In the previous [part](https://0xax.gitbook.io/linux-insides/summary/initialization/linux-initialization-1) we stopped before setting of early interrupt handlers. At this moment we are in the decompressed Linux kernel, we have basic [paging](https://en.wikipedia.org/wiki/Page_table) structure for early boot and our current goal is to finish early preparation before the main kernel code will start to work.
We already started to do this preparation in the previous [first](https://0xax.gitbook.io/linux-insides/summary/initialization/linux-initialization-1) part of this [chapter](https://0xax.gitbook.io/linux-insides/summary/initialization). We continue in this part and will know more about interrupt and exception handling.
from the [arch/x86/kernel/head64.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/head64.c) source code file. But before we start to sort out this function, we need to know about interrupts and handlers.
An interrupt is an event caused by software or hardware to the CPU. For example a user have pressed a key on keyboard. On interrupt, CPU stops the current task and transfer control to the special routine which is called - [interrupt handler](https://en.wikipedia.org/wiki/Interrupt_handler). An interrupt handler handles and interrupt and transfer control back to the previously stopped task. We can split interrupts on three types:
Every interrupt and exception is assigned a unique number which is called - `vector number`. `Vector number` can be any number from `0` to `255`. There is common practice to use first `32` vector numbers for exceptions, and vector numbers from `32` to `255` are used for user-defined interrupts.
CPU uses vector number as an index in the `Interrupt Descriptor Table` (we will see description of it soon). CPU catches interrupts from the [APIC](http://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller) or through its pins. Following table shows `0-31` exceptions:
To react on interrupt CPU uses special structure - Interrupt Descriptor Table or IDT. IDT is an array of 8-byte descriptors like Global Descriptor Table, but IDT entries are called `gates`. CPU multiplies vector number by 8 to find the IDT entry. But in 64-bit mode IDT is an array of 16-byte descriptors and CPU multiplies vector number by 16 to find the entry in the IDT. We remember from the previous part that CPU uses special `GDTR` register to locate Global Descriptor Table, so CPU uses special register `IDTR` for Interrupt Descriptor Table and `lidt` instruction for loading base address of the table into this register.
Interrupt and trap gates contain a far pointer to the entry point of the interrupt handler. Only one difference between these types is how CPU handles `IF` flag. If interrupt handler was accessed through interrupt gate, CPU clear the `IF` flag to prevent other interrupts while current interrupt handler executes. After that current interrupt handler executes, CPU sets the `IF` flag again with `iret` instruction.
`idt_setup_early_handler` is defined in the [arch/x86/kernel/idt.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/idt.c) like the following:
```C
void __init idt_setup_early_handler(void)
{
int i;
for (i = 0; i <NUM_EXCEPTION_VECTORS;i++)
set_intr_gate(i, early_idt_handler_array[i]);
load_idt(&idt_descr);
}
```
where `NUM_EXCEPTION_VECTORS` expands to `32`. As we can see, We're filling only first 32 `IDT` entries in the loop, because all of the early setup runs with interrupts disabled, so there is no need to set up interrupt handlers for vectors greater than `32`. Here we call `set_intr_gate` in the loop, which takes two parameters:
The `early_idt_handler_array` array is declared in the [arch/x86/include/asm/segment.h](https://github.com/torvalds/linux/blob/master/arch/x86/include/asm/segment.h) header file and contains addresses of the first `32` exception handlers:
The `early_idt_handler_array` is `288` bytes array which contains address of exception entry points every nine bytes. Every nine bytes of this array consist of two bytes optional instruction for pushing dummy error code if an exception does not provide it, two bytes instruction for pushing vector number to the stack and five bytes of `jump` to the common exception handler code. You will see more detail in the next paragraph.
The `set_intr_gate` function is defined in the [arch/x86/kernel/idt.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/idt.c) source file and looks:
First of all it checks that passed vector number is not greater than `255` with `BUG_ON` macro. We need to do this because we are limited to have up to `256` interrupts. After this, we fill the idt data with the given arguments and others, which will be passed to `idt_setup_from_table`. The `idt_setup_from_table` function is defined in the same file as the `set_intr_gate` function like the following:
which fill temporary idt descriptor with the given arguments and others. And then we just copy it to the certain element of the `idt_table` array. `idt_table` is an array of idt entries:
Okay, now we have filled and loaded `Interrupt Descriptor Table`, we know how the CPU acts during an interrupt. So now time to deal with interrupts handlers.
As you can read above, we filled `IDT` with the address of the `early_idt_handler_array`. In this section, we are going to look into it in detail. We can find it in the [arch/x86/kernel/head_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/head_64.S) assembly file:
We can see here, interrupt handlers generation for the first `32` exceptions. We check here, if exception has an error code then we do nothing, if exception does not return error code, we push zero to the stack. We do it for that stack was uniform. After that we push `vector number` on the stack and jump on the `early_idt_handler_common` which is generic interrupt handler for now. After all, every nine bytes of the `early_idt_handler_array` array consists of optional push of an error code, push of `vector number` and jump instruction to `early_idt_handler_common`. We can see it in the output of the `objdump` util:
As we may know, CPU pushes flag register, `CS` and `RIP` on the stack before calling interrupt handler. So before `early_idt_handler_common` will be executed, stack will contain following data:
Now let's look on the `early_idt_handler_common` implementation. It locates in the same [arch/x86/kernel/head_64.S](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/head_64.S) assembly file. First of all we increment `early_recursion_flag` to prevent recursion in the `early_idt_handler_common`:
We need to do it to prevent wrong values of registers when we return from the interrupt handler. After this we check the vector number, and if it is `#PF` or [Page Fault](https://en.wikipedia.org/wiki/Page_fault), we put value from the `cr2` to the `rdi` register and call `early_make_pgtable` (we'll see it soon):
In the previous paragraph we saw the early interrupt handler which checks if the vector number is page fault and calls `early_make_pgtable` for building new page tables if it is. We need to have `#PF` handler in this step because there are plans to add ability to load kernel above `4G` and make access to `boot_params` structure above the 4G.
You can find the implementation of `early_make_pgtable` in [arch/x86/kernel/head64.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/head64.c) and takes one parameter - the value of `cr2` register, which contains the address caused page fault. Let's look on it:
`__PAGE_OFFSET` is defined in the [arch/x86/include/asm/page_64_types.h](https://elixir.bootlin.com/linux/v3.10-rc1/source/arch/x86/include/asm/page_64_types.h#L33) header file, and the suffix `UL` forces the page offset to be a unsigned long data type.
And the `_AC` macro is defined in the [include/uapi/linux/const.h](https://elixir.bootlin.com/linux/v3.10-rc1/source/include/uapi/linux/const.h#L16) header file:
Where `__PAGE_OFFSET` expands to `0xffff888000000000`. But, why is it possible to translate a virtual address to a physical address by subtracting `__PAGE_OFFSET`? The answer is in the [Documentation/x86/x86_64/mm.rst](https://elixir.bootlin.com/linux/v5.10-rc5/source/Documentation/x86/x86_64/mm.rst#L45) documentation:
As explained above, the virtual address space `ffff888000000000-ffffc87fffffffff` is direct mapping of all physical memory. When the kernel wants to access all physical memory, it uses direct mapping.
Okay, let's get back to discussing `early_make_pgtable`. We initialize `pmd` and pass it to the `__early_make_pgtable` function along with `address`. The `__early_make_pgtable` function is defined in the same file as the `early_make_pgtable` function as follows:
It starts from the definition of some variables which have `*val_t` types. All of these types are declared as alias of `unsigned long` using `typedef`.
After we made the check that we have no invalid address, we're getting the address of the Page Global Directory entry which contains base address of Page Upper Directory and put its value to the `pgd` variable:
If `pgd` is not presented, we check if `next_early_pgt` is not greater than `EARLY_DYNAMIC_PAGE_TABLES` which is `64` and present a fixed number of buffers to set up new page tables on demand. If `next_early_pgt` is greater than `EARLY_DYNAMIC_PAGE_TABLES` we reset page tables and start again from `again` label. If `next_early_pgt` is less than `EARLY_DYNAMIC_PAGE_TABLES`, we assign the next entry of `early_dynamic_pgts` to `pud_p` and fill whole entry of the page upper directory with `0`, then fill the page global directory entry with the base address and some access rights:
In the end we assign the given `pmd` which is passed by the `early_make_pgtable` function to the certain entry of page middle directory which maps kernel text+data virtual addresses:
In early interrupt phase, exceptions other than page fault are handled by `early_fixup_exception` function which is defined in [arch/x86/mm/extable.c](https://github.com/torvalds/linux/blob/master/arch/x86/mm/extable.c) and takes two parameters - pointer to kernel stack which consists of saved registers and vector number:
The `fixup_exception` function finds the actual handler and call it. It is defined in the same file as `early_fixup_exception` function as the following:
The `search_exception_tables` function looks up the given address in the exception table (i.e. the contents of the ELF section, `__ex_table`). After that, we get the actual address by `ex_fixup_handler` function. At last we call actual handler. For more information about exception table, you can refer to [Documentation/x86/exception-tables.txt](https://github.com/torvalds/linux/blob/master/Documentation/x86/exception-tables.txt).
The `fixup_bug` function is defined in [arch/x86/kernel/traps.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/traps.c). Let's have a look on the function implementation:
All what this function does is just returns `1` if the exception is generated because `#UD` (or [Invalid Opcode](https://wiki.osdev.org/Exceptions#Invalid_Opcode)) occurred and the `report_bug` function returns `BUG_TRAP_TYPE_WARN`, otherwise returns `0`.
This is the end of the second part about Linux kernel insides. If you have questions or suggestions, ping me in twitter [0xAX](https://twitter.com/0xAX), drop me [email](mailto:anotherworldofworld@gmail.com) or just create [issue](https://github.com/0xAX/linux-insides/issues/new). In the next part we will see all steps before kernel entry point - `start_kernel` function.
**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).**
