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, BPF
 ffffffff80000000 |   -2    GB | ffffffffffffffff |    2 GB | kernel
__________________|____________|__________________|_________|____________________________________________________________