64 lines
3.5 KiB
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64 lines
3.5 KiB
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.. SPDX-License-Identifier: GPL-2.0
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=====================================
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Virtual Memory Layout on RISC-V Linux
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=====================================
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:Author: Alexandre Ghiti <alex@ghiti.fr>
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:Date: 12 February 2021
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This document describes the virtual memory layout used by the RISC-V Linux
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Kernel.
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RISC-V Linux Kernel 32bit
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=========================
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RISC-V Linux Kernel SV32
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------------------------
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TODO
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RISC-V Linux Kernel 64bit
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=========================
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The RISC-V privileged architecture document states that the 64bit addresses
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"must have bits 63–48 all equal to bit 47, or else a page-fault exception will
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occur.": that splits the virtual address space into 2 halves separated by a very
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big hole, the lower half is where the userspace resides, the upper half is where
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the RISC-V Linux Kernel resides.
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RISC-V Linux Kernel SV39
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------------------------
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::
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========================================================================================================================
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Start addr | Offset | End addr | Size | VM area description
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========================================================================================================================
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0000000000000000 | 0 | 0000003fffffffff | 256 GB | user-space virtual memory, different per mm
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__________________|____________|__________________|_________|___________________________________________________________
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| | | |
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0000004000000000 | +256 GB | ffffffbfffffffff | ~16M TB | ... huge, almost 64 bits wide hole of non-canonical
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| | | | virtual memory addresses up to the -256 GB
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| | | | starting offset of kernel mappings.
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__________________|____________|__________________|_________|___________________________________________________________
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| Kernel-space virtual memory, shared between all processes:
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____________________________________________________________|___________________________________________________________
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ffffffc000000000 | -256 GB | ffffffc7ffffffff | 32 GB | kasan
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ffffffcefea00000 | -196 GB | ffffffcefeffffff | 6 MB | fixmap
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ffffffceff000000 | -196 GB | ffffffceffffffff | 16 MB | PCI io
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ffffffcf00000000 | -196 GB | ffffffcfffffffff | 4 GB | vmemmap
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ffffffd000000000 | -192 GB | ffffffdfffffffff | 64 GB | vmalloc/ioremap space
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ffffffe000000000 | -128 GB | ffffffff7fffffff | 124 GB | direct mapping of all physical memory
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__________________|____________|__________________|_________|____________________________________________________________
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____________________________________________________________|____________________________________________________________
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ffffffff00000000 | -4 GB | ffffffff7fffffff | 2 GB | modules, BPF
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ffffffff80000000 | -2 GB | ffffffffffffffff | 2 GB | kernel
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__________________|____________|__________________|_________|____________________________________________________________
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