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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 12:00:08 +0100	[thread overview]
Message-ID: <20210506110008.GC32366@C02TD0UTHF1T.local> (raw)
In-Reply-To: <20210506104640.GA15324@willie-the-truck>

On Thu, May 06, 2021 at 11:46:41AM +0100, Will Deacon wrote:
> 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 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.
> 
> 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?

If we also could avoid mapping EFI_MEMORY_WT to anything, that'd be
nicer, but just the warning is probably good enough, yes.

Thanks,
Mark.

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  reply	other threads:[~2021-05-06 11:03 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
2021-05-06 11:00       ` Mark Rutland [this message]
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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