diff options
Diffstat (limited to 'Documentation/arm64/memory.rst')
| -rw-r--r-- | Documentation/arm64/memory.rst | 98 |
1 files changed, 98 insertions, 0 deletions
diff --git a/Documentation/arm64/memory.rst b/Documentation/arm64/memory.rst new file mode 100644 index 000000000000..464b880fc4b7 --- /dev/null +++ b/Documentation/arm64/memory.rst @@ -0,0 +1,98 @@ +============================== +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. |