linux-kernel - Re: [RESEND PATCH V4 1/3] riscv: mm: Prepare for reusing PTE RSW bit(9)

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Message-ID: <bedac8d9-4ba4-427c-9323-edcdbd7876ee@ghiti.fr>
Date: Tue, 5 Nov 2024 13:45:44 +0100
From: Alexandre Ghiti <alex@...ti.fr>
To: Chunyan Zhang <zhangchunyan@...as.ac.cn>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
 <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Chunyan Zhang <zhang.lyra@...il.com>
Subject: Re: [RESEND PATCH V4 1/3] riscv: mm: Prepare for reusing PTE RSW
 bit(9)

Hi Chunyan,

On 30/08/2024 03:10, Chunyan Zhang wrote:
> The PTE bit(9) on RISC-V is reserved for software, it is used by DEVMAP
> now which has to be disabled if we want to use bit(9) for other features,
> since there's no more free PTE bit on RISC-V now.
>
> So to make ARCH_HAS_PTE_DEVMAP selectable, this patch uses it as
> the build condition of devmap definitions.
>
> Signed-off-by: Chunyan Zhang <zhangchunyan@...as.ac.cn>
> ---
>   arch/riscv/include/asm/pgtable-64.h   | 2 +-
>   arch/riscv/include/asm/pgtable-bits.h | 6 ++++++
>   2 files changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/include/asm/pgtable-64.h b/arch/riscv/include/asm/pgtable-64.h
> index 0897dd99ab8d..babb8d2b0f0b 100644
> --- a/arch/riscv/include/asm/pgtable-64.h
> +++ b/arch/riscv/include/asm/pgtable-64.h
> @@ -398,7 +398,7 @@ static inline struct page *pgd_page(pgd_t pgd)
>   #define p4d_offset p4d_offset
>   p4d_t *p4d_offset(pgd_t *pgd, unsigned long address);
>   
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && defined(CONFIG_ARCH_HAS_PTE_DEVMAP)
>   static inline int pte_devmap(pte_t pte);
>   static inline pte_t pmd_pte(pmd_t pmd);
>   
> diff --git a/arch/riscv/include/asm/pgtable-bits.h b/arch/riscv/include/asm/pgtable-bits.h
> index a8f5205cea54..5bcc73430829 100644
> --- a/arch/riscv/include/asm/pgtable-bits.h
> +++ b/arch/riscv/include/asm/pgtable-bits.h
> @@ -19,7 +19,13 @@
>   #define _PAGE_SOFT      (3 << 8)    /* Reserved for software */
>   
>   #define _PAGE_SPECIAL   (1 << 8)    /* RSW: 0x1 */
> +
> +#ifdef CONFIG_ARCH_HAS_PTE_DEVMAP
>   #define _PAGE_DEVMAP    (1 << 9)    /* RSW, devmap */
> +#else
> +#define _PAGE_DEVMAP	0
> +#endif /* CONFIG_ARCH_HAS_PTE_DEVMAP */
> +
>   #define _PAGE_TABLE     _PAGE_PRESENT
>   
>   /*


There is a small inconsistency in our code: pte_devmap() is protected by 
a #ifdef but pte_mkdevmap() is not, I guess that's why you had to define 
_PAGE_DEVMAP if !CONFIG_ARCH_HAS_PTE_DEVMAP.

I'd be in favor of removing the #ifdef around pte_devmap() too.

Thanks,

Alex