arm64: Add a description on 48-bit address space with 4KB pages

This patch adds memory layout and translation lookup information
about 48-bit address space with 4K pages. The description is based
on 4 levels of translation tables.
Signed-off-by: Jungseok Lee <jays.lee@samsung.com>
Reviewed-by: Sungjinn Chung <sungjinn.chung@samsung.com>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Jungseok Lee <jungseoklee85@gmail.com>

Documentation/arm64/memory.txt

@@ -8,10 +8,11 @@ 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 3 levels of translation tables with the 4KB page
-configuration, allowing 39-bit (512GB) virtual addresses for both user
-and kernel. With 64KB pages, only 2 levels of translation tables are
-used but the memory layout is the same.
+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 63:39 set to 0 while the kernel addresses have
 the same bits set to 1. TTBRx selection is given by bit 63 of the
@@ -21,7 +22,7 @@ The swapper_pgd_dir address is written to TTBR1 and never written to
 TTBR0.
 
 
-AArch64 Linux memory layout with 4KB pages:
+AArch64 Linux memory layout with 4KB pages + 3 levels:
 
 Start			End			Size		Use
 -----------------------------------------------------------------------
@@ -48,7 +49,34 @@ ffffffbffc000000	ffffffbfffffffff	  64MB		modules
 ffffffc000000000	ffffffffffffffff	 256GB		kernel logical memory map
 
 
-AArch64 Linux memory layout with 64KB pages:
+AArch64 Linux memory layout with 4KB pages + 4 levels:
+
+Start			End			Size		Use
+-----------------------------------------------------------------------
+0000000000000000	0000ffffffffffff	 256TB		user
+
+ffff000000000000	ffff7bfffffeffff	~124TB		vmalloc
+
+ffff7bffffff0000	ffff7bffffffffff	  64KB		[guard page]
+
+ffff7c0000000000	ffff7dffffffffff	   2TB		vmemmap
+
+ffff7e0000000000	ffff7ffffbbfffff	  ~2TB		[guard, future vmmemap]
+
+ffff7ffffa000000	ffff7ffffaffffff	  16MB		PCI I/O space
+
+ffff7ffffb000000	ffff7ffffbbfffff	  12MB		[guard]
+
+ffff7ffffbc00000	ffff7ffffbdfffff	   2MB		fixed mappings
+
+ffff7ffffbe00000	ffff7ffffbffffff	   2MB		[guard]
+
+ffff7ffffc000000	ffff7fffffffffff	  64MB		modules
+
+ffff800000000000	ffffffffffffffff	 128TB		kernel logical memory map
+
+
+AArch64 Linux memory layout with 64KB pages + 2 levels:
 
 Start			End			Size		Use
 -----------------------------------------------------------------------
@@ -75,7 +103,7 @@ fffffdfffc000000	fffffdffffffffff	  64MB		modules
 fffffe0000000000	ffffffffffffffff	   2TB		kernel logical memory map
 
 
-Translation table lookup with 4KB pages:
+Translation table lookup with 4KB pages + 3 levels:
 
 +--------+--------+--------+--------+--------+--------+--------+--------+
 |63    56|55    48|47    40|39    32|31    24|23    16|15     8|7      0|
@@ -90,7 +118,22 @@ Translation table lookup with 4KB pages:
  +-------------------------------------------------> [63] TTBR0/1
 
 
-Translation table lookup with 64KB pages:
+Translation table lookup with 4KB pages + 4 levels:
+
++--------+--------+--------+--------+--------+--------+--------+--------+
+|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 + 2 levels:
 
 +--------+--------+--------+--------+--------+--------+--------+--------+
 |63    56|55    48|47    40|39    32|31    24|23    16|15     8|7      0|