The previous [post](https://0xax.gitbooks.io/linux-insides/content/Booting/linux-bootstrap-5.html) was a last part of the Linux kernel [booting process](https://0xax.gitbooks.io/linux-insides/content/Booting/index.html) chapter and now we are starting to dive into initialization process of the Linux kernel. After the image of the Linux kernel is decompressed and placed in a correct place in memory, it starts to work. All previous parts describe the work of the Linux kernel setup code which does preparation before the first bytes of the Linux kernel code will be executed. From now we are in the kernel and all parts of this chapter will be devoted to the initialization process of the kernel before it will launch process with [pid](https://en.wikipedia.org/wiki/Process_identifier) `1`. There are many things to do before the kernel will start first `init` process. Hope we will see all of the preparations before kernel will start in this big chapter. We will start from the kernel entry point, which is located in the [arch/x86/kernel/head_64.S](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/head_64.S) and will move further and further. We will see first preparations like early page tables initialization, switch to a new descriptor in kernel space and many many more, before we will see the `start_kernel` function from the [init/main.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/init/main.c#L489) will be called.
In the last [part](https://0xax.gitbooks.io/linux-insides/content/Booting/linux-bootstrap-5.html) of the previous [chapter](https://0xax.gitbooks.io/linux-insides/content/Booting/index.html) we stopped at the [jmp](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/boot/compressed/head_64.S) instruction from the [arch/x86/boot/compressed/head_64.S](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/boot/compressed/head_64.S) assembly source code file:
At this moment the `rax` register contains address of the Linux kernel entry point which that was obtained as a result of the call of the `decompress_kernel` function from the [arch/x86/boot/compressed/misc.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/boot/compressed/misc.c) source code file. So, our last instruction in the kernel setup code is a jump on the kernel entry point. We already know where is defined the entry point of the linux kernel, so we are able to start to learn what does the Linux kernel does after the start.
Okay, we got the address of the decompressed kernel image from the `decompress_kernel` function into `rax` register and just jumped there. As we already know the entry point of the decompressed kernel image starts in the [arch/x86/kernel/head_64.S](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/head_64.S) assembly source code file and at the beginning of it, we can see following definitions:
We can see definition of the `startup_64` routine that is defined in the `__HEAD` section, which is just a macro which expands to the definition of executable `.head.text` section:
We can see definition of this section in the [arch/x86/kernel/vmlinux.lds.S](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/vmlinux.lds.S#L93) linker script:
Besides the definition of the `.text` section, we can understand default virtual and physical addresses from the linker script. Note that address of the `_text` is location counter which is defined as:
for the [x86_64](https://en.wikipedia.org/wiki/X86-64). The definition of the `__START_KERNEL` macro is located in the [arch/x86/include/asm/page_types.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/page_types.h) header file and represented by the sum of the base virtual address of the kernel mapping and physical start:
Now we know default physical and virtual addresses of the `startup_64` routine, but to know actual addresses we must to calculate it with the following code:
Yes, it defined as `0x1000000`, but it may be different, for example if [kASLR](https://en.wikipedia.org/wiki/Address_space_layout_randomization#Linux) is enabled. So our current goal is to calculate delta between `0x1000000` and where we actually loaded. Here we just put the `rip-relative` address to the `rbp` register and then subtract `$_text - __START_KERNEL_map` from it. We know that compiled virtual address of the `_text` is `0xffffffff81000000` and the physical address of it is `0x1000000`. The `__START_KERNEL_map` macro expands to the `0xffffffff80000000` address, so at the second line of the assembly code, we will get following expression:
So, after the calculation, the `rbp` will contain `0` which represents difference between addresses where we actually loaded and where the code was compiled. In our case `zero` means that the Linux kernel was loaded by default address and the [kASLR](https://en.wikipedia.org/wiki/Address_space_layout_randomization#Linux) was disabled.
Here we just compare low part of the `rbp` register with the complemented value of the `PMD_PAGE_MASK`. The `PMD_PAGE_MASK` indicates the mask for `Page middle directory` (read [paging](http://0xax.gitbooks.io/linux-insides/content/Theory/Paging.html) about it) and defined as:
As we can easily calculate, `PMD_PAGE_SIZE` is `2` megabytes. Here we use standard formula for checking alignment and if `text` address is not aligned for `2` megabytes, we jump to `bad_address` label.
All of `early_level4_pgt`, `level3_kernel_pgt` and other address may be wrong if the `startup_64` is not equal to default `0x1000000` address. The `rbp` register contains the delta address so we add to the certain entries of the `early_level4_pgt`, the `level3_kernel_pgt` and the `level2_fixmap_pgt`. Let's try to understand what these labels mean. First of all let's look at their definition:
Looks hard, but it isn't. First of all let's look at the `early_level4_pgt`. It starts with the (4096 - 8) bytes of zeros, it means that we don't use the first `511` entries. And after this we can see one `level3_kernel_pgt` entry. Note that we subtract `__START_KERNEL_map + _PAGE_TABLE` from it. As we know `__START_KERNEL_map` is a base virtual address of the kernel text, so if we subtract `__START_KERNEL_map`, we will get physical address of the `level3_kernel_pgt`. Now let's look at `_PAGE_TABLE`, it is just page entry access rights:
The `level3_kernel_pgt` - stores two entries which map kernel space. At the start of it's definition, we can see that it is filled with zeros `L3_START_KERNEL` or `510` times. Here the `L3_START_KERNEL` is the index in the page upper directory which contains `__START_KERNEL_map` address and it equals `510`. After this, we can see the definition of the two `level3_kernel_pgt` entries: `level2_kernel_pgt` and `level2_fixmap_pgt`. First is simple, it is page table entry which contains pointer to the page middle directory which maps kernel space and it has:
access rights. The second - `level2_fixmap_pgt` is a virtual addresses which can refer to any physical addresses even under kernel space. They represented by the one `level2_fixmap_pgt` entry and `10` megabytes hole for the [vsyscalls](https://lwn.net/Articles/446528/) mapping. The next `level2_kernel_pgt` calls the `PDMS` macro which creates `512` megabytes from the `__START_KERNEL_map` for kernel `.text` (after these `512` megabytes will be modules memory space).
Now, after we saw definitions of these symbols, let's get back to the code which is described at the beginning of the section. Remember that the `rbp` register contains delta between the address of the `startup_64` symbol which was got during kernel [linking](https://en.wikipedia.org/wiki/Linker_%28computing%29) and the actual address. So, for this moment, we just need to add this delta to the base address of some page table entries, that they'll have correct addresses. In our case these entries are:
or the last entry of the `early_level4_pgt` which is the `level3_kernel_pgt`, last two entries of the `level3_kernel_pgt` which are the `level2_kernel_pgt` and the `level2_fixmap_pgt` and five hundreds seventh entry of the `level2_fixmap_pgt` which is `level1_fixmap_pgt` page directory.
Note that we didn't fixup base address of the `early_level4_pgt` and some of other page table directories, because we will see this during of building/filling of structures for these page tables. As we corrected base addresses of the page tables, we can start to build it.
Now we can see the set up of identity mapping of early page tables. In Identity Mapped Paging, virtual addresses are mapped to physical addresses that have the same value, `1 : 1`. Let's look at it in detail. First of all we get the `rip-relative` address of the `_text` and `_early_level4_pgt` and put they into `rdi` and `rbx` registers:
After this we store address of the `_text` in the `rax` and get the index of the page global directory entry which stores `_text` address, by shifting `_text` address on the `PGDIR_SHIFT`:
where `PGDIR_SHIFT` is `39`. `PGDIR_SHFT` indicates the mask for page global directory bits in a virtual address. There are macro for all types of page directories:
After this we put the address of the first entry of the `early_dynamic_pgts` page table to the `rdx` register with the `_KERNPG_TABLE` access rights (see above) and fill the `early_level4_pgt` with the 2 `early_dynamic_pgts` entries:
```assembly
leaq (4096 + _KERNPG_TABLE)(%rbx), %rdx
movq %rdx, 0(%rbx,%rax,8)
movq %rdx, 8(%rbx,%rax,8)
```
The `rbx` register contains address of the `early_level4_pgt` and `%rax * 8` here is index of a page global directory occupied by the `_text` address. So here we fill two entries of the `early_level4_pgt` with address of two entries of the `early_dynamic_pgts` which is related to `_text`. The `early_dynamic_pgts` is array of arrays:
After this we add `4096` (size of the `early_level4_pgt`) to the `rdx` (it now contains the address of the first entry of the `early_dynamic_pgts`) and put `rdi` (it now contains physical address of the `_text`) to the `rax`. Now we shift address of the `_text` ot `PUD_SHIFT` to get index of an entry from page upper directory which contains this address and clears high bits to get only `pud` related part:
As we have index of a page upper directory we write two addresses of the second entry of the `early_dynamic_pgts` array to the first entry of this temporary page directory:
In the next step we do the same operation for last page table directory, but filling not two entries, but all entries to cover full size of the kernel.
That's all for now. Our early paging is prepared and we just need to finish last preparation before we will jump into C code and kernel entry point later.
After that we jump to the label `1` we enable `PAE`, `PGE` (Paging Global Extension) and put the content of the `phys_base` (see above) to the `rax` register and fill `cr3` register with it:
We put `0x80000001` value to the `eax` and execute `cpuid` instruction for getting the extended processor info and feature bits. The result will be in the `edx` register which we put to the `edi`.
We will not see all fields in details here, but we will learn about this and other `MSRs` in a special part about it. As we read `EFER` to the `edx:eax`, we check `_EFER_SCE` or zero bit which is `System Call Extensions` with `btsl` instruction and set it to one. By the setting `SCE` bit we enable `SYSCALL` and `SYSRET` instructions. In the next step we check 20th bit in the `edi`, remember that this register stores result of the `cpuid` (see above). If `20` bit is set (`NX` bit) we just write `EFER_SCE` to the model specific register.
If the [NX](https://en.wikipedia.org/wiki/NX_bit) bit is supported we enable `_EFER_NX` and write it too, with the `wrmsr` instruction. After the [NX](https://en.wikipedia.org/wiki/NX_bit) bit is set, we set some bits in the `cr0` [control register](https://en.wikipedia.org/wiki/Control_register), namely:
*`X86_CR0_PE` - system is in protected mode;
*`X86_CR0_MP` - controls interaction of WAIT/FWAIT instructions with TS flag in CR0;
*`X86_CR0_ET` - on the 386, it allowed to specify whether the external math coprocessor was an 80287 or 80387;
*`X86_CR0_NE` - enable internal x87 floating point error reporting when set, else enables PC style x87 error detection;
*`X86_CR0_WP` - when set, the CPU can't write to read-only pages when privilege level is 0;
*`X86_CR0_AM` - alignment check enabled if AM set, AC flag (in EFLAGS register) set, and privilege level is 3;
We already know that to run any code, and even more [C](https://en.wikipedia.org/wiki/C_%28programming_language%29) code from assembly, we need to setup a stack. As always, we are doing it by the setting of [stack pointer](https://en.wikipedia.org/wiki/Stack_register) to a correct place in memory and resetting [flags](https://en.wikipedia.org/wiki/FLAGS_register) register after this:
The most interesting thing here is the `initial_stack`. This symbol is defined in the [source](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/head_64.S) code file and looks like:
The `GLOBAL` is already familiar to us from. It defined in the [arch/x86/include/asm/linkage.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/linkage.h) header file expands to the `global` symbol definition:
The `THREAD_SIZE` macro is defined in the [arch/x86/include/asm/page_64_types.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/page_64_types.h) header file and depends on value of the `KASAN_STACK_ORDER` macro:
We consider when the [kasan](http://lxr.free-electrons.com/source/Documentation/kasan.txt) is disabled and the `PAGE_SIZE` is `4096` bytes. So the `THREAD_SIZE` will expands to `16` kilobytes and represents size of the stack of a thread. Why is `thread`? You may already know that each [process](https://en.wikipedia.org/wiki/Process_%28computing%29) may have parent [processes](https://en.wikipedia.org/wiki/Parent_process) and [child](https://en.wikipedia.org/wiki/Child_process) processes. Actually, a parent process and child process differ in stack. A new kernel stack is allocated for a new process. In the Linux kernel this stack is represented by the [union](https://en.wikipedia.org/wiki/Union_type#C.2FC.2B.2B) with the `thread_info` structure.
And as we can see the `init_thread_union` is represented by the `thread_union` [union](https://en.wikipedia.org/wiki/Union_type#C.2FC.2B.2B). Earlier this union looked like:
but from the Linux kernel `4.9-rc1` release, `thread_info` was moved to the `task_struct` structure which represents a thread. So, for now `thread_union` looks like:
where the `CONFIG_THREAD_INFO_IN_TASK` kernel configuration option is enabled for `x86_64` architecture. So, as we consider only `x86_64` architecture in this book, an instance of `thread_union` will contain only stack and `thread_info` structure will be placed in the `task_struct`.
that `initial_stack` symbol points to the start of the `thread_union.stack` array + `THREAD_SIZE` which is 16 killobytes and - 8 bytes. Here we need to subtract `8` bytes at the top of stack. This is necessary to guarantee illegal access of the next page memory.
After the early boot stack is set, to update the [Global Descriptor Table](https://en.wikipedia.org/wiki/Global_Descriptor_Table) with the `lgdt` instruction:
We need to reload `Global Descriptor Table` because now kernel works in the low userspace addresses, but soon kernel will work in its own space. Now let's look at the definition of `early_gdt_descr`. Global Descriptor Table contains `32` entries:
in the [arch/x86/include/asm/desc.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/desc.h). It contains one field `gdt` which is array of the `desc_struct` structure which is defined as:
and presents familiar to us `GDT` descriptor. Also we can note that `gdt_page` structure aligned to `PAGE_SIZE` which is `4096` bytes. It means that `gdt` will occupy one page. Now let's try to understand what is `INIT_PER_CPU_VAR`. `INIT_PER_CPU_VAR` is a macro which defined in the [arch/x86/include/asm/percpu.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/include/asm/percpu.h) and just concats `init_per_cpu__` with the given parameter:
After the `INIT_PER_CPU_VAR` macro will be expanded, we will have `init_per_cpu__gdt_page`. We can see in the [linker script](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/vmlinux.lds.S):
#define INIT_PER_CPU(x) init_per_cpu__##x = x + __per_cpu_load
INIT_PER_CPU(gdt_page);
```
As we got `init_per_cpu__gdt_page` in `INIT_PER_CPU_VAR` and `INIT_PER_CPU` macro from linker script will be expanded we will get offset from the `__per_cpu_load`. After this calculations, we will have correct base address of the new GDT.
Generally per-CPU variables is a 2.6 kernel feature. You can understand what it is from its name. When we create `per-CPU` variable, each CPU will have its own copy of this variable. Here we creating `gdt_page` per-CPU variable. There are many advantages for variables of this type, like there are no locks, because each CPU works with its own copy of variable and etc... So every core on multiprocessor will have its own `GDT` table and every entry in the table will represent a memory segment which can be accessed from the thread which ran on the core. You can read in details about `per-CPU` variables in the [Theory/per-cpu](http://0xax.gitbooks.io/linux-insides/content/Concepts/per-cpu.html) post.
After all of these steps we set up `gs` register that it post to the `irqstack` which represents special stack where [interrupts](https://en.wikipedia.org/wiki/Interrupt) will be handled on:
We need to put `MSR_GS_BASE` to the `ecx` register and load data from the `eax` and `edx` (which are point to the `initial_gs`) with `wrmsr` instruction. We don't use `cs`, `fs`, `ds` and `ss` segment registers for addressing in the 64-bit mode, but `fs` and `gs` registers can be used. `fs` and `gs` have a hidden part (as we saw it in the real mode for `cs`) and this part contains descriptor which mapped to [Model Specific Registers](https://en.wikipedia.org/wiki/Model-specific_register). So we can see above `0xc0000101` is a `gs.base` MSR address. When a [system call](https://en.wikipedia.org/wiki/System_call) or [interrupt](https://en.wikipedia.org/wiki/Interrupt) occurred, there is no kernel stack at the entry point, so the value of the `MSR_GS_BASE` will store address of the interrupt stack.
In the next step we put the address of the real mode bootparam structure to the `rdi` (remember `rsi` holds pointer to this structure from the start) and jump to the C code with:
Here we put the address of the `initial_code` to the `rax` and push fake address, `__KERNEL_CS` and the address of the `initial_code` to the stack. After this we can see `lretq` instruction which means that after it return address will be extracted from stack (now there is address of the `initial_code`) and jump there. `initial_code` is defined in the same source code file and looks:
As we can see `initial_code` contains address of the `x86_64_start_kernel`, which is defined in the [arch/x86/kerne/head64.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/arch/x86/kernel/head64.c) and looks like this:
We need to see last preparations before we can see "kernel entry point" - start_kernel function from the [init/main.c](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/init/main.c#L489).
There are checks for different things like virtual addresses of modules space is not fewer than base address of the kernel text - `__STAT_KERNEL_map`, that kernel text with modules is not less than image of the kernel and etc... `BUILD_BUG_ON` is a macro which looks as:
Let's try to understand how this trick works. Let's take for example first condition: `MODULES_VADDR < __START_KERNEL_map`. `!!conditions` is the same that `condition != 0`. So it means if `MODULES_VADDR < __START_KERNEL_map` is true, we will get `1` in the `!!(condition)` or zero if not. After `2*!!(condition)` we will get or `2` or `0`. In the end of calculations we can get two different behaviors:
* We will have compilation error, because try to get size of the char array with negative index (as can be in our case, because `MODULES_VADDR` can't be less than `__START_KERNEL_map` will be in our case);
In the next step we can see call of the `cr4_init_shadow` function which stores shadow copy of the `cr4` per cpu. Context switches can change bits in the `cr4` so we need to store `cr4` for each CPU. And after this we can see call of the `reset_early_page_tables` function where we resets all page global directory entries and write new pointer to the PGT in `cr3`:
Soon we will build new page tables. Here we can see that we go through all Page Global Directory Entries (`PTRS_PER_PGD` is `512`) in the loop and make it zero. After this we set `next_early_pgt` to zero (we will see details about it in the next post) and write physical address of the `early_level4_pgt` to the `cr3`. `__pa_nodebug` is a macro which will be expanded to:
After this we clear `_bss` from the `__bss_stop` to `__bss_start` and the next step will be setup of the early `IDT` handlers, but it's big concept so we will see it in the next part.
If you have questions or suggestions, feel free to ping me in twitter [0xAX](https://twitter.com/0xAX), drop me [email](anotherworldofworld@gmail.com) or just create [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-insides).**
