arm64 page 4k和64k页面布局

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