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

From: Mark Rutland <mark.rutland@arm.com>
To: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org,
	Christoph Hellwig <hch@lst.de>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Subject: Re: [PATCH 2/3] arm64: acpi: Map EFI_MEMORY_WT memory as Normal-NC
Date: Thu, 6 May 2021 11:20:45 +0100	[thread overview]
Message-ID: <20210506102009.GA32366@C02TD0UTHF1T.local> (raw)
In-Reply-To: <20210506095034.15246-3-will@kernel.org>

On Thu, May 06, 2021 at 10:50:33AM +0100, Will Deacon 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.

The UEFI spec explicitly defines EFI_MEMORY_WT as Normal Outer-WT
Inner-WT (and even explicitly specifies the MAIR.Attr<n> value).

In the UEFI 2.9 spec, see section 2.3.6.1 "Memory types", Table 2-5
"Map: EFI Cacheability Attributes to AArch64 Memory Types".

The UEFI 2.9 spec can be found at:

  https://uefi.org/sites/default/files/resources/UEFI_Spec_2_9_2021_03_18.pdf

Given that is specified explicitly, and given that we don't know how
future CPUs will treat this equivalently, I don't think this change is
architecturally sound and I don't think there's wiggle-room to read the
spec as permitting this.

Thanks,
Mark.

> 
> 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>
> ---
>  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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> linux-arm-kernel@lists.infradead.org
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