This is the third part of the Linux kernel initialization process series. In the previous [part](https://github.com/0xAX/linux-insides/blob/master/Initialization/linux-initialization-2.md) we saw early interrupt and exception handling and will continue to dive into the linux kernel initialization process in the current part. Our next point is 'kernel entry point' - `start_kernel` function from the [init/main.c](https://github.com/torvalds/linux/blob/master/init/main.c) source code file. Yes, technically it is not kernel's entry point but the start of the generic kernel code which does not depend on certain architecture. But before we will see call of the `start_kernel` function, we must do some preparations. So let's continue.
In the previous part we stopped at setting Interrupt Descriptor Table and loading it in the `IDTR` register. At the next step after this we can see a call of the `copy_bootdata` function:
This function takes one argument - virtual address of the `real_mode_data`. Remember that we passed the address of the `boot_params` structure from [arch/x86/include/uapi/asm/bootparam.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/uapi/asm/bootparam.h#L114) to the `x86_64_start_kernel` function as first argument in [arch/x86/kernel/head_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/head_64.S):
where `PAGE_OFFSET` is `__PAGE_OFFSET` which is `0xffff880000000000` and the base virtual address of the direct mapping of all physical memory. So we're getting virtual address of the `boot_params` structure and pass it to the `copy_bootdata` function, where we copy `real_mod_data` to the `boot_params` which is declared in the [arch/x86/kernel/setup.h](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/setup.h)
First of all, note that this function is declared with `__init` prefix. It means that this function will be used only during the initialization and used memory will be freed.
We can see declaration of two variables for the kernel command line and copying `real_mode_data` to the `boot_params` with the `memcpy` function. The next call of the `sanitize_boot_params` function which fills some fields of the `boot_params` structure like `ext_ramdisk_image` and etc... if bootloaders which fail to initialize unknown fields in `boot_params` to zero. After this we're getting address of the command line with the call of the `get_cmd_line_ptr` function:
which gets the 64-bit address of the command line from the kernel boot header and returns it. In the last step we check that we got `cmd_line_pty`, getting its virtual address and copy it to the `boot_command_line` which is just an array of bytes:
After this we will have copied kernel command line and `boot_params` structure. In the next step we can see call of the `load_ucode_bsp` function which loads processor microcode, but we will not see it here.
After microcode was loaded we can see the check of the `console_loglevel` and the `early_printk` function which prints `Kernel Alive` string. But you'll never see this output because `early_printk` is not initilized yet. It is a minor bug in the kernel and i sent the patch - [commit](http://git.kernel.org/cgit/linux/kernel/git/tip/tip.git/commit/?id=91d8f0416f3989e248d3a3d3efb821eda10a85d2) and you will see it in the mainline soon. So you can skip this code.
In the next step as we have copied `boot_params` structure, we need to move from the early page tables to the page tables for initialization process. We already set early page tables for switchover, you can read about it in the previous [part](http://0xax.gitbooks.io/linux-insides/content/Initialization/linux-initialization-1.html) and dropped all it in the `reset_early_page_tables` function (you can read about it in the previous part too) and kept only kernel high mapping. After this we call:
```C
clear_page(init_level4_pgt);
```
function and pass `init_level4_pgt` which defined also in the [arch/x86/kernel/head_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/head_64.S) and looks:
which maps first 2 gigabytes and 512 megabytes for the kernel code, data and bss. `clear_page` function defined in the [arch/x86/lib/clear_page_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/lib/clear_page_64.S) let look on this function:
```assembly
ENTRY(clear_page)
CFI_STARTPROC
xorl %eax,%eax
movl $4096/64,%ecx
.p2align 4
.Lloop:
decl %ecx
#define PUT(x) movq %rax,x*8(%rdi)
movq %rax,(%rdi)
PUT(1)
PUT(2)
PUT(3)
PUT(4)
PUT(5)
PUT(6)
PUT(7)
leaq 64(%rdi),%rdi
jnz .Lloop
nop
ret
CFI_ENDPROC
.Lclear_page_end:
ENDPROC(clear_page)
```
As you can understart from the function name it clears or fills with zeros page tables. First of all note that this function starts with the `CFI_STARTPROC` and `CFI_ENDPROC` which are expands to GNU assembly directives:
```C
#define CFI_STARTPROC .cfi_startproc
#define CFI_ENDPROC .cfi_endproc
```
and used for debugging. After `CFI_STARTPROC` macro we zero out `eax` register and put 64 to the `ecx` (it will be counter). Next we can see loop which starts with the `.Lloop` label and it starts from the `ecx` decrement. After it we put zero from the `rax` register to the `rdi` which contains the base address of the `init_level4_pgt` now and do the same procedure seven times but every time move `rdi` offset on 8. After this we will have first 64 bytes of the `init_level4_pgt` filled with zeros. In the next step we put the address of the `init_level4_pgt` with 64-bytes offset to the `rdi` again and repeat all operations which `ecx` is not zero. In the end we will have `init_level4_pgt` filled with zeros.
As we have `init_level4_pgt` filled with zeros, we set the last `init_level4_pgt` entry to kernel high mapping with the:
```C
init_level4_pgt[511] = early_level4_pgt[511];
```
Remeber that we dropped all `early_level4_pgt` entries in the `reset_early_page_table` function and kept only kernel high mapping there.
The last step in the `x86_64_start_kernel` function is the call of the:
```C
x86_64_start_reservations(real_mode_data);
```
function with the `real_mode_data` as argument. The `x86_64_start_reservations` function defined in the same source code file as the `x86_64_start_kernel` function and looks:
First of all we can see in the `x86_64_start_reservations` function check for `boot_params.hdr.version`:
```C
if (!boot_params.hdr.version)
copy_bootdata(__va(real_mode_data));
```
and if it is not we call again `copy_bootdata` function with the virtual address of the `real_mode_data` (read about about it's implementation).
In the next step we can see the call of the `reserve_ebda_region` function which defined in the [arch/x86/kernel/head.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/head.c). This function reserves memory block for th `EBDA` or Extended BIOS Data Area. The Extended BIOS Data Area located in the top of conventional memory and contains data about ports, disk parameters and etc...
Let's look on the `reserve_ebda_region` function. It starts from the checking is paravirtualization enabled or not:
we exit from the `reserve_ebda_region` function if paravirtualization is enabled because if it enabled the extended bios data area is absent. In the next step we need to get the end of the low memory:
lowmem = *(unsigned short *)__va(BIOS_LOWMEM_KILOBYTES);
lowmem <<= 10;
```
We're getting the virtual address of the BIOS low memory in kilobytes and convert it to bytes with shifting it on 10 (multiply on 1024 in other words). After this we need to get the address of the extended BIOS data are with the:
```C
ebda_addr = get_bios_ebda();
```
where `get_bios_ebda` function defined in the [arch/x86/include/asm/bios_ebda.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/asm/bios_ebda.h) and looks like:
```C
static inline unsigned int get_bios_ebda(void)
{
unsigned int address = *(unsigned short *)phys_to_virt(0x40E);
address <<= 4;
return address;
}
```
Let's try to understand how it works. Here we can see that we converting physical address `0x40E` to the virtual, where `0x0040:0x000e` is the segment which contains base address of the extended BIOS data area. Don't worry that we are using `phys_to_virt` function for converting a physical address to virtual address. You can note that previously we have used `__va` macro for the same point, but `phys_to_virt` is the same:
This configuration option is enabled by `CONFIG_PHYS_ADDR_T_64BIT`. After that we got virtual address of the segment which stores the base address of the extended BIOS data area, we shift it on 4 and return. After this `ebda_addr` variables contains the base address of the extended BIOS data area.
or 128 kilobytes. In the last step we get lower part in the low memory and extended bios data area and call `memblock_reserve` function which will reserve memory region for extended bios data between low memory and one megabyte mark:
and reserves memory region for the given base address and size. `memblock_reserve` is the first function in this book from linux kernel memory manager framework. We will take a closer look on memory manager soon, but now let's look at its implementation.
In the previous paragraph we stopped at the call of the `memblock_reserve` function and as i sad before it is the first function from the memory manager framework. Let's try to understand how it works. `memblock_reserve` function just calls:
At the start of the `memblock_reserve_region` body we can see definition of the `memblock_type` structure:
```C
struct memblock_type *_rgn = &memblock.reserved;
```
which presents the type of the memory block and looks:
```C
struct memblock_type {
unsigned long cnt;
unsigned long max;
phys_addr_t total_size;
struct memblock_region *regions;
};
```
As we need to reserve memory block for extended bios data area, the type of the current memory region is reserved where `memblock` structure is:
```C
struct memblock {
bool bottom_up;
phys_addr_t current_limit;
struct memblock_type memory;
struct memblock_type reserved;
#ifdef CONFIG_HAVE_MEMBLOCK_PHYS_MAP
struct memblock_type physmem;
#endif
};
```
and describes generic memory block. You can see that we initialize `_rgn` by assigning it to the address of the `memblock.reserved`. `memblock` is the global variable which looks:
We will not dive into detail of this varaible, but we will see all details about it in the parts about memory manager. Just note that `memblock` variable defined with the `__initdata_memblock` which is:
From this we can conclude that all memory blocks will be in the `.meminit.data` section. After we defined `_rgn` we print information about it with `memblock_dbg` macros. You can enable it by passing `memblock=debug` to the kernel command line.
After debugging lines were printed next is the call of the following function:
```C
memblock_add_range(_rgn, base, size, nid, flags);
```
which adds new memory block region into the `.meminit.data` section. As we do not initlieze `_rgn` but it just contains `&memblock.reserved`, we just fill passed `_rgn` with the base address of the extended BIOS data area region, size of this region and flags:
```C
if (type->regions[0].size == 0) {
WARN_ON(type->cnt != 1 || type->total_size);
type->regions[0].base = base;
type->regions[0].size = size;
type->regions[0].flags = flags;
memblock_set_region_node(&type->regions[0], nid);
type->total_size = size;
return 0;
}
```
After we filled our region we can see the call of the `memblock_set_region_node` function with two parameters:
* address of the filled memory region;
* NUMA node id.
where our regions represented by the `memblock_region` structure:
NUMA node id depends on `MAX_NUMNODES` macro which is defined in the [include/linux/numa.h](https://github.com/torvalds/linux/blob/master/include/linux/numa.h):
After this we will have first reserved `memblock` for the extended bios data area in the `.meminit.data` section. `reserve_ebda_region` function finished its work on this step and we can go back to the [arch/x86/kernel/head64.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/head64.c).
It is the end of the third part about linux kernel internals. In next part we will see the first initialization steps in the kernel entry point - `start_kernel` function. It will be the first step before we will see launch of the first `init` process.
**Please note that English is not my first language, And I am really sorry for any inconvenience. If you will find any mistakes please send me PR to [linux-internals](https://github.com/0xAX/linux-internals).**
