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Kernel booting process. Part 2.

First steps in the kernel setup

We started to dive into linux kernel insides in the previous part and saw the initial part of the kernel setup code. We stopped at the first call to the main function (which is the first function written in C) from arch/x86/boot/main.c.

In this part we will continue to research the kernel setup code and

  • see what protected mode is,
  • some preparation for the transition into it,
  • the heap and console initialization,
  • memory detection, cpu validation, keyboard initialization
  • and much much more.

So, Let's go ahead.

Protected mode

Before we can move to the native Intel64 Long Mode, the kernel must switch the CPU into protected mode.

What is protected mode? Protected mode was first added to the x86 architecture in 1982 and was the main mode of Intel processors from the 80286 processor until Intel 64 and long mode came.

The main reason to move away from Real mode is that there is very limited access to the RAM. As you may remember from the previous part, there is only 220 bytes or 1 Megabyte, sometimes even only 640 Kilobytes of RAM available in the Real mode.

Protected mode brought many changes, but the main one is the difference in memory management. The 20-bit address bus was replaced with a 32-bit address bus. It allowed access to 4 Gigabytes of memory vs 1 Megabyte of real mode. Also paging support was added, which you can read about in the next sections.

Memory management in Protected mode is divided into two, almost independent parts:

  • Segmentation
  • Paging

Here we will only see segmentation. Paging will be discussed in the next sections.

As you can read in the previous part, addresses consist of two parts in real mode:

  • Base address of the segment
  • Offset from the segment base

And we can get the physical address if we know these two parts by:

PhysicalAddress = Segment Selector * 16 + Offset

Memory segmentation was completely redone in protected mode. There are no 64 Kilobyte fixed-size segments. Instead, the size and location of each segment is described by an associated data structure called Segment Descriptor. The segment descriptors are stored in a data structure called Global Descriptor Table (GDT).

The GDT is a structure which resides in memory. It has no fixed place in the memory so, its address is stored in the special GDTR register. Later we will see the GDT loading in the Linux kernel code. There will be an operation for loading it into memory, something like:

lgdt gdt

where the lgdt instruction loads the base address and limit(size) of global descriptor table to the GDTR register. GDTR is a 48-bit register and consists of two parts:

  • size(16-bit) of global descriptor table;
  • address(32-bit) of the global descriptor table.

As mentioned above the GDT contains segment descriptors which describe memory segments. Each descriptor is 64-bits in size. The general scheme of a descriptor is:

31          24        19      16              7            0
------------------------------------------------------------
|             | |B| |A|       | |   | |0|E|W|A|            |
| BASE 31:24  |G|/|L|V| LIMIT |P|DPL|S|  TYPE | BASE 23:16 | 4
|             | |D| |L| 19:16 | |   | |1|C|R|A|            |
------------------------------------------------------------
|                             |                            |
|        BASE 15:0            |       LIMIT 15:0           | 0
|                             |                            |
------------------------------------------------------------

Don't worry, I know it looks a little scary after real mode, but it's easy. For example LIMIT 15:0 means that bit 0-15 of the Descriptor contain the value for the limit. The rest of it is in LIMIT 19:16. So, the size of Limit is 0-19 i.e 20-bits. Let's take a closer look at it:

  1. Limit[20-bits] is at 0-15,16-19 bits. It defines length_of_segment - 1. It depends on G(Granularity) bit.
  • if G (bit 55) is 0 and segment limit is 0, the size of the segment is 1 Byte
  • if G is 1 and segment limit is 0, the size of the segment is 4096 Bytes
  • if G is 0 and segment limit is 0xfffff, the size of the segment is 1 Megabyte
  • if G is 1 and segment limit is 0xfffff, the size of the segment is 4 Gigabytes

So, it means that if

  • if G is 0, Limit is interpreted in terms of 1 Byte and the maximum size of the segment can be 1 Megabyte.
  • if G is 1, Limit is interpreted in terms of 4096 Bytes = 4 KBytes = 1 Page and the maximum size of the segment can be 4 Gigabytes. Actually when G is 1, the value of Limit is shifted to the left by 12 bits. So, 20 bits + 12 bits = 32 bits and 232 = 4 Gigabytes.

  1. Base[32-bits] is at (0-15, 32-39 and 56-63 bits). It defines the physical address of the segment's starting location.

  2. Type/Attribute (40-47 bits) defines the type of segment and kinds of access to it.

  • S flag at bit 44 specifies descriptor type. If S is 0 then this segment is a system segment, whereas if S is 1 then this is a code or data segment (Stack segments are data segments which must be read/write segments).

To determine if the segment is a code or data segment we can check its Ex(bit 43) Attribute marked as 0 in the above diagram. If it is 0, then the segment is a Data segment otherwise it is a code segment.

A segment can be of one of the following types:

|           Type Field        | Descriptor Type | Description
|-----------------------------|-----------------|------------------
| Decimal                     |                 |
|             0    E    W   A |                 |
| 0           0    0    0   0 | Data            | Read-Only
| 1           0    0    0   1 | Data            | Read-Only, accessed
| 2           0    0    1   0 | Data            | Read/Write
| 3           0    0    1   1 | Data            | Read/Write, accessed
| 4           0    1    0   0 | Data            | Read-Only, expand-down
| 5           0    1    0   1 | Data            | Read-Only, expand-down, accessed
| 6           0    1    1   0 | Data            | Read/Write, expand-down
| 7           0    1    1   1 | Data            | Read/Write, expand-down, accessed
|                  C    R   A |                 |
| 8           1    0    0   0 | Code            | Execute-Only
| 9           1    0    0   1 | Code            | Execute-Only, accessed
| 10          1    0    1   0 | Code            | Execute/Read
| 11          1    0    1   1 | Code            | Execute/Read, accessed
| 12          1    1    0   0 | Code            | Execute-Only, conforming
| 14          1    1    0   1 | Code            | Execute-Only, conforming, accessed
| 13          1    1    1   0 | Code            | Execute/Read, conforming
| 15          1    1    1   1 | Code            | Execute/Read, conforming, accessed

As we can see the first bit(bit 43) is 0 for a data segment and 1 for a code segment. The next three bits(40, 41, 42, 43) are either EWA(Expansion Writable Accessible) or CRA(Conforming Readable Accessible).

  • if E(bit 42) is 0, expand up other wise expand down. Read more here.
  • if W(bit 41)(for Data Segments) is 1, write access is allowed otherwise not. Note that read access is always allowed on data segments.
  • A(bit 40) - Whether the segment is accessed by processor or not.
  • C(bit 43) is conforming bit(for code selectors). If C is 1, the segment code can be executed from a lower level privilege e.g. user level. If C is 0, it can only be executed from the same privilege level.
  • R(bit 41)(for code segments). If 1 read access to segment is allowed otherwise not. Write access is never allowed to code segments.

  1. DPL[2-bits] (Descriptor Privilege Level) is at bits 45-46. It defines the privilege level of the segment. It can be 0-3 where 0 is the most privileged.

  2. P flag(bit 47) - indicates if the segment is present in memory or not. If P is 0, the segment will be presented as invalid and the processor will refuse to read this segment.

  3. AVL flag(bit 52) - Available and reserved bits. It is ignored in Linux.

  4. L flag(bit 53) - indicates whether a code segment contains native 64-bit code. If 1 then the code segment executes in 64 bit mode.

  5. D/B flag(bit 54) - Default/Big flag represents the operand size i.e 16/32 bits. If it is set then 32 bit otherwise 16.

Segment registers contain segment selectors as in real mode. However, in protected mode, a segment selector is handled differently. Each Segment Descriptor has an associated Segment Selector which is a 16-bit structure:

15              3  2   1  0
-----------------------------
|      Index     | TI | RPL |
-----------------------------

Where,

  • Index shows the index number of the descriptor in the GDT.
  • TI(Table Indicator) shows where to search for the descriptor. If it is 0 then search in the Global Descriptor Table(GDT) otherwise it will look in Local Descriptor Table(LDT).
  • And RPL is Requester's Privilege Level.

Every segment register has a visible and hidden part.

  • Visible - Segment Selector is stored here
  • Hidden - Segment Descriptor(base, limit, attributes, flags)

The following steps are needed to get the physical address in the protected mode:

  • The segment selector must be loaded in one of the segment registers
  • The CPU tries to find a segment descriptor by GDT address + Index from selector and load the descriptor into the hidden part of the segment register
  • Base address (from segment descriptor) + offset will be the linear address of the segment which is the physical address (if paging is disabled).

Schematically it will look like this:

linear address

The algorithm for the transition from real mode into protected mode is:

  • Disable interrupts
  • Describe and load GDT with lgdt instruction
  • Set PE (Protection Enable) bit in CR0 (Control Register 0)
  • Jump to protected mode code

We will see the complete transition to protected mode in the linux kernel in the next part, but before we can move to protected mode, we need to do some more preparations.

Let's look at arch/x86/boot/main.c. We can see some routines there which perform keyboard initialization, heap initialization, etc... Let's take a look.

Copying boot parameters into the "zeropage"

We will start from the main routine in "main.c". First function which is called in main is copy_boot_params(void). It copies the kernel setup header into the field of the boot_params structure which is defined in the arch/x86/include/uapi/asm/bootparam.h.

The boot_params structure contains the struct setup_header hdr field. This structure contains the same fields as defined in linux boot protocol and is filled by the boot loader and also at kernel compile/build time. copy_boot_params does two things:

  1. Copies hdr from header.S to the boot_params structure in setup_header field

  2. Updates pointer to the kernel command line if the kernel was loaded with the old command line protocol.

Note that it copies hdr with memcpy function which is defined in the copy.S source file. Let's have a look inside:

GLOBAL(memcpy)
    pushw   %si
    pushw   %di
    movw    %ax, %di
    movw    %dx, %si
    pushw   %cx
    shrw    $2, %cx
    rep; movsl
    popw    %cx
    andw    $3, %cx
    rep; movsb
    popw    %di
    popw    %si
    retl
ENDPROC(memcpy)

Yeah, we just moved to C code and now assembly again :) First of all we can see that memcpy and other routines which are defined here, start and end with the two macros: GLOBAL and ENDPROC. GLOBAL is described in arch/x86/include/asm/linkage.h which defines globl directive and the label for it. ENDPROC is described in include/linux/linkage.h which marks the name symbol as a function name and ends with the size of the name symbol.

Implementation of memcpy is easy. At first, it pushes values from the si and di registers to the stack to preserve their values because they will change during the memcpy. memcpy (and other functions in copy.S) use fastcall calling conventions. So it gets its incoming parameters from the ax, dx and cx registers. Calling memcpy looks like this:

memcpy(&boot_params.hdr, &hdr, sizeof hdr);

So,

  • ax will contain the address of the boot_params.hdr
  • dx will contain the address of hdr
  • cx will contain the size of hdr in bytes.

memcpy puts the address of boot_params.hdr into si and saves the size on the stack. After this it shifts to the right on 2 size (or divide on 4) and copies from si to di by 4 bytes. After this we restore the size of hdr again, align it by 4 bytes and copy the rest of the bytes from si to di byte by byte (if there is more). Restore si and di values from the stack in the end and after this copying is finished.

Console initialization

After hdr is copied into boot_params.hdr, the next step is console initialization by calling the console_init function which is defined in arch/x86/boot/early_serial_console.c.

It tries to find the earlyprintk option in the command line and if the search was successful, it parses the port address and baud rate of the serial port and initializes the serial port. Value of earlyprintk command line option can be one of these:

  • serial,0x3f8,115200
  • serial,ttyS0,115200
  • ttyS0,115200

After serial port initialization we can see the first output:

if (cmdline_find_option_bool("debug"))
    puts("early console in setup code\n");

The definition of puts is in tty.c. As we can see it prints character by character in a loop by calling the putchar function. Let's look into the putchar implementation:

void __attribute__((section(".inittext"))) putchar(int ch)
{
    if (ch == '\n')
        putchar('\r');

    bios_putchar(ch);

    if (early_serial_base != 0)
        serial_putchar(ch);
}

__attribute__((section(".inittext"))) means that this code will be in the .inittext section. We can find it in the linker file setup.ld.

First of all, putchar checks for the \n symbol and if it is found, prints \r before. After that it outputs the character on the VGA screen by calling the BIOS with the 0x10 interrupt call:

static void __attribute__((section(".inittext"))) bios_putchar(int ch)
{
    struct biosregs ireg;

    initregs(&ireg);
    ireg.bx = 0x0007;
    ireg.cx = 0x0001;
    ireg.ah = 0x0e;
    ireg.al = ch;
    intcall(0x10, &ireg, NULL);
}

Here initregs takes the biosregs structure and first fills biosregs with zeros using the memset function and then fills it with register values.

    memset(reg, 0, sizeof *reg);
    reg->eflags |= X86_EFLAGS_CF;
    reg->ds = ds();
    reg->es = ds();
    reg->fs = fs();
    reg->gs = gs();

Let's look at the memset implementation:

GLOBAL(memset)
    pushw   %di
    movw    %ax, %di
    movzbl  %dl, %eax
    imull   $0x01010101,%eax
    pushw   %cx
    shrw    $2, %cx
    rep; stosl
    popw    %cx
    andw    $3, %cx
    rep; stosb
    popw    %di
    retl
ENDPROC(memset)

As you can read above, it uses the fastcall calling conventions like the memcpy function, which means that the function gets parameters from ax, dx and cx registers.

Generally memset is like a memcpy implementation. It saves the value of the di register on the stack and puts the ax value into di which is the address of the biosregs structure. Next is the movzbl instruction, which copies the dl value to the low 2 bytes of the eax register. The remaining 2 high bytes of eax will be filled with zeros.

The next instruction multiplies eax with 0x01010101. It needs to because memset will copy 4 bytes at the same time. For example, we need to fill a structure with 0x7 with memset. eax will contain 0x00000007 value in this case. So if we multiply eax with 0x01010101, we will get 0x07070707 and now we can copy these 4 bytes into the structure. memset uses rep; stosl instructions for copying eax into es:di.

The rest of the memset function does almost the same as memcpy.

After the biosregs structure is filled with memset, bios_putchar calls the 0x10 interrupt which prints a character. Afterwards it checks if the serial port was initialized or not and writes a character there with serial_putchar and inb/outb instructions if it was set.

Heap initialization

After the stack and bss section were prepared in header.S (see previous part), the kernel needs to initialize the heap with the init_heap function.

First of all init_heap checks the CAN_USE_HEAP flag from the loadflags in the kernel setup header and calculates the end of the stack if this flag was set:

    char *stack_end;

    if (boot_params.hdr.loadflags & CAN_USE_HEAP) {
        asm("leal %P1(%%esp),%0"
            : "=r" (stack_end) : "i" (-STACK_SIZE));

or in other words stack_end = esp - STACK_SIZE.

Then there is the heap_end calculation:

    heap_end = (char *)((size_t)boot_params.hdr.heap_end_ptr + 0x200);

which means heap_end_ptr or _end + 512(0x200h). The last check is whether heap_end is greater than stack_end. If it is then stack_end is assigned to heap_end to make them equal.

Now the heap is initialized and we can use it using the GET_HEAP method. We will see how it is used, how to use it and how the it is implemented in the next posts.

CPU validation

The next step as we can see is cpu validation by validate_cpu from arch/x86/boot/cpu.c.

It calls the check_cpu function and passes cpu level and required cpu level to it and checks that the kernel launches on the right cpu level.

check_cpu(&cpu_level, &req_level, &err_flags);
if (cpu_level < req_level) {
    ...
    return -1;
}

check_cpu checks the cpu's flags, presence of long mode in case of x86_64(64-bit) CPU, checks the processor's vendor and makes preparation for certain vendors like turning off SSE+SSE2 for AMD if they are missing, etc.

Memory detection

The next step is memory detection by the detect_memory function. detect_memory basically provides a map of available RAM to the cpu. It uses different programming interfaces for memory detection like 0xe820, 0xe801 and 0x88. We will see only the implementation of 0xE820 here.

Let's look into the detect_memory_e820 implementation from the arch/x86/boot/memory.c source file. First of all, the detect_memory_e820 function initializes the biosregs structure as we saw above and fills registers with special values for the 0xe820 call:

    initregs(&ireg);
    ireg.ax  = 0xe820;
    ireg.cx  = sizeof buf;
    ireg.edx = SMAP;
    ireg.di  = (size_t)&buf;
  • ax contains the number of the function (0xe820 in our case)
  • cx register contains size of the buffer which will contain data about memory
  • edx must contain the SMAP magic number
  • es:di must contain the address of the buffer which will contain memory data
  • ebx has to be zero.

Next is a loop where data about the memory will be collected. It starts from the call of the 0x15 BIOS interrupt, which writes one line from the address allocation table. For getting the next line we need to call this interrupt again (which we do in the loop). Before the next call ebx must contain the value returned previously:

    intcall(0x15, &ireg, &oreg);
    ireg.ebx = oreg.ebx;

Ultimately, it does iterations in the loop to collect data from the address allocation table and writes this data into the e820entry array:

  • start of memory segment
  • size of memory segment
  • type of memory segment (which can be reserved, usable and etc...).

You can see the result of this in the dmesg output, something like:

[    0.000000] e820: BIOS-provided physical RAM map:
[    0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
[    0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000000f0000-0x00000000000fffff] reserved
[    0.000000] BIOS-e820: [mem 0x0000000000100000-0x000000003ffdffff] usable
[    0.000000] BIOS-e820: [mem 0x000000003ffe0000-0x000000003fffffff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000fffc0000-0x00000000ffffffff] reserved

Keyboard initialization

The next step is the initialization of the keyboard with the call of the keyboard_init() function. At first keyboard_init initializes registers using the initregs function and calling the 0x16 interrupt for getting the keyboard status.

    initregs(&ireg);
    ireg.ah = 0x02;     /* Get keyboard status */
    intcall(0x16, &ireg, &oreg);
    boot_params.kbd_status = oreg.al;

After this it calls 0x16 again to set repeat rate and delay.

    ireg.ax = 0x0305;   /* Set keyboard repeat rate */
    intcall(0x16, &ireg, NULL);

Querying

The next couple of steps are queries for different parameters. We will not dive into details about these queries, but will get back to it in later parts. Let's take a short look at these functions:

The query_mca routine calls the 0x15 BIOS interrupt to get the machine model number, sub-model number, BIOS revision level, and other hardware-specific attributes:

int query_mca(void)
{
    struct biosregs ireg, oreg;
    u16 len;

    initregs(&ireg);
    ireg.ah = 0xc0;
    intcall(0x15, &ireg, &oreg);

    if (oreg.eflags & X86_EFLAGS_CF)
        return -1;  /* No MCA present */

    set_fs(oreg.es);
    len = rdfs16(oreg.bx);

    if (len > sizeof(boot_params.sys_desc_table))
        len = sizeof(boot_params.sys_desc_table);

    copy_from_fs(&boot_params.sys_desc_table, oreg.bx, len);
    return 0;
}

It fills the ah register with 0xc0 and calls the 0x15 BIOS interruption. After the interrupt execution it checks the carry flag and if it is set to 1, the BIOS doesn't support (MCA)[https://en.wikipedia.org/wiki/Micro_Channel_architecture]. If carry flag is set to 0, ES:BX will contain a pointer to the system information table, which looks like this:

Offset  Size    Description
 00h    WORD    number of bytes following
 02h    BYTE    model (see #00515)
 03h    BYTE    submodel (see #00515)
 04h    BYTE    BIOS revision: 0 for first release, 1 for 2nd, etc.
 05h    BYTE    feature byte 1 (see #00510)
 06h    BYTE    feature byte 2 (see #00511)
 07h    BYTE    feature byte 3 (see #00512)
 08h    BYTE    feature byte 4 (see #00513)
 09h    BYTE    feature byte 5 (see #00514)
---AWARD BIOS---
 0Ah  N BYTEs   AWARD copyright notice
---Phoenix BIOS---
 0Ah    BYTE    ??? (00h)
 0Bh    BYTE    major version
 0Ch    BYTE    minor version (BCD)
 0Dh  4 BYTEs   ASCIZ string "PTL" (Phoenix Technologies Ltd)
---Quadram Quad386---
 0Ah 17 BYTEs   ASCII signature string "Quadram Quad386XT"
---Toshiba (Satellite Pro 435CDS at least)---
 0Ah  7 BYTEs   signature "TOSHIBA"
 11h    BYTE    ??? (8h)
 12h    BYTE    ??? (E7h) product ID??? (guess)
 13h  3 BYTEs   "JPN"

Next we call the set_fs routine and pass the value of the es register to it. The implementation of set_fs is pretty simple:

static inline void set_fs(u16 seg)
{
    asm volatile("movw %0,%%fs" : : "rm" (seg));
}

This function contains inline assembly which gets the value of the seg parameter and puts it into the fs register. There are many functions in boot.h like set_fs, for example set_gs, fs, gs for reading a value in it etc...

At the end of query_mca it just copies the table pointed to by es:bx to the boot_params.sys_desc_table.

The next step is getting Intel SpeedStep information by calling the query_ist function. First of all it checks the CPU level and if it is correct, calls 0x15 for getting info and saves the result to boot_params.

The following query_apm_bios function gets Advanced Power Management information from the BIOS. query_apm_bios calls the 0x15 BIOS interruption too, but with ah = 0x53 to check APM installation. After the 0x15 execution, query_apm_bios functions check the PM signature (it must be 0x504d), carry flag (it must be 0 if APM supported) and value of the cx register (if it's 0x02, protected mode interface is supported).

Next it calls 0x15 again, but with ax = 0x5304 for disconnecting the APM interface and connecting the 32-bit protected mode interface. In the end it fills boot_params.apm_bios_info with values obtained from the BIOS.

Note that query_apm_bios will be executed only if CONFIG_APM or CONFIG_APM_MODULE was set in the configuration file:

#if defined(CONFIG_APM) || defined(CONFIG_APM_MODULE)
    query_apm_bios();
#endif

The last is the query_edd function, which queries Enhanced Disk Drive information from the BIOS. Let's look into the query_edd implementation.

First of all it reads the edd option from the kernel's command line and if it was set to off then query_edd just returns.

If EDD is enabled, query_edd goes over BIOS-supported hard disks and queries EDD information in the following loop:

for (devno = 0x80; devno < 0x80+EDD_MBR_SIG_MAX; devno++) {
    if (!get_edd_info(devno, &ei) && boot_params.eddbuf_entries < EDDMAXNR) {
        memcpy(edp, &ei, sizeof ei);
        edp++;
        boot_params.eddbuf_entries++;
    }
    ...
    ...
    ...

where 0x80 is the first hard drive and the value of EDD_MBR_SIG_MAX macro is 16. It collects data into the array of edd_info structures. get_edd_info checks that EDD is present by invoking the 0x13 interrupt with ah as 0x41 and if EDD is present, get_edd_info again calls the 0x13 interrupt, but with ah as 0x48 and si containing the address of the buffer where EDD information will be stored.

Conclusion

This is the end of the second part about Linux kernel insides. In the next part we will see video mode setting and the rest of preparations before transition to protected mode and directly transitioning into it.

If you have any questions or suggestions write me a comment or ping me at twitter.

Please note that English is not my first language, And I am really sorry for any inconvenience. If you find any mistakes please send me a PR to linux-insides.

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