[Android]Memory Layout on AArch64 Linux

Documentation\arm64\memory.txt 通常配置是用4K pages, 39bits. userspace 和kernel space 各512GB ，中間部分地址不用。

相關kernel 配置： CONFIG_ARM64_4K_PAGES=y CONFIG_ARM64_64K_PAGES is not set CONFIG_ARM64_VA_BITS_39=y CONFIG_ARM64_VA_BITS_48 is not set CONFIG_ARM64_VA_BITS=39 CONFIG_ARM64_PGTABLE_LEVELS=3

Memory Layout on AArch64 Linux
	     ==============================

Author: Catalin Marinas catalin.marinas@arm.com

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 either 3 levels or 4 levels of translation tables with the 4KB page configuration, allowing 39-bit (512GB) or 48-bit (256TB) virtual addresses, respectively, for both user and kernel. With 64KB pages, only 2 levels of translation tables, allowing 42-bit (4TB) virtual address, are used but the memory layout is the same.

User addresses have bits 63:48 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_pg_dir address is written to TTBR1 and never written to TTBR0.

AArch64 Linux memory layout with 4KB pages + 3 levels:

Start End Size Use

0000000000000000 0000007fffffffff 512GB user ffffff8000000000 ffffffffffffffff 512GB kernel

AArch64 Linux memory layout with 4KB pages + 4 levels:

Start End Size Use

0000000000000000 0000ffffffffffff 256TB user ffff000000000000 ffffffffffffffff 256TB kernel

AArch64 Linux memory layout with 64KB pages + 2 levels:

Start End Size Use

0000000000000000 000003ffffffffff 4TB user fffffc0000000000 ffffffffffffffff 4TB kernel

AArch64 Linux memory layout with 64KB pages + 3 levels:

Start End Size Use

0000000000000000 0000ffffffffffff 256TB user ffff000000000000 ffffffffffffffff 256TB kernel

For details of the virtual kernel memory layout please see the kernel booting log.

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 +-------------------------------------------------> [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 | +-------------------------------> [47:42] L1 index +-------------------------------------------------> [63] TTBR0/1

When using KVM, the hypervisor maps kernel pages in EL2, at a fixed offset from the kernel VA (top 24bits of the kernel VA set to zero):

Start End Size Use

0000004000000000 0000007fffffffff 256GB kernel objects mapped in HYP