[RFC PATCH 11/12] riscv: Explicit comment about user virtual address space size

[Alexandre Ghiti]

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 <alex at ghiti.fr>
Reviewed-by: Anup Patel <anup at brainfault.org>
Reviewed-by: Palmer Dabbelt <palmerdabbelt at google.com>
---
 arch/riscv/include/asm/pgtable.h | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
index 95721016049d..360858cdbfdd 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -465,8 +465,15 @@ static inline int ptep_clear_flush_young(struct vm_area_struct *vma,
 #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.
+ * -   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)
-- 
2.20.1