Documentation/arch/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    ffffffc6fea00000 | -228    GB | ffffffc6feffffff |    6 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    ffff8d7ffea00000 |  -114.5 TB | ffff8d7ffeffffff |    6 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   __________________|____________|__________________|_________|____________________________________________________________
 100
 101
 102 RISC-V Linux Kernel SV57
 103 ------------------------
 104
 105 ::
 106
 107  ========================================================================================================================
 108       Start addr    |   Offset   |     End addr     |  Size   | VM area description
 109  ========================================================================================================================
 110                     |            |                  |         |
 111    0000000000000000 |   0        | 00ffffffffffffff |   64 PB | user-space virtual memory, different per mm
 112   __________________|____________|__________________|_________|___________________________________________________________
 113                     |            |                  |         |
 114    0100000000000000 | +64     PB | feffffffffffffff | ~16K PB | ... huge, almost 64 bits wide hole of non-canonical
 115                     |            |                  |         | virtual memory addresses up to the -64 PB
 116                     |            |                  |         | starting offset of kernel mappings.
 117   __________________|____________|__________________|_________|___________________________________________________________
 118                                                               |
 119                                                               | Kernel-space virtual memory, shared between all processes:
 120   ____________________________________________________________|___________________________________________________________
 121                     |            |                  |         |
 122    ff1bfffffea00000 | -57     PB | ff1bfffffeffffff |    6 MB | fixmap
 123    ff1bffffff000000 | -57     PB | ff1bffffffffffff |   16 MB | PCI io
 124    ff1c000000000000 | -57     PB | ff1fffffffffffff |    1 PB | vmemmap
 125    ff20000000000000 | -56     PB | ff5fffffffffffff |   16 PB | vmalloc/ioremap space
 126    ff60000000000000 | -40     PB | ffdeffffffffffff |   32 PB | direct mapping of all physical memory
 127    ffdf000000000000 |  -8     PB | fffffffeffffffff |    8 PB | kasan
 128   __________________|____________|__________________|_________|____________________________________________________________
 129                                                               |
 130                                                               | Identical layout to the 39-bit one from here on:
 131   ____________________________________________________________|____________________________________________________________
 132                     |            |                  |         |
 133    ffffffff00000000 |  -4     GB | ffffffff7fffffff |    2 GB | modules, BPF
 134    ffffffff80000000 |  -2     GB | ffffffffffffffff |    2 GB | kernel
 135   __________________|____________|__________________|_________|____________________________________________________________
 136
 137
 138 Userspace VAs
 139 --------------------
 140 To maintain compatibility with software that relies on the VA space with a
 141 maximum of 48 bits the kernel will, by default, return virtual addresses to
 142 userspace from a 48-bit range (sv48). This default behavior is achieved by
 143 passing 0 into the hint address parameter of mmap. On CPUs with an address space
 144 smaller than sv48, the CPU maximum supported address space will be the default.
 145
 146 Software can "opt-in" to receiving VAs from another VA space by providing
 147 a hint address to mmap. A hint address passed to mmap will cause the largest
 148 address space that fits entirely into the hint to be used, unless there is no
 149 space left in the address space. If there is no space available in the requested
 150 address space, an address in the next smallest available address space will be
 151 returned.
 152
 153 For example, in order to obtain 48-bit VA space, a hint address greater than
 154 :code:`1 << 47` must be provided. Note that this is 47 due to sv48 userspace
 155 ending at :code:`1 << 47` and the addresses beyond this are reserved for the
 156 kernel. Similarly, to obtain 57-bit VA space addresses, a hint address greater
 157 than or equal to :code:`1 << 56` must be provided.