xv6源码解析：现代x86多处理器上的Unix v6实现

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"xv6-rev6.pdf 是一个基于 Dennis Ritchie 和 Ken Thompson 的 Unix Version 6 (v6) 的简化实现，适用于现代 x86 多处理器系统，并使用 ANSI C 语言编写。该资源提供了 xv6 操作系统的源代码，具有行号提示，便于学习和理解操作系统的基本构造和原理。" xv6 是一个教学性质的操作系统，它旨在通过模仿经典的 Unix v6 版本，帮助学生和研究人员了解操作系统的核心概念。这个系统由 Frans Kaashoek、Robert Morris 和 Russ Cox 开发，并且受到 John Lions 的《Commentary on UNIX 6th Edition》以及 JOS、Plan9、FreeBSD 和 NetBSD 等开源项目的影响。在 xv6 的实现中，可以学习到以下关键知识点： 1. **进程管理**：xv6 实现了进程的概念，包括进程创建、调度、上下文切换等。这些功能在 `contextswitching` 和 `locking` 部分体现，由 Russ Cox 贡献。进程间通信（IPC）是操作系统中的重要部分，xv6 通过管道、信号量和共享内存等方式支持。 2. **内存管理**：xv6 使用虚拟内存系统，涉及页表和内存分配算法，如在 `mmu.h` 和 `mmu.c` 中实现。它还需要处理物理内存与虚拟内存的映射，以及内存的分配和释放。 3. **多处理器支持（MP）**：为了适应现代多核硬件，xv6 包含了多处理器支持的代码，这部分由 Cliff Frey、Xiao Yu 及其他贡献者实现。涉及的文件如 `mp.h`, `mp.c`, `lapic.c` 等，它们实现了处理器间通信、中断处理和同步机制。 4. **I/O 管理**：xv6 对 I/O 设备如 IDE 硬盘和串口进行管理，`ide.c` 和 `console.c` 文件处理输入输出操作。同时，`ioapic.c` 文件涉及高级 I/O 总线中断控制器，这是在多处理器系统中处理中断的关键。 5. **文件系统**：xv6 实现了一个简单的文件系统，支持文件的创建、读写和删除。这涉及到文件的 i 节点管理、目录结构和文件数据块的存储。 6. **系统调用接口**：xv6 提供了一套基本的系统调用，如 fork, exec, wait, open, read, write, close 等，它们是用户程序与操作系统交互的桥梁。 7. **异常和中断处理**：`entryother.S` 文件包含了异常和中断处理的初始设置，这在处理硬件故障、软件错误和其他突发事件时至关重要。 8. **锁和同步机制**：为了保证多线程环境下的数据一致性，xv6 使用了各种锁（如自旋锁、信号量）和同步原语，确保并发执行的安全性。除了这些核心组件，xv6 还包含对其他系统的兼容性和优化，如 Greg Price、Yandong Mao 和 Hitoshi Mitake 提交的补丁，进一步完善了这个教学操作系统。 xv6 是学习操作系统原理和实践的宝贵资源，其源代码清晰简洁，适合初学者理解和分析。通过阅读和研究 xv6，开发者可以深入理解操作系统设计的核心概念，并为构建自己的操作系统打下坚实基础。

Sep 5 23:39 2011 xv6/mmu.h Page 1

0700 // This file contains definitions for the

0701 // x86 memory management unit (MMU).

0702

0703 // Eflags register

0704 #define FL_CF 0x00000001 // Carry Flag

0705 #define FL_PF 0x00000004 // Parity Flag

0706 #define FL_AF 0x00000010 // Auxiliary carry Flag

0707 #define FL_ZF 0x00000040 // Zero Flag

0708 #define FL_SF 0x00000080 // Sign Flag

0709 #define FL_TF 0x00000100 // Trap Flag

0710 #define FL_IF 0x00000200 // Interrupt Enable

0711 #define FL_DF 0x00000400 // Direction Flag

0712 #define FL_OF 0x00000800 // Overflow Flag

0713 #define FL_IOPL_MASK 0x00003000 // I/O Privilege Level bitmask

0714 #define FL_IOPL_0 0x00000000 // IOPL == 0

0715 #define FL_IOPL_1 0x00001000 // IOPL == 1

0716 #define FL_IOPL_2 0x00002000 // IOPL == 2

0717 #define FL_IOPL_3 0x00003000 // IOPL == 3

0718 #define FL_NT 0x00004000 // Nested Task

0719 #define FL_RF 0x00010000 // Resume Flag

0720 #define FL_VM 0x00020000 // Virtual 8086 mode

0721 #define FL_AC 0x00040000 // Alignment Check

0722 #define FL_VIF 0x00080000 // Virtual Interrupt Flag

0723 #define FL_VIP 0x00100000 // Virtual Interrupt Pending

0724 #define FL_ID 0x00200000 // ID flag

0725

0726 // Control Register flags

0727 #define CR0_PE 0x00000001 // Protection Enable

0728 #define CR0_MP 0x00000002 // Monitor coProcessor

0729 #define CR0_EM 0x00000004 // Emulation

0730 #define CR0_TS 0x00000008 // Task Switched

0731 #define CR0_ET 0x00000010 // Extension Type

0732 #define CR0_NE 0x00000020 // Numeric Errror

0733 #define CR0_WP 0x00010000 // Write Protect

0734 #define CR0_AM 0x00040000 // Alignment Mask

0735 #define CR0_NW 0x20000000 // Not Writethrough

0736 #define CR0_CD 0x40000000 // Cache Disable

0737 #define CR0_PG 0x80000000 // Paging

0738

0739 #define CR4_PSE 0x00000010 // Page size extension

0740

0741 #define SEG_KCODE 1 // kernel code

0742 #define SEG_KDATA 2 // kernel data+stack

0743 #define SEG_KCPU 3 // kernel per−cpu data

0744 #define SEG_UCODE 4 // user code

0745 #define SEG_UDATA 5 // user data+stack

0746 #define SEG_TSS 6 // this process’s task state

0747

0748

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Sheet 07

Sep 5 23:39 2011 xv6/mmu.h Page 2

0750 #ifndef __ASSEMBLER__

0751 // Segment Descriptor

0752 struct segdesc {

0753 uint lim_15_0 : 16; // Low bits of segment limit

0754 uint base_15_0 : 16; // Low bits of segment base address

0755 uint base_23_16 : 8; // Middle bits of segment base address

0756 uint type : 4; // Segment type (see STS_ constants)

0757 uint s : 1; // 0 = system, 1 = application

0758 uint dpl : 2; // Descriptor Privilege Level

0759 uint p : 1; // Present

0760 uint lim_19_16 : 4; // High bits of segment limit

0761 uint avl : 1; // Unused (available for software use)

0762 uint rsv1 : 1; // Reserved

0763 uint db : 1; // 0 = 16−bit segment, 1 = 32−bit segment

0764 uint g : 1; // Granularity: limit scaled by 4K when set

0765 uint base_31_24 : 8; // High bits of segment base address

0766 };

0767

0768 // Normal segment

0769 #define SEG(type, base, lim, dpl) (struct segdesc) \

0770 { ((lim) >> 12) & 0xffff, (uint)(base) & 0xffff, \

0771 ((uint)(base) >> 16) & 0xff, type, 1, dpl, 1, \

0772 (uint)(lim) >> 28, 0, 0, 1, 1, (uint)(base) >> 24 }

0773 #define SEG16(type, base, lim, dpl) (struct segdesc) \

0774 { (lim) & 0xffff, (uint)(base) & 0xffff, \

0775 ((uint)(base) >> 16) & 0xff, type, 1, dpl, 1, \

0776 (uint)(lim) >> 16, 0, 0, 1, 0, (uint)(base) >> 24 }

0777 #endif

0778

0779 #define DPL_USER 0x3 // User DPL

0780

0781 // Application segment type bits

0782 #define STA_X 0x8 // Executable segment

0783 #define STA_E 0x4 // Expand down (non−executable segments)

0784 #define STA_C 0x4 // Conforming code segment (executable only)

0785 #define STA_W 0x2 // Writeable (non−executable segments)

0786 #define STA_R 0x2 // Readable (executable segments)

0787 #define STA_A 0x1 // Accessed

0788

0789 // System segment type bits

0790 #define STS_T16A 0x1 // Available 16−bit TSS

0791 #define STS_LDT 0x2 // Local Descriptor Table

0792 #define STS_T16B 0x3 // Busy 16−bit TSS

0793 #define STS_CG16 0x4 // 16−bit Call Gate

0794 #define STS_TG 0x5 // Task Gate / Coum Transmitions

0795 #define STS_IG16 0x6 // 16−bit Interrupt Gate

0796 #define STS_TG16 0x7 // 16−bit Trap Gate

0797 #define STS_T32A 0x9 // Available 32−bit TSS

0798 #define STS_T32B 0xB // Busy 32−bit TSS

0799 #define STS_CG32 0xC // 32−bit Call Gate

Sheet 07

Sep 5 23:39 2011 xv6/mmu.h Page 3

0800 #define STS_IG32 0xE // 32−bit Interrupt Gate

0801 #define STS_TG32 0xF // 32−bit Trap Gate

0802

0803 // A virtual address ’la’ has a three−part structure as follows:

0804 //

0805 // +−−−−−−−−10−−−−−−+−−−−−−−10−−−−−−−+−−−−−−−−−12−−−−−−−−−−+

0806 // | Page Directory | Page Table | Offset within Page |

0807 // | Index | Index | |

0808 // +−−−−−−−−−−−−−−−−+−−−−−−−−−−−−−−−−+−−−−−−−−−−−−−−−−−−−−−+

0809 // \−−− PDX(va) −−/ \−−− PTX(va) −−/

0810

0811 // page directory index

0812 #define PDX(va) (((uint)(va) >> PDXSHIFT) & 0x3FF)

0813

0814 // page table index

0815 #define PTX(va) (((uint)(va) >> PTXSHIFT) & 0x3FF)

0816

0817 // construct virtual address from indexes and offset

0818 #define PGADDR(d, t, o) ((uint)((d) << PDXSHIFT | (t) << PTXSHIFT | (o)))

0819

0820 // Page directory and page table constants.

0821 #define NPDENTRIES 1024 // # directory entries per page directory

0822 #define NPTENTRIES 1024 // # PTEs per page table

0823 #define PGSIZE 4096 // bytes mapped by a page

0824

0825 #define PGSHIFT 12 // log2(PGSIZE)

0826 #define PTXSHIFT 12 // offset of PTX in a linear address

0827 #define PDXSHIFT 22 // offset of PDX in a linear address

0828

0829 #define PGROUNDUP(sz) (((sz)+PGSIZE−1) & ~(PGSIZE−1))

0830 #define PGROUNDDOWN(a) (((a)) & ~(PGSIZE−1))

0831

0832 // Page table/directory entry flags.

0833 #define PTE_P 0x001 // Present

0834 #define PTE_W 0x002 // Writeable

0835 #define PTE_U 0x004 // User

0836 #define PTE_PWT 0x008 // Write−Through

0837 #define PTE_PCD 0x010 // Cache−Disable

0838 #define PTE_A 0x020 // Accessed

0839 #define PTE_D 0x040 // Dirty

0840 #define PTE_PS 0x080 // Page Size

0841 #define PTE_MBZ 0x180 // Bits must be zero

0842

0843 // Address in page table or page directory entry

0844 #define PTE_ADDR(pte) ((uint)(pte) & ~0xFFF)

0845

0846 #ifndef __ASSEMBLER__

0847 typedef uint pte_t;

0848

0849

Sheet 08

Sep 5 23:39 2011 xv6/mmu.h Page 4

0850 // Task state segment format

0851 struct taskstate {

0852 uint link; // Old ts selector

0853 uint esp0; // Stack pointers and segment selectors

0854 ushort ss0; // after an increase in privilege level

0855 ushort padding1;

0856 uint *esp1;

0857 ushort ss1;

0858 ushort padding2;

0859 uint *esp2;

0860 ushort ss2;

0861 ushort padding3;

0862 void *cr3; // Page directory base

0863 uint *eip; // Saved state from last task switch

0864 uint eflags;

0865 uint eax; // More saved state (registers)

0866 uint ecx;

0867 uint edx;

0868 uint ebx;

0869 uint *esp;

0870 uint *ebp;

0871 uint esi;

0872 uint edi;

0873 ushort es; // Even more saved state (segment selectors)

0874 ushort padding4;

0875 ushort cs;

0876 ushort padding5;

0877 ushort ss;

0878 ushort padding6;

0879 ushort ds;

0880 ushort padding7;

0881 ushort fs;

0882 ushort padding8;

0883 ushort gs;

0884 ushort padding9;

0885 ushort ldt;

0886 ushort padding10;

0887 ushort t; // Trap on task switch

0888 ushort iomb; // I/O map base address

0889 };

0890

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Sheet 08

Sep 5 23:39 2011 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

Sep 5 23:39 2011 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

Sep 5 23:39 2011 xv6/entry.S Page 1

1000 # Multiboot header, for multiboot boot loaders like GNU Grub.

1001 # http://www.gnu.org/software/grub/manual/multiboot/multiboot.html

1002 #

1003 # Using GRUB 2, you can boot xv6 from a file stored in a

1004 # Linux file system by copying kernel or kernelmemfs to /boot

1005 # and then adding this menu entry:

1006 #

1007 # menuentry "xv6" {

1008 # insmod ext2

1009 # set root=’(hd0,msdos1)’

1010 # set kernel=’/boot/kernel’

1011 # echo "Loading ${kernel}..."

1012 # multiboot ${kernel} ${kernel}

1013 # boot

1014 # }

1015

1016 #include "asm.h"

1017 #include "memlayout.h"

1018 #include "mmu.h"

1019 #include "param.h"

1020

1021 # Multiboot header. Data to direct multiboot loader.

1022 .p2align 2

1023 .text

1024 .globl multiboot_header

1025 multiboot_header:

1026 #define magic 0x1badb002

1027 #define flags 0

1028 .long magic

1029 .long flags

1030 .long (−magic−flags)

1031

1032 # By convention, the _start symbol specifies the ELF entry point.

1033 # Since we haven’t set up virtual memory yet, our entry point is

1034 # the physical address of ’entry’.

1035 .globl _start

1036 _start = V2P_WO(entry)

1037

1038 # Entering xv6 on boot processor. Machine is mostly set up.

1039 .globl entry

1040 entry:

1041 # Turn on page size extension for 4Mbyte pages

1042 movl %cr4, %eax

1043 orl $(CR4_PSE), %eax

1044 movl %eax, %cr4

1045 # Set page directory

1046 movl $(V2P_WO(entrypgdir)), %eax

1047 movl %eax, %cr3

1048 # Turn on paging.

1049 movl %cr0, %eax

Sheet 10

Sep 5 23:39 2011 xv6/entry.S Page 2

1050 orl $(CR0_PG|CR0_WP), %eax

1051 movl %eax, %cr0

1052

1053 # Set up the stack pointer.

1054 movl $(stack + KSTACKSIZE), %esp

1055

1056 # Jump to main(), and switch to executing at

1057 # high addresses. The indirect call is needed because

1058 # the assembler produces a PC−relative instruction

1059 # for a direct jump.

1060 mov $main, %eax

1061 jmp *%eax

1062

1063 .comm stack, KSTACKSIZE

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Sheet 10

Sep 5 23:39 2011 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 is identical to bootasm.S except:

1120 # − it does not need to enable A20

1121 # − it uses the address at start−4, start−8, and start−12

1122

1123 .code16

1124 .globl start

1125 start:

1126 cli

1127

1128 xorw %ax,%ax

1129 movw %ax,%ds

1130 movw %ax,%es

1131 movw %ax,%ss

1132

1133 lgdt gdtdesc

1134 movl %cr0, %eax

1135 orl $CR0_PE, %eax

1136 movl %eax, %cr0

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Sheet 11

Sep 5 23:39 2011 xv6/entryother.S Page 2

1150 ljmpl $(SEG_KCODE<<3), $(start32)

1151

1152 .code32

1153 start32:

1154 movw $(SEG_KDATA<<3), %ax

1155 movw %ax, %ds

1156 movw %ax, %es

1157 movw %ax, %ss

1158 movw $0, %ax

1159 movw %ax, %fs

1160 movw %ax, %gs

1161

1162 # Turn on page size extension for 4Mbyte pages

1163 movl %cr4, %eax

1164 orl $(CR4_PSE), %eax

1165 movl %eax, %cr4

1166 # Use enterpgdir as our initial page table

1167 movl (start−12), %eax

1168 movl %eax, %cr3

1169 # Turn on paging.

1170 movl %cr0, %eax

1171 orl $(CR0_PE|CR0_PG|CR0_WP), %eax

1172 movl %eax, %cr0

1173

1174 # Switch to the stack allocated by startothers()

1175 movl (start−4), %esp

1176 # Call mpenter()

1177 call *(start−8)

1178

1179 movw $0x8a00, %ax

1180 movw %ax, %dx

1181 outw %ax, %dx

1182 movw $0x8ae0, %ax

1183 outw %ax, %dx

1184 spin:

1185 jmp spin

1186

1187 .p2align 2

1188 gdt:

1189 SEG_NULLASM

1190 SEG_ASM(STA_X|STA_R, 0, 0xffffffff)

1191 SEG_ASM(STA_W, 0, 0xffffffff)

1192

1193

1194 gdtdesc:

1195 .word (gdtdesc − gdt − 1)

1196 .long gdt

1197

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1199

Sheet 11

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