Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   Alexandre Ghiti <alex@ghiti.fr>
To:     Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Zong Li <zong.li@sifive.com>, Anup Patel <anup@brainfault.org>,
        Christoph Hellwig <hch@lst.de>,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Cc:     Alexandre Ghiti <alex@ghiti.fr>,
        Palmer Dabbelt <palmerdabbelt@google.com>
Subject: [PATCH v2 8/8] riscv: Explicit comment about user virtual address space size
Date:   Wed,  3 Jun 2020 04:11:04 -0400
Message-Id: <20200603081104.14004-9-alex@ghiti.fr>
In-Reply-To: <20200603081104.14004-1-alex@ghiti.fr>
References: <20200603081104.14004-1-alex@ghiti.fr>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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@ghiti.fr>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@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 cb8c6863266b..86bbc2ed1cdd 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -480,8 +480,15 @@ 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