==============================
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.

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.