FreeBSD架构手册：社区协作的系统详解

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《FreeBSD架构手册》是FreeBSD Documentation Project发起的一项重要项目，旨在为FreeBSD操作系统提供深入的系统架构详解。该手册于2000年8月首次发布，版权归属于2000年至2003年的FreeBSD Documentation Project团队。手册作为一项正在进行中的工作，其内容由众多个体贡献者共同构建，部分章节尚未完成，部分现有内容也需要更新。《FreeBSD架构手册》的核心价值在于帮助用户和开发者理解FreeBSD系统的内部构造，包括其设计原则、组件、系统调用接口、内存管理、进程管理和网络通信等方面。它对于深入了解FreeBSD的操作系统原理、优化系统性能以及进行系统级编程具有重要意义。手册强调了开源社区的合作精神，鼓励对文档项目的参与，有兴趣的读者可以通过邮件列表（<http://lists.FreeBSD.org/mailman/listinfo/freebsd-doc>）加入讨论和贡献。最新版本的《FreeBSD架构手册》始终可以从FreeBSD官方网站获取，地址是<../../../../index.html>。此外，手册还支持多种格式和压缩选项下载，用户可以从FTP服务器<ftp://ftp.FreeBSD.org/pub/FreeBSD/doc/>或众多镜像站点中选择适合自己的版本。值得注意的是，FreeBSD、UNIX、Sun、Sun Microsystems、SunOS、Solaris和Java等商标均为各自公司的注册商标。阅读和学习这本手册时，读者可以期待获得关于FreeBSD底层设计的详尽信息，这对于那些希望在FreeBSD环境中进行高级开发和系统管理的人员来说，是一份不可或缺的参考资料。同时，手册也展示了FreeBSD作为一个开放源代码操作系统的强大社区支持和持续改进的精神。

Chapter 1 Bootstrapping and kernel initialization

ret

/* now running relocated at KERNBASE where the system is linked to run */

begin:

The function init386() is called, with a pointer to the ﬁrst free physical page, after that mi_startup(). init386

is an architecture dependent initialization function, and mi_startup() is an architecture independent one (the ’mi_’

preﬁx stands for Machine Independent). The kernel never returns from mi_startup(), and by calling it, the kernel

ﬁnishes booting:

sys/i386/i386/locore.s:

movl physfree, %esi

pushl %esi /* value of first for init386(first) */

call _init386 /* wire 386 chip for unix operation */

call _mi_startup /* autoconfiguration, mountroot etc */

hlt /* never returns to here */

1.7.1 init386()

init386() is deﬁned in sys/i386/i386/machdep.c and performs low-level initialization, speciﬁc to the i386

chip. The switch to protected mode was performed by the loader. The loader has created the very ﬁrst task, in which

the kernel continues to operate. Before running straight away to the code, I will enumerate the tasks the processor

must complete to initialize protected mode execution:

• Initialize the kernel tunable parameters, passed from the bootstrapping program.

• Prepare the GDT.

• Prepare the IDT.

• Initialize the system console.

• Initialize the DDB, if it is compiled into kernel.

• Initialize the TSS.

• Prepare the LDT.

• Setup proc0’s pcb.

What init386() ﬁrst does is initialize the tunable parameters passed from bootstrap. This is done by setting the

environment pointer (envp) and calling init_param1(). The envp pointer has been passed from loader in the

bootinfo structure:

sys/i386/i386/machdep.c:

kern_envp = (caddr_t)bootinfo.bi_envp + KERNBASE;

/* Init basic tunables, hz etc */

init_param1();

init_param1() is deﬁned in sys/kern/subr_param.c. That ﬁle has a number of sysctls, and two functions,

init_param1() and init_param2(), that are called from init386():

sys/kern/subr_param.c

hz = HZ;

Chapter 1 Bootstrapping and kernel initialization

TUNABLE_INT_FETCH("kern.hz", &hz);

TUNABLE_<typename>_FETCH is used to fetch the value from the environment:

/usr/src/sys/sys/kernel.h

#define TUNABLE_INT_FETCH(path, var) getenv_int((path), (var))

Sysctl kern.hz is the system clock tick. Along with this, the following sysctls are set by init_param1():

kern.maxswzone, kern.maxbcache, kern.maxtsiz, kern.dfldsiz, kern.dflssiz,

kern.maxssiz, kern.sgrowsiz.

Then init386() prepares the Global Descriptors Table (GDT). Every task on an x86 is running in its own virtual

address space, and this space is addressed by a segment:offset pair. Say, for instance, the current instruction to be

executed by the processor lies at CS:EIP, then the linear virtual address for that instruction would be “the virtual

address of code segment CS” + EIP. For convenience, segments begin at virtual address 0 and end at a 4Gb boundary.

Therefore, the instruction’s linear virtual address for this example would just be the value of EIP. Segment registers

such as CS, DS etc are the selectors, i.e. indexes, into GDT (to be more precise, an index is not a selector itself, but

the INDEX ﬁeld of a selector). FreeBSD’s GDT holds descriptors for 15 selectors per CPU:

sys/i386/i386/machdep.c:

union descriptor gdt[NGDT * MAXCPU]; /* global descriptor table */

sys/i386/include/segments.h:

* Entries in the Global Descriptor Table (GDT)

#define GNULL_SEL 0 /* Null Descriptor */

#define GCODE_SEL 1 /* Kernel Code Descriptor */

#define GDATA_SEL 2 /* Kernel Data Descriptor */

#define GPRIV_SEL 3 /* SMP Per-Processor Private Data */

#define GPROC0_SEL 4 /* Task state process slot zero and up */

#define GLDT_SEL 5 /* LDT - eventually one per process */

#define GUSERLDT_SEL 6 /* User LDT */

#define GTGATE_SEL 7 /* Process task switch gate */

#define GBIOSLOWMEM_SEL 8 /* BIOS low memory access (must be entry 8) */

#define GPANIC_SEL 9 /* Task state to consider panic from */

#define GBIOSCODE32_SEL 10 /* BIOS interface (32bit Code) */

#define GBIOSCODE16_SEL 11 /* BIOS interface (16bit Code) */

#define GBIOSDATA_SEL 12 /* BIOS interface (Data) */

#define GBIOSUTIL_SEL 13 /* BIOS interface (Utility) */

#define GBIOSARGS_SEL 14 /* BIOS interface (Arguments) */

Note that those #deﬁnes are not selectors themselves, but just a ﬁeld INDEX of a selector, so they are exactly the

indices of the GDT. for example, an actual selector for the kernel code (GCODE_SEL) has the value 0x08.

The next step is to initialize the Interrupt Descriptor Table (IDT). This table is to be referenced by the processor

when a software or hardware interrupt occurs. For example, to make a system call, user application issues the INT

0x80 instruction. This is a software interrupt, so the processor’s hardware looks up a record with index 0x80 in the

IDT. This record points to the routine that handles this interrupt, in this particular case, this will be the kernel’s

syscall gate. The IDT may have a maximum of 256 (0x100) records. The kernel allocates NIDT records for the IDT,

where NIDT is the maximum (256):

sys/i386/i386/machdep.c:

Chapter 1 Bootstrapping and kernel initialization

#endif

#define TEXT_SET(set, sym) MAKE_SET(set, sym)

#define DATA_SET(set, sym) MAKE_SET(set, sym)

In our case, the following declaration will occur:

static struct sysinit announce_sys_init = {

SI_SUB_COPYRIGHT,

SI_ORDER_FIRST,

(sysinit_cfunc_t)(sysinit_nfunc_t) print_caddr_t,

(void *) copyright

};

static void const *const __set_sysinit_set_sym_announce_sys_init =

&announce_sys_init;

__asm(".section .set.sysinit_set" ",\"aw\"");

__asm(".long " "announce_sys_init");

__asm(".previous");

The ﬁrst __asm instruction will create an ELF section within the kernel’s executable. This will happen at kernel link

time. The section will have the name .set.sysinit_set. The content of this section is one 32-bit value, the

address of announce_sys_init structure, and that is what the second __asm is. The third __asm instruction marks the

end of a section. If a directive with the same section name occurred before, the content, i.e. the 32-bit value, will be

appended to the existing section, so forming an array of 32-bit pointers.

Running objdump on a kernel binary, you may notice the presence of such small sections:

% objdump -h /kernel

7 .set.cons_set 00000014 c03164c0 c03164c0 002154c0 2**2

CONTENTS, ALLOC, LOAD, DATA

8 .set.kbddriver_set 00000010 c03164d4 c03164d4 002154d4 2**2

CONTENTS, ALLOC, LOAD, DATA

9 .set.scrndr_set 00000024 c03164e4 c03164e4 002154e4 2**2

CONTENTS, ALLOC, LOAD, DATA

10 .set.scterm_set 0000000c c0316508 c0316508 00215508 2**2

CONTENTS, ALLOC, LOAD, DATA

11 .set.sysctl_set 0000097c c0316514 c0316514 00215514 2**2

CONTENTS, ALLOC, LOAD, DATA

12 .set.sysinit_set 00000664 c0316e90 c0316e90 00215e90 2**2

CONTENTS, ALLOC, LOAD, DATA

This screen dump shows that the size of .set.sysinit_set section is 0x664 bytes, so 0x664/sizeof(void *) sysinit

objects are compiled into the kernel. The other sections such as .set.sysctl_set represent other linker sets.

By deﬁning a variable of type struct linker_set the content of .set.sysinit_set section will be “collected”

into that variable:

sys/kern/init_main.c:

extern struct linker_set sysinit_set; /* XXX */

The struct linker_set is deﬁned as follows:

/usr/include/linker_set.h:

struct linker_set {

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