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   ffffffc000000000 | -256    GB | ffffffc7ffffffff |   32 GB | kasan
51   ffffffcefee00000 | -196    GB | ffffffcefeffffff |    2 MB | fixmap
52   ffffffceff000000 | -196    GB | ffffffceffffffff |   16 MB | PCI io
53   ffffffcf00000000 | -196    GB | ffffffcfffffffff |    4 GB | vmemmap
54   ffffffd000000000 | -192    GB | ffffffdfffffffff |   64 GB | vmalloc/ioremap space
55   ffffffe000000000 | -128    GB | ffffffff7fffffff |  124 GB | direct mapping of all physical memory
56  __________________|____________|__________________|_________|____________________________________________________________
57                                                              |
58                                                              |
59  ____________________________________________________________|____________________________________________________________
60                    |            |                  |         |
61   ffffffff00000000 |   -4    GB | ffffffff7fffffff |    2 GB | modules
62   ffffffff80000000 |   -2    GB | ffffffffffffffff |    2 GB | kernel, BPF
63  __________________|____________|__________________|_________|____________________________________________________________
64