Unverified Commit 2a433cf8 authored by Alexandre Ghiti's avatar Alexandre Ghiti Committed by Palmer Dabbelt
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Documentation: riscv: Add documentation that describes the VM layout 



This new document presents the RISC-V virtual memory layout and is based
one the x86 one: it describes the different limits of the different regions
of the virtual address space.

Signed-off-by: default avatarAlexandre Ghiti <alex@ghiti.fr>
Signed-off-by: default avatarPalmer Dabbelt <palmerdabbelt@google.com>
parent 2bfc6cd8

Documentation/riscv/index.rst

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    :maxdepth: 1



    boot-image-header

    vm-layout

    pmu

    patch-acceptance

Documentation/riscv/vm-layout.rst

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.. SPDX-License-Identifier: GPL-2.0



=====================================

Virtual Memory Layout on RISC-V Linux

=====================================



:Author: Alexandre Ghiti <alex@ghiti.fr>

:Date: 12 February 2021



This document describes the virtual memory layout used by the RISC-V Linux

Kernel.



RISC-V Linux Kernel 32bit

=========================



RISC-V Linux Kernel SV32

------------------------



TODO



RISC-V Linux Kernel 64bit

=========================



The RISC-V privileged architecture document states that the 64bit addresses

"must have bits 63–48 all equal to bit 47, or else a page-fault exception will

occur.": that splits the virtual address space into 2 halves separated by a very

big hole, the lower half is where the userspace resides, the upper half is where

the RISC-V Linux Kernel resides.



RISC-V Linux Kernel SV39

------------------------



::



  ========================================================================================================================

      Start addr    |   Offset   |     End addr     |  Size   | VM area description

  ========================================================================================================================

                    |            |                  |         |

   0000000000000000 |    0       | 0000003fffffffff |  256 GB | user-space virtual memory, different per mm

  __________________|____________|__________________|_________|___________________________________________________________

                    |            |                  |         |

   0000004000000000 | +256    GB | ffffffbfffffffff | ~16M TB | ... huge, almost 64 bits wide hole of non-canonical

                    |            |                  |         |     virtual memory addresses up to the -256 GB

                    |            |                  |         |     starting offset of kernel mappings.

  __________________|____________|__________________|_________|___________________________________________________________

                                                              |

                                                              | Kernel-space virtual memory, shared between all processes:

  ____________________________________________________________|___________________________________________________________

                    |            |                  |         |

   ffffffc000000000 | -256    GB | ffffffc7ffffffff |   32 GB | kasan

   ffffffcefee00000 | -196    GB | ffffffcefeffffff |    2 MB | fixmap

   ffffffceff000000 | -196    GB | ffffffceffffffff |   16 MB | PCI io

   ffffffcf00000000 | -196    GB | ffffffcfffffffff |    4 GB | vmemmap

   ffffffd000000000 | -192    GB | ffffffdfffffffff |   64 GB | vmalloc/ioremap space

   ffffffe000000000 | -128    GB | ffffffff7fffffff |  124 GB | direct mapping of all physical memory

  __________________|____________|__________________|_________|____________________________________________________________

                                                              |

                                                              |

  ____________________________________________________________|____________________________________________________________

                    |            |                  |         |

   ffffffff00000000 |   -4    GB | ffffffff7fffffff |    2 GB | modules

   ffffffff80000000 |   -2    GB | ffffffffffffffff |    2 GB | kernel, BPF

  __________________|____________|__________________|_________|____________________________________________________________