[v3,09/13] riscv: Explicit comment about user virtual address space size

Commit Message

Alexandre Ghiti Dec. 6, 2021, 10:46 a.m. UTC

Define precisely the size of the user accessible virtual space size
for sv32/39/48 mmu types and explain why the whole virtual address
space is split into 2 equal chunks between kernel and user space.

Signed-off-by: Alexandre Ghiti <[e-mail]>
Reviewed-by: Anup Patel <[e-mail]>
Reviewed-by: Palmer Dabbelt <[e-mail]>
---
 arch/riscv/include/asm/pgtable.h | 9 +++++++++
 1 file changed, 9 insertions(+)

Patch

diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
index e1c74ef4ead2..fe1701329237 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -677,6 +677,15 @@  static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
 /*
  * 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.
+ * -   0x4000000000 ( 256GB) for RV64 using SV39 mmu
+ * - 0x800000000000 ( 128TB) for RV64 using SV48 mmu
+ *
+ * Note that PGDIR_SIZE must evenly divide TASK_SIZE since "RISC-V
+ * Instruction Set Manual Volume II: Privileged Architecture" states that
+ * "load and store effective addresses, which are 64bits, must have bits
+ * 63–48 all equal to bit 47, or else a page-fault exception will occur."
  */
 #ifdef CONFIG_64BIT
 #define TASK_SIZE      (PGDIR_SIZE * PTRS_PER_PGD / 2)