Re: [PATCH 2/3] arm64: acpi: Map EFI_MEMORY_WT memory as Normal-NC

[Ard Biesheuvel <ardb@kernel.org>]

On Thu, 6 May 2021 at 11:50, Will Deacon <will@kernel.org> wrote:
>
> The only user we have of Normal Write-Through memory is in the ACPI code
> when mapping memory regions advertised as EFI_MEMORY_WT. Since most (all?)
> CPUs treat write-through as non-cacheable under the hood, don't bother
> with the extra memory type here and just treat EFI_MEMORY_WT the same way
> as EFI_MEMORY_WC by mapping it to the Normal-NC memory type instead.
>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Will Deacon <will@kernel.org>

I don't have any objections to this change per se, but I will point
out that the UEFI spec describes the MAIR encodings, paragraph 2.3.6.1
(in revision 2.8B). However, the paragraph in question provides no
context whatsoever, and so it is not clear whether it is normative,
and whether it applies to the boot time firmware only or to the OS as
well.

So in summary, given that EFI_MEMORY_WT (which I have never seen being
used on ARM) should behave as expected when using the same MAIR
attributes as EFI_MEMORY_WC, with only a theoretical performance
impact, the change looks reasonable to me.

Acked-by: Ard Biesheuvel <ardb@kernel.org>




> ---
>  arch/arm64/kernel/acpi.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
>
> diff --git a/arch/arm64/kernel/acpi.c b/arch/arm64/kernel/acpi.c
> index cada0b816c8a..aca5ee2a9e64 100644
> --- a/arch/arm64/kernel/acpi.c
> +++ b/arch/arm64/kernel/acpi.c
> @@ -246,7 +246,7 @@ pgprot_t __acpi_get_mem_attribute(phys_addr_t addr)
>          * types" of UEFI 2.5 section 2.3.6.1, each EFI memory type is
>          * mapped to a corresponding MAIR attribute encoding.
>          * The EFI memory attribute advises all possible capabilities
> -        * of a memory region. We use the most efficient capability.
> +        * of a memory region.
>          */
>
>         u64 attr;
> @@ -254,9 +254,7 @@ pgprot_t __acpi_get_mem_attribute(phys_addr_t addr)
>         attr = efi_mem_attributes(addr);
>         if (attr & EFI_MEMORY_WB)
>                 return PAGE_KERNEL;
> -       if (attr & EFI_MEMORY_WT)
> -               return __pgprot(PROT_NORMAL_WT);
> -       if (attr & EFI_MEMORY_WC)
> +       if (attr & (EFI_MEMORY_WC | EFI_MEMORY_WT))
>                 return __pgprot(PROT_NORMAL_NC);
>         return __pgprot(PROT_DEVICE_nGnRnE);
>  }
> @@ -340,9 +338,7 @@ void __iomem *acpi_os_ioremap(acpi_physical_address phys, acpi_size size)
>                 default:
>                         if (region->attribute & EFI_MEMORY_WB)
>                                 prot = PAGE_KERNEL;
> -                       else if (region->attribute & EFI_MEMORY_WT)
> -                               prot = __pgprot(PROT_NORMAL_WT);
> -                       else if (region->attribute & EFI_MEMORY_WC)
> +                       else if (region->attribute & (EFI_MEMORY_WC | EFI_MEMORY_WT))
>                                 prot = __pgprot(PROT_NORMAL_NC);
>                 }
>         }
> --
> 2.31.1.607.g51e8a6a459-goog
>

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