docs: arm64: Move arm64 documentation under Documentation/arch/

Architecture-specific documentation is being moved into Documentation/arch/
as a way of cleaning up the top-level documentation directory and making
the docs hierarchy more closely match the source hierarchy.  Move
Documentation/arm64 into arch/ (along with the Chinese equvalent
translations) and fix up documentation references.

Cc: Will Deacon <will@kernel.org>
Cc: Alex Shi <alexs@kernel.org>
Cc: Hu Haowen <src.res@email.cn>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Yantengsi <siyanteng@loongson.cn>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 167 insertions, 0 deletions

diff --git a/Documentation/arch/arm64/memory.rst b/Documentation/arch/arm64/memory.rst
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+==============================
+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.
+
+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 + 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	ffff800007ffffff	 128MB		modules
+  ffff800008000000	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	ffff800007ffffff	 128MB		modules
+  ffff800008000000	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
+   +-------------------------------------------------> [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 (48-bit)
+   |                                                   [51:42] L1 index (52-bit)
+   +-------------------------------------------------> [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_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.