└── README.md


/README.md:
--------------------------------------------------------------------------------
 1 | # Translate Virtual Address To Physical Address
 2 | 
 3 | The followings are some ways to translate a virtual address to the corresponding physical address, if exist.
 4 | 
 5 | Step 1. Walk the page tables to get `pte_t *`.
 6 | 
 7 | ```C
 8 | // struct mm_struct *mm : The memory descriptor of the current process.
 9 | // unsigned long address: The virtual address to be translated.
10 | pgd_t *pgd = pgd_offset(mm, address);
11 | if (!pgd_none(*pgd) && !pgd_bad(*pgd)) {
12 |     pud_t *pud = pud_offset(pgd, address);
13 |     if (!pud_none(*pud) && !pud_bad(*pud)) {
14 |         pmd_t *pmd = pmd_offset(pud, address);
15 |         if (!pmd_none(*pmd) && !pmd_bad(*pmd)) {
16 |             pte_t *pte = pte_offset_map(pmd, address);
17 |             if (!pte_none(*pte)) {
18 |                 // now you get a pointer pointing to a pte entry!
19 |             }
20 |             pte_unmap(pte);
21 |         }
22 |     }
23 | }       
24 | ```
25 | 
26 | Step 2.
27 | 
28 | - A) Calculate the bits directly.
29 | 
30 | ```C
31 | // Assume it is on 32-bit machine with 4 KB pages.
32 | unsigned long phys = pte_val(*pte) & 0xfffff000 + address & 0x00000fff;
33 | ```
34 | 
35 | - B) With the help of a page descriptor.
36 | 
37 | ```C
38 | struct page *pg = pte_page(*pte);
39 | unsigned long phys = page_to_phys(pg);
40 | ```
41 | 
42 | ## Userspace:
43 | 
44 | `cat /proc/<pid>/maps` to get virtual address intervals.
45 | 
46 | `xxd /proc/<pid>/pagemap` to get virtual - physical address mappings.
47 | [documention](https://www.mjmwired.net/kernel/Documentation/vm/pagemap.txt)
48 | 
49 | 
50 | ## Others:
51 | 
52 | - `virt_to_page(unsigned long address)` seems to be a faster way to get `struct page*`, but you have to make sure the return value is valid. ([code](https://elixir.free-electrons.com/linux/v3.9/source/arch/x86/include/asm/page.h#L63))
53 | 
54 | - `virt_to_phys(volatile void *address)` is only suitable for addresses directly mapped or mapped by kmalloc. ([code](https://elixir.free-electrons.com/linux/v3.9/source/arch/x86/include/asm/io.h#L111))
55 | 
56 | 
57 | ## Reference:
58 | - Understanding the Linux Kernel 3rd (Daniel P. Bovet, Marco Cesati), Sections 2.5, 8.1, 9.2, etc.
59 | - [page_to_phys](https://elixir.free-electrons.com/linux/v3.9/source/arch/x86/include/asm/io.h#L137) from <asm/io.h>
60 | - How to get the physical address from the logical one in a Linux kernel module? [Stack Overflow](https://stackoverflow.com/questions/6252063/how-to-get-the-physical-address-from-the-logical-one-in-a-linux-kernel-module)
61 | - How to find the physical address of a variable from user-space in Linux? [Stack Overflow](https://stackoverflow.com/questions/2440385/how-to-find-the-physical-address-of-a-variable-from-user-space-in-linux/13949855)
62 | 


--------------------------------------------------------------------------------