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

[Chunyan Zhang]

Hi Alex,

On Tue, 5 Nov 2024 at 20:45, Alexandre Ghiti <alex at ghiti.fr> wrote:
>
> 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 at iscas.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

pte_devmap() is defined in include/linux/mm.h as well, we have to use
#ifdef to avoid compile error since we will change the
CONFIG_ARCH_HAS_PTE_DEVMAP to be selectable on riscv.

> _PAGE_DEVMAP if !CONFIG_ARCH_HAS_PTE_DEVMAP.
>
> I'd be in favor of removing the #ifdef around pte_devmap() too.

So we cannot remove the #ifdef here, the compiler would report
redefinition errors if CONFIG_ARCH_HAS_PTE_DEVMAP is not selected.

Thanks for the review,
Chunyan