/hello.elf: file format elf32-littleriscv
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 00000050 00000000 00000000 00001000 2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
1 .riscv.attributes 0000001a 00000000 00000000 00001050 2**0
CONTENTS, READONLY
Contents of section .text:
0000 93020000 13035000 6f004000 13000000 ......P.o.@.....
0010 63826202 93080004 13051000 97050000 c.b.............
0020 93854502 1306d000 73000000 93821200 ..E.....s.......
0030 6ff01ffe 9308d005 13050000 73000000 o...........s...
0040 48656c6c 6f20576f 726c6421 0a000000 Hello World!....
Contents of section .riscv.attributes:
0000 41190000 00726973 63760001 0f000000 A....riscv......
0010 05727633 32693270 3100 .rv32i2p1.
Disassembly of section .text:
00000000 <.text>:
0: 00000293 li t0,0
4: 00500313 li t1,5
8: 0040006f j 0xc
c: 00000013 nop
10: 02628263 beq t0,t1,0x34
14: 04000893 li a7,64
18: 00100513 li a0,1
1c: 00000597 auipc a1,0x0
20: 02458593 addi a1,a1,36 # 0x40
24: 00d00613 li a2,13
28: 00000073 ecall
2c: 00128293 addi t0,t0,1
30: fe1ff06f j 0x10
34: 05d00893 li a7,93
38: 00000513 li a0,0
3c: 00000073 ecall
40: 6548 flw fa0,12(a0)
42: 6c6c flw fa1,92(s0)
44: 6f57206f j 0x72f38
48: 6c72 flw fs8,28(sp)
4a: 2164 fld fs1,192(a0)
4c: 000a c.slli zero,0x2
...
```
### elf open
Run `riscv32-unknown-elf-readelf -h build/hello.elf`
```text=
$ riscv32-unknown-elf-readelf -h build/hello.elf
ELF Header:
Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
Class: ELF32
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: EXEC (Executable file)
Machine: RISC-V
Version: 0x1
Entry point address: 0x0
Start of program headers: 52 (bytes into file)
Start of section headers: 4240 (bytes into file)
Flags: 0x0
Size of this header: 52 (bytes)
Size of program headers: 32 (bytes)
Number of program headers: 2
Size of section headers: 40 (bytes)
Number of section headers: 4
Section header string table index: 3
```
上面的 Elf Header 是要在 rv32emu `elf_*` 的 function 處理的。
### Start using debugging tool (gdb or lldb)
我在 Apple Silicom M1 上跑，無法使用 gdb。但 lldb 和 gdb 大同小異，請自行轉換 Command
Navigate to main function
```text
(lldb) b main
Breakpoint 1: 25 locations.
(lldb) r
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
frame #0: 0x000000010000bca4 rv32emu`main at main.c:112:5 [opt]
109 static bool parse_args(int argc, char **args)
110 {
111 /* parse each argument in turn */
-> 112 for (int i = 1; i < argc; ++i) {
113 const char *arg = args[i];
114 /* parse flags */
115 if (arg[0] == '-') {
(lldb) finish
Process 28894 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = step out
frame #0: 0x000000010000bdf4 rv32emu`main(argc=<unavailable>, args=0x000000016fdff368) at main.c:209:18 [opt]
206 }
207
208 /* open the ELF file from the file system */
-> 209 elf_t *elf = elf_new();
210 if (!elf_open(elf, opt_prog_name)) {
211 fprintf(stderr, "Unable to open ELF file '%s'\n", opt_prog_name);
212 return 1;
```
Initialize `elf_t` for saving elf data
```text
(lldb) s
Process 28894 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = step in
frame #0: 0x000000010000a81c rv32emu`elf_new at elf.c:137:16 [opt]
134
135 elf_t *elf_new()
136 {
-> 137 elf_t *e = malloc(sizeof(elf_t));
138 e->hdr = NULL;
139 e->raw_size = 0;
140 e->symbols = map_init(int, char *, map_cmp_uint);
141 e->raw_data = NULL;
142 return e;
143 }
```
`elf_t` declared in `elf.c`
```cpp=
typedef struct elf_internal elf_t; // fron elf.h
struct elf_internal {
const struct Elf32_Ehdr *hdr;
uint32_t raw_size;
uint8_t *raw_data;
/* symbol table map: uint32_t -> (const char *) */
map_t symbols;
};
```
```cpp=
bool elf_open(elf_t *e, const char *path)
{
bool elf_open(elf_t *e, const char *path)
{
/* free previous memory */
if (e->raw_data)
release(e);
#if defined(USE_MMAP)
int fd = open(path, O_RDONLY);
if (fd < 0)
return false;
/* get file size */
struct stat st;
fstat(fd, &st);
e->raw_size = st.st_size;
/* map or unmap files or devices into memory.
* The beginning of the file is ELF header.
*/
e->raw_data = mmap(0, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
if (e->raw_data == MAP_FAILED) {
release(e);
return false;
}
#else /* fallback to standard I/O text stream */
FILE *f = fopen(path, "rb");
if (!f)
return false;
/* get file size */
fseek(f, 0, SEEK_END);
e->raw_size = ftell(f);
fseek(f, 0, SEEK_SET);
if (e->raw_size == 0) {
fclose(f);
return false;
}
/* allocate memory */
free(e->raw_data);
e->raw_data = malloc(e->raw_size);
/* read data into memory */
const size_t r = fread(e->raw_data, 1, e->raw_size, f);
fclose(f);
if (r != e->raw_size) {
release(e);
return false;
}
#endif /* USE_MMAP */
/* point to the header */
e->hdr = (const struct Elf32_Ehdr *) e->raw_data;
/* check it is a valid ELF file */
if (!is_valid(e)) {
release(e);
return false;
}
return true;
}
```
Q&A (2023/09/11)
* Question: 為什麼原本使用 fopen 現在使用 `mmap`. [man mmap](https://man7.org/linux/man-pages/man2/mmap.2.html) ?
* Jserv: `mmap` 在開啟大檔案時會有優勢。On-demand paging。反之，fopen 遇到大檔案時，開啟時間較慢。
* Question: man `mmap` 説，mmap 是利用 virtual memory。Virtual Memory 在 Disk 不是會比較慢嗎。
* Jserv: virtual memory 不見得跟 disk 有關，請「不要」讀恐龍書來。[這本書可自由下載，品質比恐龍書好多了](https://pages.cs.wisc.edu/~remzi/OSTEP/)
[understanding mmap, the workhorse behind keeping memory access efficient in linux](https://www.youtube.com/watch?v=8hVLcyBkSXY)
`struct stat` - [man](https://man7.org/linux/man-pages/man2/stat.2.html)
```cpp=
struct stat {
dev_t st_dev; /* ID of device containing file */
ino_t st_ino; /* inode number */
mode_t st_mode; /* protection */
nlink_t st_nlink; /* number of hard links */
uid_t st_uid; /* user ID of owner */
gid_t st_gid; /* group ID of owner */
dev_t st_rdev; /* device ID (if special file) */
off_t st_size; /* total size, in bytes */
blksize_t st_blksize; /* blocksize for file system I/O */
blkcnt_t st_blocks; /* number of 512B blocks allocated */
time_t st_atime; /* time of last access */
time_t st_mtime; /* time of last modification */
time_t st_ctime; /* time of last status change */
};
```
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