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:
____________________________________________________________|___________________________________________________________
                  |            |                  |         |
 ffffffc6fee00000 | -228    GB | ffffffc6feffffff |    2 MB | fixmap
 ffffffc6ff000000 | -228    GB | ffffffc6ffffffff |   16 MB | PCI io
 ffffffc700000000 | -228    GB | ffffffc7ffffffff |    4 GB | vmemmap
 ffffffc800000000 | -224    GB | ffffffd7ffffffff |   64 GB | vmalloc/ioremap space
 ffffffd800000000 | -160    GB | fffffff6ffffffff |  124 GB | direct mapping of all physical memory
 fffffff700000000 |  -36    GB | fffffffeffffffff |   32 GB | kasan
__________________|____________|__________________|_________|____________________________________________________________
                                                            |
                                                            |
____________________________________________________________|____________________________________________________________
                  |            |                  |         |
 ffffffff00000000 |   -4    GB | ffffffff7fffffff |    2 GB | modules, BPF
 ffffffff80000000 |   -2    GB | ffffffffffffffff |    2 GB | kernel
__________________|____________|__________________|_________|____________________________________________________________

RISC-V Linux Kernel SV48

========================================================================================================================
     Start addr    |   Offset   |     End addr     |  Size   | VM area description
========================================================================================================================
                   |            |                  |         |
  0000000000000000 |    0       | 00007fffffffffff |  128 TB | user-space virtual memory, different per mm
 __________________|____________|__________________|_________|___________________________________________________________
                   |            |                  |         |
  0000800000000000 | +128    TB | ffff7fffffffffff | ~16M TB | ... huge, almost 64 bits wide hole of non-canonical
                   |            |                  |         | virtual memory addresses up to the -128 TB
                   |            |                  |         | starting offset of kernel mappings.
 __________________|____________|__________________|_________|___________________________________________________________
                                                             |
                                                             | Kernel-space virtual memory, shared between all processes:
 ____________________________________________________________|___________________________________________________________
                   |            |                  |         |
  ffff8d7ffee00000 |  -114.5 TB | ffff8d7ffeffffff |    2 MB | fixmap
  ffff8d7fff000000 |  -114.5 TB | ffff8d7fffffffff |   16 MB | PCI io
  ffff8d8000000000 |  -114.5 TB | ffff8f7fffffffff |    2 TB | vmemmap
  ffff8f8000000000 |  -112.5 TB | ffffaf7fffffffff |   32 TB | vmalloc/ioremap space
  ffffaf8000000000 |  -80.5  TB | ffffef7fffffffff |   64 TB | direct mapping of all physical memory
  ffffef8000000000 |  -16.5  TB | fffffffeffffffff | 16.5 TB | kasan
 __________________|____________|__________________|_________|____________________________________________________________
                                                             |
                                                             | Identical layout to the 39-bit one from here on:
 ____________________________________________________________|____________________________________________________________
                   |            |                  |         |
  ffffffff00000000 |   -4    GB | ffffffff7fffffff |    2 GB | modules, BPF
  ffffffff80000000 |   -2    GB | ffffffffffffffff |    2 GB | kernel
 __________________|____________|__________________|_________|____________________________________________________________