This new document presents the RISC-V virtual memory layout and is based
one the x86 one: it describes the different limits of the different regions
of the virtual address space.
Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
---
Documentation/riscv/index.rst | 1 +
Documentation/riscv/vm-layout.rst | 61 +++++++++++++++++++++++++++++++
2 files changed, 62 insertions(+)
create mode 100644 Documentation/riscv/vm-layout.rst
diff --git a/Documentation/riscv/index.rst b/Documentation/riscv/index.rst
index 6e6e39482502..ea915c196048 100644
--- a/Documentation/riscv/index.rst
+++ b/Documentation/riscv/index.rst
@@ -6,6 +6,7 @@ RISC-V architecture
:maxdepth: 1
boot-image-header
+ vm-layout
pmu
patch-acceptance
diff --git a/Documentation/riscv/vm-layout.rst b/Documentation/riscv/vm-layout.rst
new file mode 100644
index 000000000000..e8e569e2686a
--- /dev/null
+++ b/Documentation/riscv/vm-layout.rst
@@ -0,0 +1,61 @@
+=====================================
+Virtual Memory Layout on RISC-V Linux
+=====================================
+
+:Author: Alexandre Ghiti <alex@xxxxxxxx>
+: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 | 126 GB | direct mapping of all physical memory
+ __________________|____________|__________________|_________|____________________________________________________________
+ |
+ |
+ ____________________________________________________________|____________________________________________________________
+ | | | |
+ ffffffff00000000 | -4 GB | ffffffff7fffffff | 2 GB | modules
+ ffffffff80000000 | -2 GB | ffffffffffffffff | 2 GB | kernel, BPF
+ __________________|____________|__________________|_________|____________________________________________________________
--
2.20.1
