ARM Linux kernel page table

Question

Ref. Linux kernel ARM Translation table base (TTB0 and TTB1)

I have father doubt/query on topic discussed in previous link:

1. 0 to 0xbfffff

Answer 1

Because of how the ARM MMU was designed, both the translations tables (TTB0 and TTB1) can only be used in a 1:1 mapping kernel mapping.

Most Linux Kernels have a 3:1 mapping (3GB User space : 1GB Kernel space for ARM). This means that 0-0xBFFFFFFF is user space while 0xC0000000 - 0xFFFFFFFF is kernel space.

Now for the HW memory translations, only TTBR0 is used. TTBR1 only holds the address of the initial swapper page (which contains all the kernel mappings) and isn't really used for virtual address translations. TTBR0 hold the address for the current used page directory (the page table that the HW is using for translations). Now each user process has their own page tables, and for each process switch, TTBR0 changes to the current user process page table (they are all located in kernel space).

For example, for each new user process, the kernel creates a new page directory, copies all the kernel mappings from the swapper page(page frames from 3-4GB) to the new page table and clears the user pages(page frames from 0-3GB). It then sets TTB0 to the base address of this page directory and flushes cache to install the new address space. The swapper page is also always kept up to date with changes to the mappings.

For your question:

1. Simplified, hardwarewise the first level page have 4096 entries. Each entry represent 1MB of virtual address, totalling 4GB of ram. Entry 0-3071 represent user space and entry 3072-4095 represent kernel space.

2. The swapper page is usually located at address 0xC0004000 - 0xc0008000 (4096 entries *4bytes each entry = 16384 =16kb in hex = 0x4000 ). By examing the memory at 0xc0004000-0xc0007000 you can find entries for user space (empty) and from 0xc0007000-0xc0008000 you can find kernel entries. I use gdb with the command line x /100x 0xc0007000 in order to examine the first 100 kernel entries. You can then examine the technical reference manual for your current platform in order to decipher the page table attributes.

If you want to learn more about the Linux kernel, I recommend you to use Qemu to simulate the Beagleboard together with gdb to examine and debug the source code. I did this to learn how the kernel builds the page table during initialization.