diff --git a/Initialization/linux-initialization-2.md b/Initialization/linux-initialization-2.md index 34fd00b..e21ecc1 100644 --- a/Initialization/linux-initialization-2.md +++ b/Initialization/linux-initialization-2.md @@ -442,9 +442,17 @@ And the `_AC` macro is defined in the [include/uapi/linux/const.h](https://elixi #define _AC(X,Y) __AC(X,Y) #endif ``` -So, where `__PAGE_OFFSET` expands to `0xffff880000000000`. +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: -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 the following: +``` +... +ffff888000000000 | -119.5 TB | ffffc87fffffffff | 64 TB | direct mapping of all physical memory (page_offset_base) +... +``` + +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: ```C int __init __early_make_pgtable(unsigned long address, pmdval_t pmd)