Re: [PATCH V6] target/riscv: Ignore reserved bits in PTE for RV64

[Palmer Dabbelt <palmer@sifive.com>]

On Wed, 25 Sep 2019 17:14:21 PDT (-0700), guoren@kernel.org wrote:
> From: Guo Ren <ren_guo@c-sky.com>
>
> Highest 10 bits of PTE are reserved in riscv-privileged, ref: [1], so we
> need to ignore them. They cannot be a part of ppn.
>
> 1: The RISC-V Instruction Set Manual, Volume II: Privileged Architecture
>    4.4 Sv39: Page-Based 39-bit Virtual-Memory System
>    4.5 Sv48: Page-Based 48-bit Virtual-Memory System
>
> Signed-off-by: Guo Ren <ren_guo@c-sky.com>
> Tested-by: Bin Meng <bmeng.cn@gmail.com>
> Reviewed-by: Liu Zhiwei <zhiwei_liu@c-sky.com>
> Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
> Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
> ---
>  target/riscv/cpu_bits.h   | 7 +++++++
>  target/riscv/cpu_helper.c | 2 +-
>  2 files changed, 8 insertions(+), 1 deletion(-)
>
>  Changelog V6:
>   - Add Reviewer: Alistair Francis
>
>  Changelog V5:
>   - Add Reviewer and Tester: Bin Meng
>
>  Changelog V4:
>   - Change title to Ignore not Bugfix
>   - Use PTE_PPN_MASK for RV32 and RV64
>
>  Changelog V3:
>   - Use UUL define for PTE_RESERVED
>   - Keep ppn >> PTE_PPN_SHIFT
>
>  Changelog V2:
>   - Bugfix pte destroyed cause boot fail
>   - Change to AND with a mask instead of shifting both directions
>
> diff --git a/target/riscv/cpu_bits.h b/target/riscv/cpu_bits.h
> index e998348..399c2c6 100644
> --- a/target/riscv/cpu_bits.h
> +++ b/target/riscv/cpu_bits.h
> @@ -473,6 +473,13 @@
>  /* Page table PPN shift amount */
>  #define PTE_PPN_SHIFT       10
>
> +/* Page table PPN mask */
> +#if defined(TARGET_RISCV32)
> +#define PTE_PPN_MASK        0xffffffffUL
> +#elif defined(TARGET_RISCV64)
> +#define PTE_PPN_MASK        0x3fffffffffffffULL
> +#endif
> +
>  /* Leaf page shift amount */
>  #define PGSHIFT             12
>
> diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c
> index 87dd6a6..9961b37 100644
> --- a/target/riscv/cpu_helper.c
> +++ b/target/riscv/cpu_helper.c
> @@ -261,7 +261,7 @@ restart:
>  #elif defined(TARGET_RISCV64)
>          target_ulong pte = ldq_phys(cs->as, pte_addr);
>  #endif
> -        hwaddr ppn = pte >> PTE_PPN_SHIFT;
> +        hwaddr ppn = (pte & PTE_PPN_MASK) >> PTE_PPN_SHIFT;
>
>          if (!(pte & PTE_V)) {
>              /* Invalid PTE */

I know I'm a bit late to the party here, but I don't like this.  There's ample 
evidence that wrapping the physical address space is a bad idea, and just 
because the ISA allows implementations to do this doesn't mean we should.