[RFC PATCH 0/2] riscv: document the Sv32 virtual memory layout

[RFC PATCH 0/2] riscv: document the Sv32 virtual memory layout

[Yu Jin]

The RISC-V virtual memory layout documentation describes the 64-bit
paging modes but has left the Sv32 section as a TODO. The layout is
already defined by the RV32 page-table macros and can be observed in the
boot-time layout printed with CONFIG_DEBUG_VM.

RV32 remains relevant to current hardware rather than only legacy
systems. For example, Espressif announced the ESP32-S31 in March 2026 as
a dual-core 32-bit RISC-V microcontroller with MMU support.

This series documents the existing rv32_defconfig layout and corrects a
nearby comment that gives a stale fixed value for RV32 TASK_SIZE. The
implementation is unchanged.

The main point on which I would appreciate feedback is whether this is
the clearest way to present the configuration-dependent part of the
layout. This is my first Linux kernel contribution, so I would also
appreciate guidance on any process or presentation issues.

Tested on QEMU with a kernel based on commit 08dbfad3f504:

  - rv32_defconfig plus CONFIG_DEBUG_VM=y
  - full RV32 kernel and modules build with GCC 13.3.0
  - boot with qemu-system-riscv32 8.2.2 and 512 MB or 4 GB RAM
  - userspace reached using a minimal initramfs
  - runtime layout matched the documented addresses
  - the 4 GB boot exposed 1 GB of managed RAM, matching the current
    direct-map and no-highmem limitations
  - make SPHINXDIRS=arch/riscv htmldocs
  - scripts/checkpatch.pl --strict, no errors or warnings per patch

The boot-time layout was:

  fixmap  : 0x9c800000 - 0x9d000000
  pci io  : 0x9d000000 - 0x9e000000
  vmemmap : 0x9e000000 - 0xa0000000
  vmalloc : 0xa0000000 - 0xc0000000
  lowmem  : 0xc0000000 - 0xdfc00000

Yu Jin (2):
  docs: riscv: document the Sv32 virtual memory layout
  riscv: mm: correct the stale RV32 TASK_SIZE comment

 Documentation/arch/riscv/vm-layout.rst | 28 +++++++++++++++++++++++++-
 arch/riscv/include/asm/pgtable.h       |  6 ++----
 2 files changed, 29 insertions(+), 5 deletions(-)


base-commit: 08dbfad3f5040f5bdb6c529da20d6d4e81fefd72
-- 
2.43.0

[RFC PATCH 1/2] docs: riscv: document the Sv32 virtual memory layout

[Yu Jin]

The RISC-V virtual memory layout documentation describes the 64-bit
paging modes but leaves the Sv32 section as a TODO.

Document the rv32_defconfig layout and clarify which boundaries depend
on the configured size of struct page. Distinguish the maximum direct-map
range from the portion backed by physical memory. Also explain why the
34-bit physical addresses encoded by Sv32 page table entries do not make
all of that address space usable as RAM by the current RV32 kernel.

Signed-off-by: Yu Jin <lambda.jinyu@gmail.com>
---
 Documentation/arch/riscv/vm-layout.rst | 28 +++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/Documentation/arch/riscv/vm-layout.rst b/Documentation/arch/riscv/vm-layout.rst
index eabec99b5852..8e1447cf574e 100644
--- a/Documentation/arch/riscv/vm-layout.rst
+++ b/Documentation/arch/riscv/vm-layout.rst
@@ -16,7 +16,33 @@ RISC-V Linux Kernel 32bit
 RISC-V Linux Kernel SV32
 ------------------------
 
-TODO
+Sv32 provides a 4GB virtual address space, with the upper part used by the
+kernel. Unlike the 64-bit layouts, the kernel image is part of the direct
+mapping. The following layout is produced by ``rv32_defconfig``.
+
+::
+
+  ========================================================================================================================
+      Start addr    |   Offset   |     End addr     |  Size   | VM area description
+  ========================================================================================================================
+                    |            |                  |         |
+           00000000 |    0       |         9c7fffff | 2504 MB | user-space virtual memory, different per mm
+  __________________|____________|__________________|_________|___________________________________________________________
+                                                              |
+                                                              | Kernel-space virtual memory, shared between all processes:
+  ____________________________________________________________|___________________________________________________________
+                    |            |                  |         |
+           9c800000 | -1592   MB |         9cffffff |    8 MB | fixmap
+           9d000000 | -1584   MB |         9dffffff |   16 MB | PCI io
+           9e000000 | -1568   MB |         9fffffff |   32 MB | vmemmap-sized reserved area
+           a0000000 |   -1.5  GB |         bfffffff |  512 MB | vmalloc/ioremap/modules; BPF JIT uses the last 128 MB
+           c0000000 |   -1    GB |         ffffffff |    1 GB | direct mapping of physical memory
+  __________________|____________|__________________|_________|___________________________________________________________
+
+The layout before ``VMALLOC_START`` depends on the configured size of
+``struct page``. The direct mapping row shows its maximum range; usable RAM is
+limited by its mapped portion and the 32-bit ``phys_addr_t``, despite the
+34-bit physical addresses encoded by Sv32 page table entries.
 
 RISC-V Linux Kernel 64bit
 =========================
-- 
2.43.0

[RFC PATCH 2/2] riscv: mm: correct the stale RV32 TASK_SIZE comment

[Yu Jin]

RV32 defines TASK_SIZE as FIXADDR_START. Its value depends on
VMEMMAP_SIZE, which in turn depends on the configured size of struct
page. The fixed 0x9fc00000 value in the comment is therefore stale.

Describe the macro relationship instead of replacing one
configuration-specific address with another.

Signed-off-by: Yu Jin <lambda.jinyu@gmail.com>
---
 arch/riscv/include/asm/pgtable.h | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
index 40b1ed4f3ea8..32348787462c 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -1231,10 +1231,8 @@ static inline pte_t pte_swp_clear_exclusive(pte_t pte)
 #endif
 
 /*
- * Task size is 0x4000000000 for RV64 or 0x9fc00000 for RV32.
- * Note that PGDIR_SIZE must evenly divide TASK_SIZE.
- * Task size is:
- * -        0x9fc00000	(~2.5GB) for RV32.
+ * For RV32, TASK_SIZE is FIXADDR_START and depends on VMEMMAP_SIZE.
+ * For RV64, task size is:
  * -      0x4000000000	( 256GB) for RV64 using SV39 mmu
  * -    0x800000000000	( 128TB) for RV64 using SV48 mmu
  * - 0x100000000000000	(  64PB) for RV64 using SV57 mmu
-- 
2.43.0