Documentation/riscv/vm-layout.rst

   1 .. SPDX-License-Identifier: GPL-2.0
   2
   3 =====================================
   4 Virtual Memory Layout on RISC-V Linux
   5 =====================================
   6
   7 :Author: Alexandre Ghiti <alex@ghiti.fr>
   8 :Date: 12 February 2021
   9
  10 This document describes the virtual memory layout used by the RISC-V Linux
  11 Kernel.
  12
  13 RISC-V Linux Kernel 32bit
  14 =========================
  15
  16 RISC-V Linux Kernel SV32
  17 ------------------------
  18
  19 TODO
  20
  21 RISC-V Linux Kernel 64bit
  22 =========================
  23
  24 The 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
  26 occur.": that splits the virtual address space into 2 halves separated by a very
  27 big hole, the lower half is where the userspace resides, the upper half is where
  28 the RISC-V Linux Kernel resides.
  29
  30 RISC-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
  66 RISC-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   __________________|____________|__________________|_________|____________________________________________________________