| ============================== |
| 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. |
| |
| ARMv8.2 adds optional support for Large Virtual Address space. This is |
| only available when running with a 64KB page size and expands the |
| number of descriptors in the first level of translation. |
| |
| TTBRx selection is given by bit 55 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 + 4 levels (48-bit):: |
| |
| Start End Size Use |
| ----------------------------------------------------------------------- |
| 0000000000000000 0000ffffffffffff 256TB user |
| ffff000000000000 ffff7fffffffffff 128TB kernel logical memory map |
| [ffff600000000000 ffff7fffffffffff] 32TB [kasan shadow region] |
| ffff800000000000 ffff80007fffffff 2GB modules |
| ffff800080000000 fffffbffefffffff 124TB vmalloc |
| fffffbfff0000000 fffffbfffdffffff 224MB fixed mappings (top down) |
| fffffbfffe000000 fffffbfffe7fffff 8MB [guard region] |
| fffffbfffe800000 fffffbffff7fffff 16MB PCI I/O space |
| fffffbffff800000 fffffbffffffffff 8MB [guard region] |
| fffffc0000000000 fffffdffffffffff 2TB vmemmap |
| fffffe0000000000 ffffffffffffffff 2TB [guard region] |
| |
| |
| AArch64 Linux memory layout with 64KB pages + 3 levels (52-bit with HW support):: |
| |
| Start End Size Use |
| ----------------------------------------------------------------------- |
| 0000000000000000 000fffffffffffff 4PB user |
| fff0000000000000 ffff7fffffffffff ~4PB kernel logical memory map |
| [fffd800000000000 ffff7fffffffffff] 512TB [kasan shadow region] |
| ffff800000000000 ffff80007fffffff 2GB modules |
| ffff800080000000 fffffbffefffffff 124TB vmalloc |
| fffffbfff0000000 fffffbfffdffffff 224MB fixed mappings (top down) |
| fffffbfffe000000 fffffbfffe7fffff 8MB [guard region] |
| fffffbfffe800000 fffffbffff7fffff 16MB PCI I/O space |
| fffffbffff800000 fffffbffffffffff 8MB [guard region] |
| fffffc0000000000 ffffffdfffffffff ~4TB vmemmap |
| ffffffe000000000 ffffffffffffffff 128GB [guard region] |
| |
| |
| 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 |
| +----------------------------------------> [55] 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 (48-bit) |
| | [51:42] L1 index (52-bit) |
| +----------------------------------------> [55] 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_SPECTRE_V3A is enabled for particular CPUs. |
| |
| When using KVM with the Virtualization Host Extensions, no additional |
| mappings are created, since the host kernel runs directly in EL2. |
| |
| 52-bit VA support in the kernel |
| ------------------------------- |
| If the ARMv8.2-LVA optional feature is present, and we are running |
| with a 64KB page size; then it is possible to use 52-bits of address |
| space for both userspace and kernel addresses. However, any kernel |
| binary that supports 52-bit must also be able to fall back to 48-bit |
| at early boot time if the hardware feature is not present. |
| |
| This fallback mechanism necessitates the kernel .text to be in the |
| higher addresses such that they are invariant to 48/52-bit VAs. Due |
| to the kasan shadow being a fraction of the entire kernel VA space, |
| the end of the kasan shadow must also be in the higher half of the |
| kernel VA space for both 48/52-bit. (Switching from 48-bit to 52-bit, |
| the end of the kasan shadow is invariant and dependent on ~0UL, |
| whilst the start address will "grow" towards the lower addresses). |
| |
| In order to optimise phys_to_virt and virt_to_phys, the PAGE_OFFSET |
| is kept constant at 0xFFF0000000000000 (corresponding to 52-bit), |
| this obviates the need for an extra variable read. The physvirt |
| offset and vmemmap offsets are computed at early boot to enable |
| this logic. |
| |
| As a single binary will need to support both 48-bit and 52-bit VA |
| spaces, the VMEMMAP must be sized large enough for 52-bit VAs and |
| also must be sized large enough to accommodate a fixed PAGE_OFFSET. |
| |
| Most code in the kernel should not need to consider the VA_BITS, for |
| code that does need to know the VA size the variables are |
| defined as follows: |
| |
| VA_BITS constant the *maximum* VA space size |
| |
| VA_BITS_MIN constant the *minimum* VA space size |
| |
| vabits_actual variable the *actual* VA space size |
| |
| |
| Maximum and minimum sizes can be useful to ensure that buffers are |
| sized large enough or that addresses are positioned close enough for |
| the "worst" case. |
| |
| 52-bit userspace VAs |
| -------------------- |
| To maintain compatibility with software that relies on the ARMv8.0 |
| VA space maximum size of 48-bits, the kernel will, by default, |
| return virtual addresses to userspace from a 48-bit range. |
| |
| Software can "opt-in" to receiving VAs from a 52-bit space by |
| specifying an mmap hint parameter that is larger than 48-bit. |
| |
| For example: |
| |
| .. code-block:: c |
| |
| maybe_high_address = mmap(~0UL, size, prot, flags,...); |
| |
| It is also possible to build a debug kernel that returns addresses |
| from a 52-bit space by enabling the following kernel config options: |
| |
| .. code-block:: sh |
| |
| CONFIG_EXPERT=y && CONFIG_ARM64_FORCE_52BIT=y |
| |
| Note that this option is only intended for debugging applications |
| and should not be used in production. |
