archive mirror
 help / color / mirror / Atom feed
From: Will Deacon <>
To: Mark Rutland <>
	Christoph Hellwig <>,
	Catalin Marinas <>,
	Ard Biesheuvel <>,
	Lorenzo Pieralisi <>
Subject: Re: [PATCH 2/3] arm64: acpi: Map EFI_MEMORY_WT memory as Normal-NC
Date: Thu, 6 May 2021 11:46:41 +0100	[thread overview]
Message-ID: <20210506104640.GA15324@willie-the-truck> (raw)
In-Reply-To: <20210506102009.GA32366@C02TD0UTHF1T.local>

On Thu, May 06, 2021 at 11:20:45AM +0100, Mark Rutland wrote:
> 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).

I wonder if they just did that because the names match :(

> In the UEFI 2.9 spec, see section "Memory types", Table 2-5
> "Map: EFI Cacheability Attributes to AArch64 Memory Types".
> The UEFI 2.9 spec can be found at:
> 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.

At the same time, allocating a MAIR for this memory type just because the
UEFI spec permits some theoretical future firmware to use it on some
theoretical CPU design is pretty farcical in my opinion. Looking through
current TRMs I've not been able to find a CPU that doesn't just emit
Normal-NC for this memory type.

How about I add a pr_warn() in this case, so that we can revisit it in the
unlikely event that it ever comes up as an issue?


linux-arm-kernel mailing list

  reply	other threads:[~2021-05-06 10:49 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
2021-05-06 17:45       ` Ard Biesheuvel
2021-05-06 10:20   ` Mark Rutland
2021-05-06 10:46     ` Will Deacon [this message]
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20210506104640.GA15324@willie-the-truck \ \ \ \ \ \ \ \
    --subject='Re: [PATCH 2/3] arm64: acpi: Map EFI_MEMORY_WT memory as Normal-NC' \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
on how to clone and mirror all data and code used for this inbox