Understanding the kernel code, and CR3 register

Recently, I was working on the kernel code and CR3 register. This article is going to record what I have learn.

First Question: How can we get the kernel code of Operating System?

Get Virtual Address of kernel code:

$ cat /boot/System.map-2.6.32-33-generic | grep text

c0100000 T_text

The first one is start virtual address of kernel code

Get Physical Address of kernel code:

$ cat /proc/iomem

00100000-00590de6 : Kernel Code

More info: please see

http://fengweizhang.blogspot.com/2010/12/boot-cat-system.html

http://jianggmulab.blogspot.com/2009/09/blog-post.html

Second Question: How to translate virtual address to physical address? 

Control Register 3 (CR3) stores the base address of Page Directory Table. More info please see page 47 in ULK book. 

Third Question: Which CR3 I should use to translate VA to PA of kernel code?

Any CR3. For each process, it has its own address spaces and CR3. No matter which processes the operating system is running, the virtual address and physical address of kernel code will never change. Although the CR3 is different depends on the running process, same VA will map to same PA by CR3.

Each process has its own address space. From the virtual address point of view, it has some address space at 0-3G region, and it also has some address space at 3-4G region (this is for kernel mode). All Processes get same mappings for the kernel address space (3-4G). Each process also has its own page directory table and page tables. CR3 is the base address of page directory table. The address space at 3-4G region must have its own translation entries in page directory table and page tables. These entries are always same in different page directory table and page tables. 

Process 1 has virtual address space from 0xc0100000-0xc0590de6 (kernel code), translate to physical address is 0x00100000-0x00590de6

Process 2 has virtual address space from 0xc0100000-0xc0590de6 (kernel code), translate to physical address is 0x00100000-0x00590de6

Thus, process 1 and process 2 has same kernel memory mappings. (CR3 is different)