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diff --git a/Documentation/riscv/vm-layout.rst b/Documentation/riscv/vm-layout.rst new file mode 100644 index 000000000000..329d32098af4 --- /dev/null +++ b/Documentation/riscv/vm-layout.rst @@ -0,0 +1,63 @@ +.. 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 + __________________|____________|__________________|_________|____________________________________________________________ |