1.. SPDX-License-Identifier: GPL-2.0
2
3=====================================
4Virtual Memory Layout on RISC-V Linux
5=====================================
6
7:Author: Alexandre Ghiti <alex@ghiti.fr>
8:Date: 12 February 2021
9
10This document describes the virtual memory layout used by the RISC-V Linux
11Kernel.
12
13RISC-V Linux Kernel 32bit
14=========================
15
16RISC-V Linux Kernel SV32
17------------------------
18
19TODO
20
21RISC-V Linux Kernel 64bit
22=========================
23
24The RISC-V privileged architecture document states that the 64bit addresses
25"must have bits 63–48 all equal to bit 47, or else a page-fault exception will
26occur.": that splits the virtual address space into 2 halves separated by a very
27big hole, the lower half is where the userspace resides, the upper half is where
28the RISC-V Linux Kernel resides.
29
30RISC-V Linux Kernel SV39
31------------------------
32
33::
34
35  ========================================================================================================================
36      Start addr    |   Offset   |     End addr     |  Size   | VM area description
37  ========================================================================================================================
38                    |            |                  |         |
39   0000000000000000 |    0       | 0000003fffffffff |  256 GB | user-space virtual memory, different per mm
40  __________________|____________|__________________|_________|___________________________________________________________
41                    |            |                  |         |
42   0000004000000000 | +256    GB | ffffffbfffffffff | ~16M TB | ... huge, almost 64 bits wide hole of non-canonical
43                    |            |                  |         |     virtual memory addresses up to the -256 GB
44                    |            |                  |         |     starting offset of kernel mappings.
45  __________________|____________|__________________|_________|___________________________________________________________
46                                                              |
47                                                              | Kernel-space virtual memory, shared between all processes:
48  ____________________________________________________________|___________________________________________________________
49                    |            |                  |         |
50   ffffffc6fee00000 | -228    GB | ffffffc6feffffff |    2 MB | fixmap
51   ffffffc6ff000000 | -228    GB | ffffffc6ffffffff |   16 MB | PCI io
52   ffffffc700000000 | -228    GB | ffffffc7ffffffff |    4 GB | vmemmap
53   ffffffc800000000 | -224    GB | ffffffd7ffffffff |   64 GB | vmalloc/ioremap space
54   ffffffd800000000 | -160    GB | fffffff6ffffffff |  124 GB | direct mapping of all physical memory
55   fffffff700000000 |  -36    GB | fffffffeffffffff |   32 GB | kasan
56  __________________|____________|__________________|_________|____________________________________________________________
57                                                              |
58                                                              |
59  ____________________________________________________________|____________________________________________________________
60                    |            |                  |         |
61   ffffffff00000000 |   -4    GB | ffffffff7fffffff |    2 GB | modules, BPF
62   ffffffff80000000 |   -2    GB | ffffffffffffffff |    2 GB | kernel
63  __________________|____________|__________________|_________|____________________________________________________________
64
65
66RISC-V Linux Kernel SV48
67------------------------
68
69::
70
71 ========================================================================================================================
72      Start addr    |   Offset   |     End addr     |  Size   | VM area description
73 ========================================================================================================================
74                    |            |                  |         |
75   0000000000000000 |    0       | 00007fffffffffff |  128 TB | user-space virtual memory, different per mm
76  __________________|____________|__________________|_________|___________________________________________________________
77                    |            |                  |         |
78   0000800000000000 | +128    TB | ffff7fffffffffff | ~16M TB | ... huge, almost 64 bits wide hole of non-canonical
79                    |            |                  |         | virtual memory addresses up to the -128 TB
80                    |            |                  |         | starting offset of kernel mappings.
81  __________________|____________|__________________|_________|___________________________________________________________
82                                                              |
83                                                              | Kernel-space virtual memory, shared between all processes:
84  ____________________________________________________________|___________________________________________________________
85                    |            |                  |         |
86   ffff8d7ffee00000 |  -114.5 TB | ffff8d7ffeffffff |    2 MB | fixmap
87   ffff8d7fff000000 |  -114.5 TB | ffff8d7fffffffff |   16 MB | PCI io
88   ffff8d8000000000 |  -114.5 TB | ffff8f7fffffffff |    2 TB | vmemmap
89   ffff8f8000000000 |  -112.5 TB | ffffaf7fffffffff |   32 TB | vmalloc/ioremap space
90   ffffaf8000000000 |  -80.5  TB | ffffef7fffffffff |   64 TB | direct mapping of all physical memory
91   ffffef8000000000 |  -16.5  TB | fffffffeffffffff | 16.5 TB | kasan
92  __________________|____________|__________________|_________|____________________________________________________________
93                                                              |
94                                                              | Identical layout to the 39-bit one from here on:
95  ____________________________________________________________|____________________________________________________________
96                    |            |                  |         |
97   ffffffff00000000 |   -4    GB | ffffffff7fffffff |    2 GB | modules, BPF
98   ffffffff80000000 |   -2    GB | ffffffffffffffff |    2 GB | kernel
99  __________________|____________|__________________|_________|____________________________________________________________
100