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riscv: Explicit comment about user virtual address space size

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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 <alexandre.ghiti@canonical.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
This commit is contained in:
Alexandre Ghiti 2021-12-06 11:46:53 +01:00 committed by Palmer Dabbelt
parent 73c7c8f68e
commit c774de22c4
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1 changed files with 9 additions and 0 deletions
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9
arch/riscv/include/asm/pgtable.h
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@ -685,6 +685,15 @@ 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)