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 without the Virtualization Host Extensions, the hypervisor maps kernel pages in EL2 at a fixed (and potentially random) offset from the linear mapping. See the kern_hyp_va macro and kvm_update_va_mask function for more details. MMIO devices such as GICv2 gets mapped next to the HYP idmap page, as do vectors when ARM64_HARDEN_EL2_VECTORS is selected for particular CPUs.
When using KVM with the Virtualization Host Extensions, no additional mappings are created, since the host kernel runs directly in EL2.