MIT教学操作系统xv6详解与源码分析

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“mit教学用的操作系统最新版源代码”是一个基于Unix V6版本的简化实现，名为xv6。这个操作系统源代码适用于现代x86架构的多处理器系统，使用ANSI C语言编写，旨在教育目的，让学生了解操作系统的基本原理和实现。在深入探讨xv6之前，我们先理解一下Unix V6。Unix V6是Dennis Ritchie和Ken Thompson合作开发的早期Unix版本之一，对后来的操作系统设计产生了深远影响。xv6则是为了复现Unix V6的核心概念，但它并非直接移植，而是根据现代硬件和编程标准进行了调整。 xv6的设计思路紧密遵循Unix V6，但包含了一些现代操作系统的特点，如多处理器支持（MP）。它借鉴了多个开源项目，包括来自JOS（用于底层硬件交互的代码）、Plan 9（多处理器相关的代码，如entryother.S, mp.h, mp.c, lapic.c）、FreeBSD（ioapic.c）以及NetBSD（console.c）的部分组件。此外，还有多位贡献者，如Russ Cox、Cliff Frey、Xiao Yu等，为xv6的实现提供了代码或技术支持。学习xv6的源代码，学生可以了解到操作系统的基本组成部分，例如： 1. **内存管理**：xv6如何分配和回收内存，以及如何使用页表进行地址转换。 2. **进程管理**：包括进程创建、上下文切换、同步原语和锁的实现。 3. **文件系统**：理解如何实现简单的文件系统结构，包括文件的读写、目录操作等。 4. **设备驱动**：如何与硬件设备交互，例如IDE硬盘驱动和键盘鼠标等输入设备的处理。 5. **中断处理**：理解中断服务例程和中断向量的概念，以及如何处理中断请求。 6. **多处理器支持**：如何在多核CPU环境下协调进程执行，以及中断处理的MP兼容性。 xv6的简洁性和教育性质使得它成为教授操作系统原理的理想教材。通过阅读和分析xv6的源代码，学生不仅可以了解操作系统的核心机制，还能实际动手修改和调试，从而获得更深入的理解。由于xv6的代码量较小（不到10000行），这对于初学者来说是可管理的，能够帮助他们在相对较短的时间内掌握操作系统设计的基础。此外，mit的教学网站提供了更多的在线资源和支持，包括文档、讨论和问题解答，进一步增强了学习体验。对于想要进入操作系统领域的开发者，研究xv6是一个非常有价值的起点。

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Aug 29 15:52 2017 xv6/mmu.h Page 5

0900 // Gate descriptors for interrupts and traps

0901 struct gatedesc {

0902 uint off_15_0 : 16; // low 16 bits of offset in segment

0903 uint cs : 16; // code segment selector

0904 uint args : 5; // # args, 0 for interrupt/trap gates

0905 uint rsv1 : 3; // reserved(should be zero I guess)

0906 uint type : 4; // type(STS_{TG,IG32,TG32})

0907 uint s : 1; // must be 0 (system)

0908 uint dpl : 2; // descriptor(meaning new) privilege level

0909 uint p : 1; // Present

0910 uint off_31_16 : 16; // high bits of offset in segment

0911 };

0912

0913 // Set up a normal interrupt/trap gate descriptor.

0914 // − istrap: 1 for a trap (= exception) gate, 0 for an interrupt gate.

0915 // interrupt gate clears FL_IF, trap gate leaves FL_IF alone

0916 // − sel: Code segment selector for interrupt/trap handler

0917 // − off: Offset in code segment for interrupt/trap handler

0918 // − dpl: Descriptor Privilege Level −

0919 // the privilege level required for software to invoke

0920 // this interrupt/trap gate explicitly using an int instruction.

0921 #define SETGATE(gate, istrap, sel, off, d) \

0922 { \

0923 (gate).off_15_0 = (uint)(off) & 0xffff; \

0924 (gate).cs = (sel); \

0925 (gate).args = 0; \

0926 (gate).rsv1 = 0; \

0927 (gate).type = (istrap) ? STS_TG32 : STS_IG32; \

0928 (gate).s = 0; \

0929 (gate).dpl = (d); \

0930 (gate).p = 1; \

0931 (gate).off_31_16 = (uint)(off) >> 16; \

0932 }

0933

0934 #endif

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Sheet 09

Aug 29 15:52 2017 xv6/elf.h Page 1

0950 // Format of an ELF executable file

0951

0952 #define ELF_MAGIC 0x464C457FU // "\x7FELF" in little endian

0953

0954 // File header

0955 struct elfhdr {

0956 uint magic; // must equal ELF_MAGIC

0957 uchar elf[12];

0958 ushort type;

0959 ushort machine;

0960 uint version;

0961 uint entry;

0962 uint phoff;

0963 uint shoff;

0964 uint flags;

0965 ushort ehsize;

0966 ushort phentsize;

0967 ushort phnum;

0968 ushort shentsize;

0969 ushort shnum;

0970 ushort shstrndx;

0971 };

0972

0973 // Program section header

0974 struct proghdr {

0975 uint type;

0976 uint off;

0977 uint vaddr;

0978 uint paddr;

0979 uint filesz;

0980 uint memsz;

0981 uint flags;

0982 uint align;

0983 };

0984

0985 // Values for Proghdr type

0986 #define ELF_PROG_LOAD 1

0987

0988 // Flag bits for Proghdr flags

0989 #define ELF_PROG_FLAG_EXEC 1

0990 #define ELF_PROG_FLAG_WRITE 2

0991 #define ELF_PROG_FLAG_READ 4

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Sheet 09

Aug 29 15:52 2017 xv6/entry.S Page 1

1000 # The xv6 kernel starts executing in this file. This file is linked with

1001 # the kernel C code, so it can refer to kernel symbols such as main().

1002 # The boot block (bootasm.S and bootmain.c) jumps to entry below.

1003

1004 # Multiboot header, for multiboot boot loaders like GNU Grub.

1005 # http://www.gnu.org/software/grub/manual/multiboot/multiboot.html

1006 #

1007 # Using GRUB 2, you can boot xv6 from a file stored in a

1008 # Linux file system by copying kernel or kernelmemfs to /boot

1009 # and then adding this menu entry:

1010 #

1011 # menuentry "xv6" {

1012 # insmod ext2

1013 # set root=’(hd0,msdos1)’

1014 # set kernel=’/boot/kernel’

1015 # echo "Loading ${kernel}..."

1016 # multiboot ${kernel} ${kernel}

1017 # boot

1018 # }

1019

1020 #include "asm.h"

1021 #include "memlayout.h"

1022 #include "mmu.h"

1023 #include "param.h"

1024

1025 # Multiboot header. Data to direct multiboot loader.

1026 .p2align 2

1027 .text

1028 .globl multiboot_header

1029 multiboot_header:

1030 #define magic 0x1badb002

1031 #define flags 0

1032 .long magic

1033 .long flags

1034 .long (−magic−flags)

1035

1036 # By convention, the _start symbol specifies the ELF entry point.

1037 # Since we haven’t set up virtual memory yet, our entry point is

1038 # the physical address of ’entry’.

1039 .globl _start

1040 _start = V2P_WO(entry)

1041

1042 # Entering xv6 on boot processor, with paging off.

1043 .globl entry

1044 entry:

1045 # Turn on page size extension for 4Mbyte pages

1046 movl %cr4, %eax

1047 orl $(CR4_PSE), %eax

1048 movl %eax, %cr4

1049 # Set page directory

Sheet 10

Aug 29 15:52 2017 xv6/entry.S Page 2

1050 movl $(V2P_WO(entrypgdir)), %eax

1051 movl %eax, %cr3

1052 # Turn on paging.

1053 movl %cr0, %eax

1054 orl $(CR0_PG|CR0_WP), %eax

1055 movl %eax, %cr0

1056

1057 # Set up the stack pointer.

1058 movl $(stack + KSTACKSIZE), %esp

1059

1060 # Jump to main(), and switch to executing at

1061 # high addresses. The indirect call is needed because

1062 # the assembler produces a PC−relative instruction

1063 # for a direct jump.

1064 mov $main, %eax

1065 jmp *%eax

1066

1067 .comm stack, KSTACKSIZE

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Sheet 10

Aug 29 15:52 2017 xv6/entryother.S Page 1

1100 #include "asm.h"

1101 #include "memlayout.h"

1102 #include "mmu.h"

1103

1104 # Each non−boot CPU ("AP") is started up in response to a STARTUP

1105 # IPI from the boot CPU. Section B.4.2 of the Multi−Processor

1106 # Specification says that the AP will start in real mode with CS:IP

1107 # set to XY00:0000, where XY is an 8−bit value sent with the

1108 # STARTUP. Thus this code must start at a 4096−byte boundary.

1109 #

1110 # Because this code sets DS to zero, it must sit

1111 # at an address in the low 2^16 bytes.

1112 #

1113 # Startothers (in main.c) sends the STARTUPs one at a time.

1114 # It copies this code (start) at 0x7000. It puts the address of

1115 # a newly allocated per−core stack in start−4,the address of the

1116 # place to jump to (mpenter) in start−8, and the physical address

1117 # of entrypgdir in start−12.

1118 #

1119 # This code combines elements of bootasm.S and entry.S.

1120

1121 .code16

1122 .globl start

1123 start:

1124 cli

1125

1126 # Zero data segment registers DS, ES, and SS.

1127 xorw %ax,%ax

1128 movw %ax,%ds

1129 movw %ax,%es

1130 movw %ax,%ss

1131

1132 # Switch from real to protected mode. Use a bootstrap GDT that makes

1133 # virtual addresses map directly to physical addresses so that the

1134 # effective memory map doesn’t change during the transition.

1135 lgdt gdtdesc

1136 movl %cr0, %eax

1137 orl $CR0_PE, %eax

1138 movl %eax, %cr0

1139

1140 # Complete the transition to 32−bit protected mode by using a long jmp

1141 # to reload %cs and %eip. The segment descriptors are set up with no

1142 # translation, so that the mapping is still the identity mapping.

1143 ljmpl $(SEG_KCODE<<3), $(start32)

1144

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Sheet 11

Aug 29 15:52 2017 xv6/entryother.S Page 2

1150 .code32 # Tell assembler to generate 32−bit code now.

1151 start32:

1152 # Set up the protected−mode data segment registers

1153 movw $(SEG_KDATA<<3), %ax # Our data segment selector

1154 movw %ax, %ds # −> DS: Data Segment

1155 movw %ax, %es # −> ES: Extra Segment

1156 movw %ax, %ss # −> SS: Stack Segment

1157 movw $0, %ax # Zero segments not ready for use

1158 movw %ax, %fs # −> FS

1159 movw %ax, %gs # −> GS

1160

1161 # Turn on page size extension for 4Mbyte pages

1162 movl %cr4, %eax

1163 orl $(CR4_PSE), %eax

1164 movl %eax, %cr4

1165 # Use entrypgdir as our initial page table

1166 movl (start−12), %eax

1167 movl %eax, %cr3

1168 # Turn on paging.

1169 movl %cr0, %eax

1170 orl $(CR0_PE|CR0_PG|CR0_WP), %eax

1171 movl %eax, %cr0

1172

1173 # Switch to the stack allocated by startothers()

1174 movl (start−4), %esp

1175 # Call mpenter()

1176 call *(start−8)

1177

1178 movw $0x8a00, %ax

1179 movw %ax, %dx

1180 outw %ax, %dx

1181 movw $0x8ae0, %ax

1182 outw %ax, %dx

1183 spin:

1184 jmp spin

1185

1186 .p2align 2

1187 gdt:

1188 SEG_NULLASM

1189 SEG_ASM(STA_X|STA_R, 0, 0xffffffff)

1190 SEG_ASM(STA_W, 0, 0xffffffff)

1191

1192

1193 gdtdesc:

1194 .word (gdtdesc − gdt − 1)

1195 .long gdt

1196

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Sheet 11

Aug 29 15:52 2017 xv6/main.c Page 1

1200 #include "types.h"

1201 #include "defs.h"

1202 #include "param.h"

1203 #include "memlayout.h"

1204 #include "mmu.h"

1205 #include "proc.h"

1206 #include "x86.h"

1207

1208 static void startothers(void);

1209 static void mpmain(void) __attribute__((noreturn));

1210 extern pde_t *kpgdir;

1211 extern char end[]; // first address after kernel loaded from ELF file

1212

1213 // Bootstrap processor starts running C code here.

1214 // Allocate a real stack and switch to it, first

1215 // doing some setup required for memory allocator to work.

1216 int

1217 main(void)

1218 {

1219 kinit1(end, P2V(4*1024*1024)); // phys page allocator

1220 kvmalloc(); // kernel page table

1221 mpinit(); // detect other processors

1222 lapicinit(); // interrupt controller

1223 seginit(); // segment descriptors

1224 picinit(); // disable pic

1225 ioapicinit(); // another interrupt controller

1226 consoleinit(); // console hardware

1227 uartinit(); // serial port

1228 pinit(); // process table

1229 tvinit(); // trap vectors

1230 binit(); // buffer cache

1231 fileinit(); // file table

1232 ideinit(); // disk

1233 startothers(); // start other processors

1234 kinit2(P2V(4*1024*1024), P2V(PHYSTOP)); // must come after startothers()

1235 userinit(); // first user process

1236 mpmain(); // finish this processor’s setup

1237 }

1238

1239 // Other CPUs jump here from entryother.S.

1240 static void

1241 mpenter(void)

1242 {

1243 switchkvm();

1244 seginit();

1245 lapicinit();

1246 mpmain();

1247 }

1248

1249

Sheet 12

Aug 29 15:52 2017 xv6/main.c Page 2

1250 // Common CPU setup code.

1251 static void

1252 mpmain(void)

1253 {

1254 cprintf("cpu%d: starting %d\n", cpuid(), cpuid());

1255 idtinit(); // load idt register

1256 xchg(&(mycpu()−>started), 1); // tell startothers() we’re up

1257 scheduler(); // start running processes

1258 }

1259

1260 pde_t entrypgdir[]; // For entry.S

1261

1262 // Start the non−boot (AP) processors.

1263 static void

1264 startothers(void)

1265 {

1266 extern uchar _binary_entryother_start[], _binary_entryother_size[];

1267 uchar *code;

1268 struct cpu *c;

1269 char *stack;

1270

1271 // Write entry code to unused memory at 0x7000.

1272 // The linker has placed the image of entryother.S in

1273 // _binary_entryother_start.

1274 code = P2V(0x7000);

1275 memmove(code, _binary_entryother_start, (uint)_binary_entryother_size);

1276

1277 for(c = cpus; c < cpus+ncpu; c++){

1278 if(c == mycpu()) // We’ve started already.

1279 continue;

1280

1281 // Tell entryother.S what stack to use, where to enter, and what

1282 // pgdir to use. We cannot use kpgdir yet, because the AP processor

1283 // is running in low memory, so we use entrypgdir for the APs too.

1284 stack = kalloc();

1285 *(void**)(code−4) = stack + KSTACKSIZE;

1286 *(void**)(code−8) = mpenter;

1287 *(int**)(code−12) = (void *) V2P(entrypgdir);

1288

1289 lapicstartap(c−>apicid, V2P(code));

1290

1291 // wait for cpu to finish mpmain()

1292 while(c−>started == 0)

1293 ;

1294 }

1295 }

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Sheet 12

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