<< Back to man.ChinaUnix.net

1.7 Kernel initialization

To where exactly is the execution passed by the loader, i.e. what is the kernel's actual entry point. Let us take a look at the command that links the kernel:

ld -elf -Bdynamic -T /usr/src/sys/conf/ldscript.i386  -export-dynamic \
-dynamic-linker /red/herring -o kernel -X locore.o \
<lots of kernel .o files>

A few interesting things can be seen in this line. First, the kernel is an ELF dynamically linked binary, but the dynamic linker for kernel is /red/herring, which is definitely a bogus file. Second, taking a look at the file sys/conf/ldscript.i386 gives an idea about what ld options are used when compiling a kernel. Reading through the first few lines, the string


says that a kernel's entry point is the symbol `btext'. This symbol is defined in locore.s:

 * This is where the bootblocks start us, set the ball rolling...

First what is done is the register EFLAGS is set to a predefined value of 0x00000002, and then all the segment registers are initialized:

/* Don't trust what the BIOS gives for eflags. */
    pushl   $PSL_KERNEL

 * Don't trust what the BIOS gives for %fs and %gs.  Trust the bootstrap
 * to set %cs, %ds, %es and %ss.
    mov %ds, %ax
    mov %ax, %fs
    mov %ax, %gs

btext calls the routines recover_bootinfo(), identify_cpu(), create_pagetables(), which are also defined in locore.s. Here is a description of what they do:

recover_bootinfo This routine parses the parameters to the kernel passed from the bootstrap. The kernel may have been booted in 3 ways: by the loader, described above, by the old disk boot blocks, and by the old diskless boot procedure. This function determines the booting method, and stores the struct bootinfo structure into the kernel memory.
identify_cpu This functions tries to find out what CPU it is running on, storing the value found in a variable _cpu.
create_pagetables This function allocates and fills out a Page Table Directory at the top of the kernel memory area.

The next steps are enabling VME, if the CPU supports it:

   testl   $CPUID_VME, R(_cpu_feature)
    jz  1f
    movl    %cr4, %eax
    orl $CR4_VME, %eax
    movl    %eax, %cr4

Then, enabling paging:

/* Now enable paging */
    movl    R(_IdlePTD), %eax
    movl    %eax,%cr3           /* load ptd addr into mmu */
    movl    %cr0,%eax           /* get control word */
    orl $CR0_PE|CR0_PG,%eax     /* enable paging */
    movl    %eax,%cr0           /* and let's page NOW! */

The next three lines of code are because the paging was set, so the jump is needed to continue the execution in virtualized address space:

   pushl   $begin              /* jump to high virtualized address */

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

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:

    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:

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:

        kern_envp = (caddr_t)bootinfo.bi_envp + KERNBASE;

    /* Init basic tunables, hz etc */

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():

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

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

#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:

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

 * 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):

static struct gate_descriptor idt0[NIDT];
struct gate_descriptor *idt = &idt0[0]; /* interrupt descriptor table */

For each interrupt, an appropriate handler is set. The syscall gate for INT 0x80 is set as well:

    setidt(0x80, &IDTVEC(int0x80_syscall),

So when a userland application issues the INT 0x80 instruction, control will transfer to the function _Xint0x80_syscall, which is in the kernel code segment and will be executed with supervisor privileges.

Console and DDB are then initialized:

/* skipped */
#ifdef DDB
    if (boothowto & RB_KDB)
        Debugger("Boot flags requested debugger");

The Task State Segment is another x86 protected mode structure, the TSS is used by the hardware to store task information when a task switch occurs.

The Local Descriptors Table is used to reference userland code and data. Several selectors are defined to point to the LDT, they are the system call gates and the user code and data selectors:

#define LSYS5CALLS_SEL  0   /* forced by intel BCS */
#define LSYS5SIGR_SEL   1
#define L43BSDCALLS_SEL 2   /* notyet */
#define LUCODE_SEL  3
#define LSOL26CALLS_SEL 4   /* Solaris >= 2.6 system call gate */
#define LUDATA_SEL  5
/* separate stack, es,fs,gs sels ? */
/* #define  LPOSIXCALLS_SEL 5*/ /* notyet */
#define LBSDICALLS_SEL  16  /* BSDI system call gate */
#define NLDT        (LBSDICALLS_SEL + 1)

Next, proc0's Process Control Block (struct pcb) structure is initialized. proc0 is a struct proc structure that describes a kernel process. It is always present while the kernel is running, therefore it is declared as global:

    struct  proc proc0;

The structure struct pcb is a part of a proc structure. It is defined in /usr/include/machine/pcb.h and has a process's information specific to the i386 architecture, such as registers values.

1.7.2 mi_startup()

This function performs a bubble sort of all the system initialization objects and then calls the entry of each object one by one:

    for (sipp = sysinit; *sipp; sipp++) {

        /* ... skipped ... */

        /* Call function */
        /* ... skipped ... */

Although the sysinit framework is described in the Developers' Handbook, I will discuss the internals of it.

Every system initialization object (sysinit object) is created by calling a SYSINIT() macro. Let us take as example an announce sysinit object. This object prints the copyright message:

static void
print_caddr_t(void *data __unused)
    printf("%s", (char *)data);
SYSINIT(announce, SI_SUB_COPYRIGHT, SI_ORDER_FIRST, print_caddr_t, copyright)

The subsystem ID for this object is SI_SUB_COPYRIGHT (0x0800001), which comes right after the SI_SUB_CONSOLE (0x0800000). So, the copyright message will be printed out first, just after the console initialization.

Let us take a look at what exactly the macro SYSINIT() does. It expands to a C_SYSINIT() macro. The C_SYSINIT() macro then expands to a static struct sysinit structure declaration with another DATA_SET macro call:

      #define C_SYSINIT(uniquifier, subsystem, order, func, ident) \
      static struct sysinit uniquifier ## _sys_init = { \ subsystem, \
      order, \ func, \ ident \ }; \ DATA_SET(sysinit_set,uniquifier ##

#define SYSINIT(uniquifier, subsystem, order, func, ident)  \
    C_SYSINIT(uniquifier, subsystem, order,         \
    (sysinit_cfunc_t)(sysinit_nfunc_t)func, (void *)ident)

The DATA_SET() macro expands to a MAKE_SET(), and that macro is the point where the all sysinit magic is hidden:

#define MAKE_SET(set, sym)                      \
    static void const * const __set_##set##_sym_##sym = &sym;   \
    __asm(".section .set." #set ",\"aw\"");             \
    __asm(".long " #sym);                       \
#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 = {
    (sysinit_cfunc_t)(sysinit_nfunc_t)  print_caddr_t,
    (void *) copyright

static void const *const __set_sysinit_set_sym_announce_sys_init =
__asm(".section .set.sysinit_set" ",\"aw\"");
__asm(".long " "announce_sys_init");

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:

      extern struct linker_set sysinit_set; /* XXX */

The struct linker_set is defined as follows:

  struct linker_set {
    int ls_length;
    void    *ls_items[1];       /* really ls_length of them, trailing NULL */

The first node will be equal to the number of a sysinit objects, and the second node will be a NULL-terminated array of pointers to them.

Returning to the mi_startup() discussion, it is must be clear now, how the sysinit objects are being organized. The mi_startup() function sorts them and calls each. The very last object is the system scheduler:

enum sysinit_sub_id {
    SI_SUB_DUMMY        = 0x0000000,    /* not executed; for linker*/
    SI_SUB_DONE     = 0x0000001,    /* processed*/
    SI_SUB_CONSOLE      = 0x0800000,    /* console*/
    SI_SUB_COPYRIGHT    = 0x0800001,    /* first use of console*/
    SI_SUB_RUN_SCHEDULER    = 0xfffffff /* scheduler: no return*/

The system scheduler sysinit object is defined in the file sys/vm/vm_glue.c, and the entry point for that object is scheduler(). That function is actually an infinite loop, and it represents a process with PID 0, the swapper process. The proc0 structure, mentioned before, is used to describe it.

The first user process, called init, is created by the sysinit object init:

static void
create_init(const void *udata __unused)
    int error;
    int s;

    s = splhigh();
    error = fork1(&proc0, RFFDG | RFPROC, &initproc);
    if (error)
        panic("cannot fork init: %d\n", error);
    initproc->p_flag |= P_INMEM | P_SYSTEM;
    cpu_set_fork_handler(initproc, start_init, NULL);

The create_init() allocates a new process by calling fork1(), but does not mark it runnable. When this new process is scheduled for execution by the scheduler, the start_init() will be called. That function is defined in init_main.c. It tries to load and exec the init binary, probing /sbin/init first, then /sbin/oinit, /sbin/init.bak, and finally /stand/sysinstall:

static char init_path[MAXPATHLEN] =
#ifdef  INIT_PATH

This, and other documents, can be downloaded from ftp://ftp.FreeBSD.org/pub/FreeBSD/doc/.

For questions about FreeBSD, read the documentation before contacting <questions@FreeBSD.org>.
For questions about this documentation, e-mail <doc@FreeBSD.org>.