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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 __________________|____________|__________________|_________|____________________________________________________________