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 set, we jump to the label `1f` or update segment registers with `__BOOT_DS` if this flag is not 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`.
First of all we put the address of `boot_stack_end` into the `eax` register and add to it value of `ebp` (remember that `ebp` now contains the address where we loaded - `0x100000`). In the end we just put the `eax` value into `esp` and that's all, we have a correct stack pointer.
It just calls `verify_cpu` function from [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 to the `cpuid` instruction. `cpuid` is the instruction which is used for getting information about the 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 a `hlt` instruction while any hardware interrupt will not happen.
First of all note on `CONFIG_RELOCATABLE` macro. This configuration option is 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:
In short words, this code calculates the address to move the kernel to for decompression put it into the `ebx` register if the kernel is relocatable or bzimage will decompress itself above `LOAD_PHYSICAL_ADDR`.
Let's look at the code. If we have `CONFIG_RELOCATABLE=n` in our kernel configuration file, it just puts `LOAD_PHYSICAL_ADDR` into 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 [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 calculate the aligned address where the kernel is loaded (`0x100000` or 1 megabyte in our case). `PHYSICAL_ALIGN` is an alignment value to which the kernel should be aligned, and it ranges from `0x200000` to `0x1000000` for x86_64. With the default values we will get 2 megabytes in the `LOAD_PHYSICAL_ADDR`:
After we retrieve the alignment unit, we add `z_extract_offset` (which is `0xe5c000` in my case) to the 2 megabytes. In the end we will get 17154048 bytes offset. You can find `z_extract_offset` in `arch/x86/boot/compressed/piggy.S`. This file is generated in compile time by the [mkpiggy](https://github.com/torvalds/linux/blob/master/arch/x86/boot/compressed/mkpiggy.c) program.
First of all we put the `ebp` value into `ebx` (remember that `ebp` contains address where we loaded) and `kernel_alignment` field from kernel setup header into 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 the value of the `kernel_alignment` field as the align unit and `ebx` (address where we loaded) as the base address instead of `CONFIG_PHYSICAL_ALIGN` and `LOAD_PHYSICAL_ADDR`.
After we calculate the 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 the calculated address is less than we need.
After all of this calculation we will have `ebp` which contains the address where we loaded and `ebx` with the address to which the kernel will be moved for decompression.
Now we need to do the last preparations before we can see the transition to 64-bit mode. At first we need to update the Global Descriptor Table for this:
Here we put the address from `ebp` with `gdt` offset into the `eax` register, next we put this address into `ebp` with offset `gdt+2` and load the Global Descriptor Table with the `lgdt` instruction.
It is defined in the same file as the `.data` section. It contains 5 descriptors: null descriptor, for kernel code segment, kernel data segment and two task descriptors. We already loaded the 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 64 bit mode.
After we have loaded the Global Descriptor Table, we must enable [PAE](http://en.wikipedia.org/wiki/Physical_Address_Extension) mode by putting the value of the `cr4` register into `eax`, setting 5 bit in it and loading it again into `cr4` :
Now we are almost finished with all preparations before we can move into 64-bit mode. The last step is to build page tables, but before that, here is some information about long mode.
Let's look at the implementation of it. First of all we clear the buffer for the page tables in memory. Every table is 4096 bytes, so we need 24 kilobytes buffer:
We put the address stored in `ebx` (remember that `ebx` contains the address to relocate the kernel for decompression) with `pgtable` offset to the `edi` register. `pgtable` is defined in the end of `head_64.S` and looks:
It is in the `.pgtable` section and its size is 24 kilobytes. After we put the address in `edi`, we zero out the `eax` register and write zeros to the buffer with the `rep stosl` instruction.
Here we get the address stored in the `ebx` with `pgtable` offset and put it in `edi`. Next we put this address with offset `0x1007` in the `eax` register. `0x1007` is 4096 bytes (size of the PML4) + 7 (PML4 entry flags - `PRESENT+RW+USER`) and puts `eax` in `edi`. After this manipulation `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 entries in the Page Directory Pointer table with `0x7` flags or present, write, userspace (`PRESENT WRITE | USER`):
We put the base address of the page directory pointer table in `edi` and the address of the first page directory pointer entry in `eax`. Put `4` in the `ecx` register, it will be a 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 the address of the first page directory pointer entry with flags `0x7`. Next we just calculate the address of following page directory pointer entries where each entry is 8 bytes, and write their addresses to `eax`.
Here we do almost the same as in the previous example, all entries will be with flags - `$0x00000183` - `PRESENT + WRITE + MBZ`. In the end we will have 2048 pages with 2-MByte page.
Our early page table structure are done, it maps 4 gigabytes of memory and now we can put the address of the high-level page table - `PML4` - in `cr3` control register:
Here we put the `MSR_EFER` flag (which is defined in [arch/x86/include/uapi/asm/msr-index.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/uapi/asm/msr-index.h#L7)) in the `ecx` register and call `rdmsr` instruction which reads the [MSR](http://en.wikipedia.org/wiki/Model-specific_register) register. After `rdmsr` executes, we will have the resulting data in `edx:eax` which depends on the `ecx` value. We check the `EFER_LME` bit with the `btsl` instruction and write data from `eax` to the `MSR` register with the `wrmsr` instruction.
In the next step we push the address of the kernel segment code to the stack (we defined it in the GDT) and put the address of the `startup_64` routine in `eax`.
Remember that we pushed the address of the `startup_64` function to the stack in the previous step, and after the `lret` instruction, the CPU extracts the address of it and jumps there.
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-insides/issues/new).
**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-insides](https://github.com/0xAX/linux-internals).**
