Subject: Re: [RFC PATCH] powerpc/book3s/hash: Drop _PAGE_PRIVILEGED from PAGE_NONE

From: "Nicholas Piggin" <[email-masked]>
Date: Mon, 13 Nov 2023 20:16:41 +1000

On Thu Nov 2, 2023 at 11:23 PM AEST, Aneesh Kumar K.V wrote:
> There used to be a dependency on _PAGE_PRIVILEGED with pte_savedwrite.
> But that got dropped by
> commit 6a56ccbcf6c6 ("mm/autonuma: use can_change_(pte|pmd)_writable() to replace savedwrite")
>
> With this change numa fault pte (pte_protnone()) gets mapped as regular user pte
> with RWX cleared (no-access).

You mean "that" above change (not *this* change), right?

> This also remove pte_user() from
> book3s/64.

Nice cleanup. That was an annoying hack.

> pte_access_permitted() now checks for _PAGE_EXEC because we now support
> EXECONLY mappings.

AFAIKS pte_exec() is not required, GUP is really only for read or
write access. It should be a separate patch if you think it's needed.

>
> Signed-off-by: Aneesh Kumar K.V <[email-masked]>
> ---
>  arch/powerpc/include/asm/book3s/64/pgtable.h | 23 +++++---------------
>  arch/powerpc/mm/book3s64/hash_utils.c        | 17 +++++++++++++++
>  2 files changed, 23 insertions(+), 17 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
> index cb77eddca54b..7c7de7b56df0 100644
> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
> @@ -17,12 +17,6 @@
>  #define _PAGE_EXEC		0x00001 /* execute permission */
>  #define _PAGE_WRITE		0x00002 /* write access allowed */
>  #define _PAGE_READ		0x00004	/* read access allowed */
> -#define _PAGE_NA		_PAGE_PRIVILEGED
> -#define _PAGE_NAX		_PAGE_EXEC
> -#define _PAGE_RO		_PAGE_READ
> -#define _PAGE_ROX		(_PAGE_READ | _PAGE_EXEC)
> -#define _PAGE_RW		(_PAGE_READ | _PAGE_WRITE)
> -#define _PAGE_RWX		(_PAGE_READ | _PAGE_WRITE | _PAGE_EXEC)
>  #define _PAGE_PRIVILEGED	0x00008 /* kernel access only */
>  #define _PAGE_SAO		0x00010 /* Strong access order */
>  #define _PAGE_NON_IDEMPOTENT	0x00020 /* non idempotent memory */

Did you leave PAGE_NONE as _PAGE_BASE | _PAGE_PRIVILEGED below?
Shouldn't that be changed too? Then this patch is not only hash
but also radix.

Why is the hash change required? Previously PAGE_NONE relied on
privileged bit to prevent access, now you need to handle a PTE
without that bit? In that case could that be patch 1, then the
rest patch 2?

__pte_flags_need_flush() should be updated after this too,
basically revert commit 1abce0580b894.

> @@ -119,9 +113,9 @@
>  /*
>   * user access blocked by key
>   */
> -#define _PAGE_KERNEL_RW		(_PAGE_PRIVILEGED | _PAGE_RW | _PAGE_DIRTY)
>  #define _PAGE_KERNEL_RO		 (_PAGE_PRIVILEGED | _PAGE_READ)
>  #define _PAGE_KERNEL_ROX	 (_PAGE_PRIVILEGED | _PAGE_READ | _PAGE_EXEC)
> +#define _PAGE_KERNEL_RW		(_PAGE_PRIVILEGED | _PAGE_RW | _PAGE_DIRTY)
>  #define _PAGE_KERNEL_RWX	(_PAGE_PRIVILEGED | _PAGE_DIRTY | _PAGE_RW | _PAGE_EXEC)
>  /*
>   * _PAGE_CHG_MASK masks of bits that are to be preserved across

No need to reorder defines.

Thanks,
Nick

> @@ -523,19 +517,14 @@ static inline bool arch_pte_access_permitted(u64 pte, bool write, bool execute)
>  }
>  #endif /* CONFIG_PPC_MEM_KEYS */
>  
> -static inline bool pte_user(pte_t pte)
> -{
> -	return !(pte_raw(pte) & cpu_to_be64(_PAGE_PRIVILEGED));
> -}
> -
>  #define pte_access_permitted pte_access_permitted
>  static inline bool pte_access_permitted(pte_t pte, bool write)
>  {
> -	/*
> -	 * _PAGE_READ is needed for any access and will be
> -	 * cleared for PROT_NONE
> -	 */
> -	if (!pte_present(pte) || !pte_user(pte) || !pte_read(pte))
> +
> +	if (!pte_present(pte))
> +		return false;
> +
> +	if (!(pte_read(pte) || pte_exec(pte)))
>  		return false;
>  
>  	if (write && !pte_write(pte))
> diff --git a/arch/powerpc/mm/book3s64/hash_utils.c b/arch/powerpc/mm/book3s64/hash_utils.c
> index ad2afa08e62e..b2eda22195f0 100644
> --- a/arch/powerpc/mm/book3s64/hash_utils.c
> +++ b/arch/powerpc/mm/book3s64/hash_utils.c
> @@ -310,9 +310,26 @@ unsigned long htab_convert_pte_flags(unsigned long pteflags, unsigned long flags
>  			else
>  				rflags |= 0x3;
>  		}
> +		WARN_ON(!(pteflags & _PAGE_RWX));
>  	} else {
>  		if (pteflags & _PAGE_RWX)
>  			rflags |= 0x2;
> +		else {
> +			/*
> +			 * PAGE_NONE will get mapped to 0b110 (slb key 1 no access)
> +			 * We picked 0b110 instead of 0b000 so that slb key 0 will
> +			 * get only read only access for the same rflags.
> +			 */
> +			if (mmu_has_feature(MMU_FTR_KERNEL_RO))
> +				rflags |= (HPTE_R_PP0 | 0x2);
> +			/*
> +			 * rflags = HPTE_R_N
> +			 * Without KERNEL_RO feature this will result in slb
> +			 * key 0 with read/write. But ISA only supports that.
> +			 * There is no key 1 no-access and key 0 read-only
> +			 * pp bit support.
> +			 */
> +		}
>  		if (!((pteflags & _PAGE_WRITE) && (pteflags & _PAGE_DIRTY)))
>  			rflags |= 0x1;
>  	}