linux/Documentation/riscv/vm-layout.rst
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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   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, BPF
  62   ffffffff80000000 |   -2    GB | ffffffffffffffff |    2 GB | kernel
  63  __________________|____________|__________________|_________|____________________________________________________________
  64