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From: Catalin Marinas <>
To: Ard Biesheuvel <>
Cc: Will Deacon <>,
	Linux ARM <>,
	Christoph Hellwig <>,
	Lorenzo Pieralisi <>
Subject: Re: [PATCH 2/3] arm64: acpi: Map EFI_MEMORY_WT memory as Normal-NC
Date: Thu, 6 May 2021 14:25:23 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Thu, May 06, 2021 at 12:14:55PM +0200, Ard Biesheuvel wrote:
> On Thu, 6 May 2021 at 11:50, 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.
> >
> > Cc: Ard Biesheuvel <>
> > Cc: Lorenzo Pieralisi <>
> > Cc: Christoph Hellwig <>
> > Signed-off-by: Will Deacon <>
> 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
> (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.

In theory there's a slight difference between WT and WC/Normal-NC as
reads are allowed to hit into the cache while for WC/Normal-NC they
aren't (somehow implied on page B2-168 in the ARMv8 ARM G.a). Whether
they must hit in the cache is not entirely clear, I don't think they
have to. The mismatched aliases section doesn't guarantee coherency
between accesses using different WC and WB attributes (point 1,
attributes must be the same for both reads and writes). Appendix K4.1.1
also suggest that WT could be implemented as Normal-NC.

So I don't think EFI can rely on any specific WT behaviour other than
maybe slightly better performance.


Acked-by: Catalin Marinas <>

linux-arm-kernel mailing list

  reply	other threads:[~2021-05-06 13:28 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-06  9:50 [PATCH 0/3] Free up a couple of MAIRs Will Deacon
2021-05-06  9:50 ` [PATCH 1/3] arm64: mm: Remove unused support for Device-GRE memory type Will Deacon
2021-05-06 13:25   ` Catalin Marinas
2021-05-06  9:50 ` [PATCH 2/3] arm64: acpi: Map EFI_MEMORY_WT memory as Normal-NC Will Deacon
2021-05-06 10:14   ` Ard Biesheuvel
2021-05-06 13:25     ` Catalin Marinas [this message]
2021-05-06 17:45       ` Ard Biesheuvel
2021-05-06 10:20   ` Mark Rutland
2021-05-06 10:46     ` Will Deacon
2021-05-06 11:00       ` Mark Rutland
2021-05-06 11:30         ` Ard Biesheuvel
2021-05-06  9:50 ` [PATCH 3/3] arm64: mm: Remove unused support for Normal-WT memory type Will Deacon
2021-05-06 13:26   ` Catalin Marinas

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