Re: [RFC PATCH v2 08/11] ARM64: mm: Swap PTE_FILE and PTE_PROT_NONE bits.

[Christopher Covington <cov@codeaurora.org>]

Hi Steve,

On 05/08/2013 05:52 AM, Steve Capper wrote:
> Under ARM64, PTEs can be broadly categorised as follows:
>    - Present and valid: Bit #0 is set. The PTE is valid and memory
>      access to the region may fault.
> 
>    - Present and invalid: Bit #0 is clear and bit #1 is set.
>      Represents present memory with PROT_NONE protection. The PTE
>      is an invalid entry, and the user fault handler will raise a
>      SIGSEGV.
> 
>    - Not present (file): Bits #0 and #1 are clear, bit #2 is set.
>      Memory represented has been paged out. The PTE is an invalid
>      entry, and the fault handler will try and re-populate the
>      memory where necessary.
> 
> Huge PTEs are block descriptors that have bit #1 clear. If we wish
> to represent PROT_NONE huge PTEs we then run into a problem as
> there is no way to distinguish between regular and huge PTEs if we
> set bit #1.
> 
> As huge PTEs are always present, the meaning of bits #1 and #2 can
> be swapped for invalid PTEs. This patch swaps the PTE_FILE and
> PTE_PROT_NONE constants, allowing us to represent PROT_NONE huge
> PTEs.

[...]

> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h

[...]

> @@ -306,8 +306,8 @@ extern pgd_t idmap_pg_dir[PTRS_PER_PGD];
>  
>  /*
>   * Encode and decode a file entry:
> - *	bits 0-1:	present (must be zero)
> - *	bit  2:		PTE_FILE
> + *	bits 0 & 2:	present (must be zero)

Consider using punctuation like "bits 0, 2" here to disambiguate from the
binary and operation.

[...]

Christopher

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