It is the fourth part of the `Kernel booting process` and we will see first steps in the [protected mode](http://en.wikipedia.org/wiki/Protected_mode), like checking that cpu supports the [long mode](http://en.wikipedia.org/wiki/Long_mode) and [SSE](http://en.wikipedia.org/wiki/Streaming_SIMD_Extensions), [paging](http://en.wikipedia.org/wiki/Paging) and initialization of the page tables and transition to the long mode in in the end of this part.
**NOTE: will be much assembly code in this part, so if you have poor knowledge, read a book about it**
In the previous [part](https://github.com/0xAX/linux-insides/blob/master/Booting/linux-bootstrap-3.md) we stopped at the jump to the 32-bit entry point in the [arch/x86/boot/pmjump.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/pmjump.S):
When using bzImage, the protected-mode kernel was relocated to 0x100000
```
And now we can make sure that it is true. Let's look on registers value in 32-bit entry point:
```
eax 0x100000 1048576
ecx 0x0 0
edx 0x0 0
ebx 0x0 0
esp 0x1ff5c 0x1ff5c
ebp 0x0 0x0
esi 0x14470 83056
edi 0x0 0
eip 0x100000 0x100000
eflags 0x46 [ PF ZF ]
cs 0x10 16
ss 0x18 24
ds 0x18 24
es 0x18 24
fs 0x18 24
gs 0x18 24
```
We can see here that `cs` register contains - `0x10` (as you can remember from the previous part, it is the second index in the Global Descriptor Table), `eip` register is `0x100000` and base address of the all segments include code segment is zero. So we can get physical address, it will be `0:0x100000` or just `0x100000`, as in boot protocol. Now let's start with 32-bit entry point.
We can find the definition of the 32-bit entry point in [arch/x86/boot/compressed/head_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/head_64.S):
First of all why `compressed` directory? Actually `bzimage` is a gzipped `vmlinux + header + kernel setup code`. We saw the kernel setup code in all of the previous parts. So, the main goal of the `head_64.S` is to prepare for entering long mode, enter into it and decompress the kernel. We will see all of these steps besides kernel decompression in this part.
We will see only `head_64.S` because we are learning linux kernel for `x86_64`. `head_32.S` even not compiled in our case. Let's look at [arch/x86/boot/compressed/Makefile](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/Makefile). We can see there the following target:
Note on `$(obj)/head_$(BITS).o`. It means that compilation of the head_{32,64}.o depends on value of the `$(BITS)`. We can find it in the other Makefile - [arch/x86/kernel/Makefile](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/Makefile):
As I wrote above, we start in [arch/x86/boot/compressed/head_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/head_64.S). First of all we can see before the `startup_32` definition:
We can find this section in the [arch/x86/boot/compressed/vmlinux.lds.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/vmlinux.lds.S) linker script:
Note on `. = 0;`. `.` is a special variable of linker - location counter. The value assigned to it is an offset relative to the offset of the segment. As we assign zero to it, we can read from comments:
In the start of `startup_32` we can see the `cld` instruction which clears the `DF` flag. After this, string operations like `stosb` and others will increment the index registers `esi` or `edi`.
Next we can see the check of the `KEEP_SEGMENTS` flag from `loadflags`. If you remember we already saw `loadflags` in the `arch/x86/boot/head.S` (there we checked flag `CAN_USE_HEAP`). Now we need to check the `KEEP_SEGMENTS` flag. We can find a description of this flag in the linux boot protocol:
remember that `__BOOT_DS` is `0x18` (index of data segment in the Global Descriptor Table). If `KEEP_SEGMENTS` is not set, we jump to the label `1f` or update segment registers with `__BOOT_DS` if this flag is set.
If you read the previous [part](https://github.com/0xAX/linux-insides/blob/master/Booting/linux-bootstrap-3.md), you can remember that we already updated segment registers in the [arch/x86/boot/pmjump.S](https://github.com/torvalds/linux/blob/master/arch/x86/boot/pmjump.S), so why do we need to set up it again? Actually linux kernel also has the 32-bit boot protocol, so `startup_32` can be the first function which will be executed right after a bootloader transfers control to the kernel.
As we checked the `KEEP_SEGMENTS` flag and put the correct value to the segment registers, the next step is to calculate difference between where we loaded and compiled to run (remember that `setup.ld.S` contains `. = 0` at the start of the section):
Here the `esi` register contains the address of the [boot_params](https://github.com/torvalds/linux/blob/master/arch/x86/include/uapi/asm/bootparam.h#L113) structure. `boot_params` contains a special field `scratch` with offset `0x1e4`. We are getting the address of the `scratch` field + 4 bytes and puting it in the `esp` register (we will use it as stack for these calculations). After this we can see the call instruction and `1f` label as its operand. What does `call` mean? It means that it pushes the `ebp` value into the stack, then the `esp` value, then the function arguments and returns the address in the end. After this we pop return address from the stack into `ebp` register (`ebp` will contain return address) and subtract address of the previous label `1`.
The next we can see assembly code which setups new stack for kernel decompression:
```assembly
movl $boot_stack_end, %eax
addl %ebp, %eax
movl %eax, %esp
```
`boots_stack_end` is in the `.bss` section, we can see definition of it in the end of `head_64.S`:
```assembly
.bss
.balign 4
boot_heap:
.fill BOOT_HEAP_SIZE, 1, 0
boot_stack:
.fill BOOT_STACK_SIZE, 1, 0
boot_stack_end:
```
First of all we put address of the `boot_stack_end` into `eax` register and add to it value of the `ebp` (remember that `ebp` now contains address where we loaded - `0x100000`). In the end we just put `eax` value into `esp` and that's all, we have correct stack pointer.
The next step is CPU verification. Need to check that CPU has support of `long mode` and `SSE`:
```assembly
call verify_cpu
testl %eax, %eax
jnz no_longmode
```
It just calls `verify_cpu` function from the [arch/x86/kernel/verify_cpu.S](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/verify_cpu.S) which contains a couple of calls of the `cpuid` instruction. `cpuid` is instruction which is used for getting information about processor. In our case it checks long mode and SSE support and returns `0` on success or `1` on fail in the `eax` register.
If `eax` is not zero, we jump to the `no_longmode` label which just stops the CPU with `hlt` instruction while any hardware interrupt will not happen.
The next step is calculating relocation address for decompression if need. We can see following assembly code:
```assembly
#ifdef CONFIG_RELOCATABLE
movl %ebp, %ebx
movl BP_kernel_alignment(%esi), %eax
decl %eax
addl %eax, %ebx
notl %eax
andl %eax, %ebx
cmpl $LOAD_PHYSICAL_ADDR, %ebx
jge 1f
#endif
movl $LOAD_PHYSICAL_ADDR, %ebx
1:
addl $z_extract_offset, %ebx
```
First of all note on `CONFIG_RELOCATABLE` macro. This configuration option defined in the [arch/x86/Kconfig](https://github.com/torvalds/linux/blob/master/arch/x86/Kconfig) and as we can read from it's description:
```
This builds a kernel image that retains relocation information
so it can be loaded someplace besides the default 1MB.
Note: If CONFIG_RELOCATABLE=y, then the kernel runs from the address
it has been loaded at and the compile time physical address
(CONFIG_PHYSICAL_START) is used as the minimum location.
```
In short words, this code calculates address where to move kernel for decompression put it to `ebx` register if the kernel is relocatable or bzimage will decompress itself above `LOAD_PHYSICAL_ADDR`.
Let's look on the code. If we have `CONFIG_RELOCATABLE=n` in our kernel configuration file, it just puts `LOAD_PHYSICAL_ADDR` to the `ebx` register and adds `z_extract_offset` to `ebx`. As `ebx` is zero for now, it will contain `z_extract_offset`. Now let's try to understand these two values.
`LOAD_PHYSICAL_ADDR` is the macro which defined in the [arch/x86/include/asm/boot.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/asm/boot.h) and it looks like this:
Here we calculates aligned address where kernel is loaded (`0x100000` or 1 megabyte in our case). `PHYSICAL_ALIGN` is an alignment value to which kernel should be aligned, it ranges from `0x200000` to `0x1000000` for x86_64. With the default values we will get 2 megabytes in the `LOAD_PHYSICAL_ADDR`:
```python
>>> 0x100000 + (0x200000 - 1) & ~(0x200000 - 1)
2097152
```
After that we got alignment unit, we adds `z_extract_offset` (which is `0xe5c000` in my case) to the 2 megabytes. In the end we will get 17154048 byte offset. You can find `z_extract_offset` in the `arch/x86/boot/compressed/piggy.S`. This file generated in compile time by [mkpiggy](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/mkpiggy.c) program.
Now let's try to understand the code if `CONFIG_RELOCATABLE` is `y`.
First of all we put `ebp` value to the `ebx` (remember that `ebp` contains address where we loaded) and `kernel_alignment` field from kernel setup header to the `eax` register. `kernel_alignment` is a physical address of alignment required for the kernel. Next we do the same as in the previous case (when kernel is not relocatable), but we just use value of the `kernel_alignment` field as align unit and `ebx` (address where we loaded) as base address instead of `CONFIG_PHYSICAL_ALIGN` and `LOAD_PHYSICAL_ADDR`.
After that we calculated address, we compare it with `LOAD_PHYSICAL_ADDR` and add `z_extract_offset` to it again or put `LOAD_PHYSICAL_ADDR` in the `ebx` if calculated address is less than we need.
After all of this calculation we will have `ebp` which contains address where we loaded and `ebx` with address where to move kernel for decompression.
Now we need to do the last preparations before we can see transition to the 64-bit mode. At first we need to update Global Descriptor Table for this:
```assembly
leal gdt(%ebp), %eax
movl %eax, gdt+2(%ebp)
lgdt gdt(%ebp)
```
Here we put the address from `ebp` with `gdt` offset to `eax` register, next we put this address into `ebp` with offset `gdt+2` and load Global Descriptor Table with the `lgdt` instruction.
Let's look on Global Descriptor Table definition:
```assembly
.data
gdt:
.word gdt_end - gdt
.long gdt
.word 0
.quad 0x0000000000000000 /* NULL descriptor */
.quad 0x00af9a000000ffff /* __KERNEL_CS */
.quad 0x00cf92000000ffff /* __KERNEL_DS */
.quad 0x0080890000000000 /* TS descriptor */
.quad 0x0000000000000000 /* TS continued */
```
It defined in the same file in the `.data` section. It contains 5 descriptors: null descriptor, for kernel code segment, kernel data segment and two task descriptors. We already loaded GDT in the previous [part](https://github.com/0xAX/linux-insides/blob/master/Booting/linux-bootstrap-3.md), we're doing almost the same here, but descriptors with `CS.L = 1` and `CS.D = 0` for execution in the 64 bit mode.
After we have loaded Global Descriptor Table, we must enable [PAE](http://en.wikipedia.org/wiki/Physical_Address_Extension) mode with putting value of `cr4` register into `eax`, setting 5 bit in it and load it again in the `cr4` :
```assembly
movl %cr4, %eax
orl $X86_CR4_PAE, %eax
movl %eax, %cr4
```
Now we finished almost with all preparations before we can move into 64-bit mode. The last step is to build page tables, but before some information about long mode.
Before we can move in the 64-bit mode, we need to build page tables, so, let's look on building of early 4G boot page tables.
**NOTE: I will not describe theory of virtual memory here, if you need to know more about it, see links in the end**
Linux kernel uses 4-level paging, and generally we build 6 page tables:
* One PML4 table
* One PDP table
* Four Page Directory tables
Let's look on the implementation of it. First of all we clear buffer for the page tables in the memory. Every table is 4096 bytes, so we need 24 kilobytes buffer:
```assembly
leal pgtable(%ebx), %edi
xorl %eax, %eax
movl $((4096*6)/4), %ecx
rep stosl
```
We put address which stored in `ebx` (remember that `ebx` contains the address where to relocate kernel for decompression) with `pgtable` offset to the `edi` register. `pgtable` defined in the end of `head_64.S` and looks:
```assembly
.section ".pgtable","a",@nobits
.balign 4096
pgtable:
.fill 6*4096, 1, 0
```
It is in the `.pgtable` section and it size is 24 kilobytes. After we put address to the `edi`, we zero out `eax` register and writes zeros to the buffer with `rep stosl` instruction.
Now we can build top level page table - `PML4` with:
```assembly
leal pgtable + 0(%ebx), %edi
leal 0x1007 (%edi), %eax
movl %eax, 0(%edi)
```
Here we get address which stored in the `ebx` with `pgtable` offset and put it to the `edi`. Next we put this address with offset `0x1007` to the `eax` register. `0x1007` is 4096 bytes (size of the PML4) + 7 (PML4 entry flags - `PRESENT+RW+USER`) and puts `eax` to the `edi`. After this manipulations `edi` will contain the address of the first Page Directory Pointer Entry with flags - `PRESENT+RW+USER`.
In the next step we build 4 Page Directory entry in the Page Directory Pointer table, where first entry will be with `0x7` flags and other with `0x8`:
We put base address of the page directory pointer table to the `edi` and address of the first page directory pointer entry to the `eax`. Put `4` to the `ecx` register, it will be counter in the following loop and write the address of the first page directory pointer table entry to the `edi` register.
After this `edi` will contain address of the first page directory pointer entry with flags `0x7`. Next we just calculates address of following page directory pointer entries with flags `0x8` and writes their addresses to the `edi`.
The next step is building of `2048` page table entries by 2 megabytes:
Here we do almost the same that in the previous example, just first entry will be with flags - `$0x00000183` - `PRESENT + WRITE + MBZ` and all another with `0x8`. In the end we will have 2048 pages by 2 megabytes.
Our early page table structure are done, it maps 4 gigabytes of memory and now we can put address of the high-level page table - `PML4` to the `cr3` control register:
```assembly
leal pgtable(%ebx), %eax
movl %eax, %cr3
```
That's all now we can see transition to the long mode.
First of all we need to set `EFER.LME` flag in the [MSR](http://en.wikipedia.org/wiki/Model-specific_register) to `0xC0000080`:
```assembly
movl $MSR_EFER, %ecx
rdmsr
btsl $_EFER_LME, %eax
wrmsr
```
Here we put `MSR_EFER` flag (which defined in the [arch/x86/include/uapi/asm/msr-index.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/uapi/asm/msr-index.h#L7)) to the `ecx` register and call `rdmsr` instruction which reads [MSR](http://en.wikipedia.org/wiki/Model-specific_register) register. After `rdmsr` executed, we will have result data in the `edx:eax` which depends on `ecx` value. We check `EFER_LME` bit with `btsl` instruction and write data from `eax` to the `MSR` register with `wrmsr` instruction.
In next step we push address of the kernel segment code to the stack (we defined it in the GDT) and put address of the `startup_64` routine to the `eax`.
```assembly
pushl $__KERNEL_CS
leal startup_64(%ebp), %eax
```
After this we push this address to the stack and enable paging with setting `PG` and `PE` bits in the `cr0` register:
```assembly
movl $(X86_CR0_PG | X86_CR0_PE), %eax
movl %eax, %cr0
```
and call:
```assembly
lret
```
Remember that we pushed address of the `startup_64` function to the stack in the previous step, and after `lret` instruction, CPU extracts address of it and jumps there.
After all of these steps we're finally in the 64-bit mode:
This is the end of the fourth part linux kernel booting process. If you have questions or suggestions, ping me in twitter [0xAX](https://twitter.com/0xAX), drop me [email](anotherworldofworld@gmail.com) or just create an [issue](https://github.com/0xAX/linux-internals/issues/new).
In the next part we will see kernel decompression and many more.
**Please note that English is not my first language and I am really sorry for any inconvenience. If you found any mistakes please send me PR to [linux-internals](https://github.com/0xAX/linux-internals).**
