Memory layout on aarch64 linux

This document describes the virtual memory layout used by the AArch64
Linux kernel. The architecture allows up to 4 levels of translation
tables with a 4KB page size and up to 3 levels with a 64KB page size.

AArch64 Linux uses 3 levels of translation tables with the 4KB page
configuration, allowing 39-bit (512GB) virtual addresses for both user
and kernel. With 64KB pages, only 2 levels of translation tables are
used but the memory layout is the same.

User addresses have bits 63:39 set to 0 while the kernel addresses have
the same bits set to 1. TTBRx selection is given by bit 63 of the
virtual address. The swapper_pg_dir contains only kernel (global)
mappings while the user pgd contains only user (non-global) mappings.
The swapper_pgd_dir address is written to TTBR1 and never written to
TTBR0.


AArch64 Linux memory layout:

Start           End         Size        Use
-----------------------------------------------------------------------
0000000000000000    0000007fffffffff     512GB      user

ffffff8000000000    ffffffbbfffeffff    ~240GB      vmalloc

ffffffbbffff0000    ffffffbbffffffff      64KB      [guard page]

ffffffbc00000000    ffffffbdffffffff       8GB      vmemmap

ffffffbe00000000    ffffffbffbbfffff      ~8GB      [guard, future vmmemap]

ffffffbffbc00000    ffffffbffbdfffff       2MB      earlyprintk device

ffffffbffbe00000    ffffffbffbe0ffff      64KB      PCI I/O space

ffffffbbffff0000    ffffffbcffffffff      ~2MB      [guard]

ffffffbffc000000    ffffffbfffffffff      64MB      modules

ffffffc000000000    ffffffffffffffff     256GB      kernel logical memory map


Translation table lookup with 4KB pages:

+--------+--------+--------+--------+--------+--------+--------+--------+
|63    56|55    48|47    40|39    32|31    24|23    16|15     8|7      0|
+--------+--------+--------+--------+--------+--------+--------+--------+
 |                 |         |         |         |         |
 |                 |         |         |         |         v
 |                 |         |         |         |   [11:0]  in-page offset
 |                 |         |         |         +-> [20:12] L3 index
 |                 |         |         +-----------> [29:21] L2 index
 |                 |         +---------------------> [38:30] L1 index
 |                 +-------------------------------> [47:39] L0 index (not used)
 +-------------------------------------------------> [63] TTBR0/1


Translation table lookup with 64KB pages:

+--------+--------+--------+--------+--------+--------+--------+--------+
|63    56|55    48|47    40|39    32|31    24|23    16|15     8|7      0|
+--------+--------+--------+--------+--------+--------+--------+--------+
 |                 |    |               |              |
 |                 |    |               |              v
 |                 |    |               |            [15:0]  in-page offset
 |                 |    |               +----------> [28:16] L3 index
 |                 |    +--------------------------> [41:29] L2 index (only 38:29 used)
 |                 +-------------------------------> [47:42] L1 index (not used)
 +-------------------------------------------------> [63] TTBR0/1