The previous [post](https://0xax.gitbook.io/linux-insides/summary/booting/linux-bootstrap-6) was a last part of the Linux kernel [booting process](https://0xax.gitbook.io/linux-insides/summary/booting) 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/master/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/master/init/main.c) will be called.
In the last [part](https://0xax.gitbook.io/linux-insides/summary/booting/linux-bootstrap-6) of the previous [chapter](https://0xax.gitbook.io/linux-insides/summary/booting) we stopped at the jmp instruction from the [arch/x86/boot/compressed/head_64.S](https://github.com/torvalds/linux/blob/master/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 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/master/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 the entry point of the Linux kernel is defined, so we are able to start to learn what 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/master/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/master/arch/x86/kernel/vmlinux.lds.S) linker script:
The ADDR keyword above returns the absolute address (here means virtual address) of the named section. The AT keyword above specifies the load address (here means physical address) of the section. The full syntax of section definition is defined in the [Using ld The GNU linker](https://ftp.gnu.org/old-gnu/Manuals/ld-2.9.1/html_node/ld_21.html).
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 [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/master/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:
After we sanitized CPU configuration, we call `__startup_64` function which is defined in [arch/x86/kernel/head64.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/head64.c):
Since [kASLR](https://en.wikipedia.org/wiki/Address_space_layout_randomization#Linux) is enabled, the address `startup_64` routine was loaded may be different from the address compiled to run at, so we need to calculate the delta with the following code:
If `_text` address is not aligned for `2` megabytes, we enter infinite loop. The `PMD_PAGE_MASK` indicates the mask for `Page middle directory` (read [Paging](https://0xax.gitbook.io/linux-insides/summary/theory/linux-theory-1) about it) and is defined as:
If [SME](https://en.wikipedia.org/wiki/Zen_%28microarchitecture%29#Enhanced_security_and_virtualization_support) is supported and enabled, we activate it and include the SME encryption mask in `load_delta`:
So, let's look at the definition of `fixup_pointer` function which returns physical address of the passed argument:
```C
static void __head *fixup_pointer(void *ptr, unsigned long physaddr)
{
return ptr - (void *)_text + (void *)physaddr;
}
```
Next we'll focus on `early_top_pgt` and the other page table symbols which we saw above. Let's try to understand what these symbols 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_top_pgt`. It starts with the `4096` bytes of zeros (or `8192` bytes if `CONFIG_PAGE_TABLE_ISOLATION` is enabled), it means that we don't use the first `512` entries. And after this we can see `level3_kernel_pgt` entry. At the start of its definition, we can see that it is filled with the `4080` bytes of zeros (`L3_START_KERNEL` equals `510`). Subsequently, it stores two entries which map kernel space. Note that we subtract `__START_KERNEL_map` from `level2_kernel_pgt` and `level2_fixmap_pgt`. 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 addresses of the `level2_kernel_pgt` and `level2_fixmap_pgt`.
Next let's look at `_KERNPG_TABLE_NOENC` and `_PAGE_TABLE_NOENC`, these are just page entry access rights:
The `level2_kernel_pgt` is page table entry which contains pointer to the page middle directory which maps kernel space. It calls the `PDMS` macro which creates `512` megabytes from the `__START_KERNEL_map` for kernel `.text` (after these `512` megabytes will be module memory space).
The `level2_fixmap_pgt` is a virtual addresses which can refer to any physical addresses even under kernel space. They are represented by the `4048` bytes of zeros, the `level1_fixmap_pgt` entry, `8` megabytes reserved for [vsyscalls](https://lwn.net/Articles/446528/) mapping and `2` megabytes of hole.
All of `p*d` addresses may be wrong if the `startup_64` is not equal to default `0x1000000` address. Remember that the `load_delta` 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 we add the delta to the certain entries of the `p*d`.
Note that we didn't fixup base address of the `early_top_pgt` and some of other page table directories, because we will see this when building/filling structures of 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 identically. Let's look at it in detail. First of all we replace `pud` and `pmd` with the pointer to first and second entry of `early_dynamic_pgts`:
`sme_get_me_mask` function returns `sme_me_mask` which was initialized in `sme_enable` function.
Next we fill two entries of `pgd` with `pud` plus `pgtable_flags` which we initialized above:
```C
i = (physaddr >> PGDIR_SHIFT) % PTRS_PER_PGD;
pgd[i + 0] = (pgdval_t)pud + pgtable_flags;
pgd[i + 1] = (pgdval_t)pud + pgtable_flags;
```
`PGDIR_SHFT` indicates the mask for page global directory bits in a virtual address. Here we calculate modulo with `PTRS_PER_PGD` (which expands to `512`) so as not to access the index greater than `512`. There are macro for all types of page directories:
Next we fixup the kernel text+data virtual addresses. Note that we might write invalid pmds, when the kernel is relocated (`cleanup_highmap` function fixes this up along with the mappings beyond `_end`).
As final step of `__startup_64` function, we encrypt the kernel (if SME is active) and return the SME encryption mask to be used as a modifier for the initial page directory entry programmed into `cr3` register:
```C
sme_encrypt_kernel(bp);
return sme_get_me_mask();
```
Now let's get back to assembly code. We prepare for next paragraph with following code:
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) with following assembly code:
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/master/arch/x86/kernel/head_64.S) code file and looks like:
The `THREAD_SIZE` macro is defined in the [arch/x86/include/asm/page_64_types.h](https://github.com/torvalds/linux/blob/master/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](https://github.com/torvalds/linux/blob/master/Documentation/dev-tools/kasan.rst) 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 processes](https://en.wikipedia.org/wiki/Child_process). 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.
The `init_thread_union` is represented by the `thread_union`. And the `thread_union` is defined in the [include/linux/sched.h](https://github.com/torvalds/linux/blob/master/include/linux/sched.h) file like the following:
The `CONFIG_ARCH_TASK_STRUCT_ON_STACK` kernel configuration option is only enabled for `ia64` architecture, and the `CONFIG_THREAD_INFO_IN_TASK` kernel configuration option is enabled for `x86_64` architecture. Thus the `thread_info` structure will be placed in `task_struct` structure instead of the `thread_union` union.
The `init_thread_union` is placed in the [include/asm-generic/vmlinux.lds.h](https://github.com/torvalds/blob/master/include/asm-generic/vmlinux.lds.h) file as part of the `INIT_TASK_DATA` macro like the following:
This macro is used in the [arch/x86/kernel/vmlinux.lds.S](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/vmlinux.lds.S) file like the following:
that `initial_stack` symbol points to the start of the `thread_union.stack` array + `THREAD_SIZE` which is 16 killobytes and - `SIZEOF_PTREGS` which is convention which helps the in-kernel unwinder reliably detect the end of the stack.
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:
Now let's look at the definition of `early_gdt_descr`. `GDT_ENTRIES` expands to `32` so that Global Descriptor Table contains `32` entries for kernel code, data, thread local storage segments and etc...
Now let's look at the definition of `early_gdt_descr_base`. The `gdt_page` structure is defined in the [arch/x86/include/asm/desc.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/asm/desc.h) as:
which looks familiar `GDT` descriptor. Note that `gdt_page` structure is aligned to `PAGE_SIZE` which is `4096` bytes. Which means that `gdt` will occupy one page.
Now let's try to understand what `INIT_PER_CPU_VAR` is. `INIT_PER_CPU_VAR` is a macro which is defined in the [arch/x86/include/asm/percpu.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/asm/percpu.h) and just concatenates `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 the initialization of `init_per_cpu__gdt_page` in the [linker script](https://github.com/torvalds/linux/blob/master/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 are 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 [Concepts/per-cpu](https://0xax.gitbook.io/linux-insides/summary/concepts/linux-cpu-1) 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 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 a descriptor which is 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) occurs, 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 the return 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/master/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/master/init/main.c).
There are checks for different things like virtual address of module 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 zero all Page Global Directory entries. 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_top_pgt` to the `cr3`.
After this we clear `_bss` from the `__bss_stop` to `__bss_start` and also clear `init_top_pgt`. `init_top_pgt` is defined in the [arch/x86/kerne/head_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/head_64.S) like the following:
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).**
