| .. 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: |
| ____________________________________________________________|___________________________________________________________ |
| | | | | |
| ffffffc6fea00000 | -228 GB | ffffffc6feffffff | 6 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: |
| ____________________________________________________________|___________________________________________________________ |
| | | | | |
| ffff8d7ffea00000 | -114.5 TB | ffff8d7ffeffffff | 6 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 |
| __________________|____________|__________________|_________|____________________________________________________________ |
| |
| |
| RISC-V Linux Kernel SV57 |
| ------------------------ |
| |
| :: |
| |
| ======================================================================================================================== |
| Start addr | Offset | End addr | Size | VM area description |
| ======================================================================================================================== |
| | | | | |
| 0000000000000000 | 0 | 00ffffffffffffff | 64 PB | user-space virtual memory, different per mm |
| __________________|____________|__________________|_________|___________________________________________________________ |
| | | | | |
| 0100000000000000 | +64 PB | feffffffffffffff | ~16K PB | ... huge, almost 64 bits wide hole of non-canonical |
| | | | | virtual memory addresses up to the -64 PB |
| | | | | starting offset of kernel mappings. |
| __________________|____________|__________________|_________|___________________________________________________________ |
| | |
| | Kernel-space virtual memory, shared between all processes: |
| ____________________________________________________________|___________________________________________________________ |
| | | | | |
| ff1bfffffea00000 | -57 PB | ff1bfffffeffffff | 6 MB | fixmap |
| ff1bffffff000000 | -57 PB | ff1bffffffffffff | 16 MB | PCI io |
| ff1c000000000000 | -57 PB | ff1fffffffffffff | 1 PB | vmemmap |
| ff20000000000000 | -56 PB | ff5fffffffffffff | 16 PB | vmalloc/ioremap space |
| ff60000000000000 | -40 PB | ffdeffffffffffff | 32 PB | direct mapping of all physical memory |
| ffdf000000000000 | -8 PB | fffffffeffffffff | 8 PB | 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 |
| __________________|____________|__________________|_________|____________________________________________________________ |
| |
| |
| Userspace VAs |
| -------------------- |
| To maintain compatibility with software that relies on the VA space with a |
| maximum of 48 bits the kernel will, by default, return virtual addresses to |
| userspace from a 48-bit range (sv48). This default behavior is achieved by |
| passing 0 into the hint address parameter of mmap. On CPUs with an address space |
| smaller than sv48, the CPU maximum supported address space will be the default. |
| |
| Software can "opt-in" to receiving VAs from another VA space by providing |
| a hint address to mmap. A hint address passed to mmap will cause the largest |
| address space that fits entirely into the hint to be used, unless there is no |
| space left in the address space. If there is no space available in the requested |
| address space, an address in the next smallest available address space will be |
| returned. |
| |
| For example, in order to obtain 48-bit VA space, a hint address greater than |
| :code:`1 << 47` must be provided. Note that this is 47 due to sv48 userspace |
| ending at :code:`1 << 47` and the addresses beyond this are reserved for the |
| kernel. Similarly, to obtain 57-bit VA space addresses, a hint address greater |
| than or equal to :code:`1 << 56` must be provided. |
