Re: [PATCH v2 3/4] arm: mm: introduce L_PTE_SPECIAL

[Russell King - ARM Linux admin]

On Fri, Oct 23, 2020 at 05:14:36PM +0800, Miles Chen wrote:
> From: Minchan Kim <minchan@xxxxxxxxxx>
> 
> This patch introduces L_PTE_SPECIAL and pte functions for supporting
> get_user_pages_fast.
> 
> Cc: Russell King <linux@xxxxxxxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Cc: Steve Capper <steve.capper@xxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> Signed-off-by: Miles Chen <miles.chen@xxxxxxxxxxxx>
> ---
>  arch/arm/Kconfig                      |  4 ++--
>  arch/arm/include/asm/pgtable-2level.h |  1 +
>  arch/arm/include/asm/pgtable-3level.h |  6 ------
>  arch/arm/include/asm/pgtable.h        | 13 +++++++++++++
>  4 files changed, 16 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index c18fa9d382b7..1f75864b7c7a 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -13,7 +13,7 @@ config ARM
>  	select ARCH_HAS_KCOV
>  	select ARCH_HAS_MEMBARRIER_SYNC_CORE
>  	select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
> -	select ARCH_HAS_PTE_SPECIAL if ARM_LPAE
> +	select ARCH_HAS_PTE_SPECIAL if (ARM_LPAE || CPU_V7 || CPU_V6 || CPUV6K)
>  	select ARCH_HAS_PHYS_TO_DMA
>  	select ARCH_HAS_SETUP_DMA_OPS
>  	select ARCH_HAS_SET_MEMORY
> @@ -82,7 +82,7 @@ config ARM
>  	select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE
>  	select HAVE_EFFICIENT_UNALIGNED_ACCESS if (CPU_V6 || CPU_V6K || CPU_V7) && MMU
>  	select HAVE_EXIT_THREAD
> -	select HAVE_FAST_GUP if ARM_LPAE
> +	select HAVE_FAST_GUP if (ARM_LPAE || CPU_V7 || CPU_V6 || CPUV6K)
>  	select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
>  	select HAVE_FUNCTION_GRAPH_TRACER if !THUMB2_KERNEL && !CC_IS_CLANG
>  	select HAVE_FUNCTION_TRACER if !XIP_KERNEL
> diff --git a/arch/arm/include/asm/pgtable-2level.h b/arch/arm/include/asm/pgtable-2level.h
> index cdcd55cca37d..385e7a32394e 100644
> --- a/arch/arm/include/asm/pgtable-2level.h
> +++ b/arch/arm/include/asm/pgtable-2level.h
> @@ -117,6 +117,7 @@
>  #define L_PTE_VALID		(_AT(pteval_t, 1) << 0)		/* Valid */
>  #define L_PTE_PRESENT		(_AT(pteval_t, 1) << 0)
>  #define L_PTE_YOUNG		(_AT(pteval_t, 1) << 1)
> +#define L_PTE_SPECIAL		(_AT(pteval_t, 1) << 5)

How does this work?  Bits 2 through 5 are already in use for the memory
type.

Repurposing this bit means that L_PTE_MT_DEV_NONSHARED,
L_PTE_MT_DEV_WC, L_PTE_MT_DEV_CACHED and L_PTE_MT_VECTORS clash with
it.

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