From mboxrd@z Thu Jan  1 00:00:00 1970
From: Catalin Marinas <catalin.marinas@arm.com>
Subject: Re: [PATCH v3 21/23] arm64: mte: Check the DT memory nodes for MTE
 support
Date: Fri, 24 Apr 2020 14:57:36 +0100
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Cc: Will Deacon <will@kernel.org>, Vincenzo Frascino <vincenzo.frascino@arm.com>, Szabolcs Nagy <szabolcs.nagy@arm.com>, Richard Earnshaw <Richard.Earnshaw@arm.com>, Kevin Brodsky <kevin.brodsky@arm.com>, Andrey Konovalov <andreyknvl@google.com>, Peter Collingbourne <pcc@google.com>, linux-mm@kvack.org, linux-arch@vger.kernel.org, Rob Herring <Rob.Herring@arm.com>, Mark Rutland <mark.rutland@arm.com>, Suzuki K Poulose <Suzuki.Poulose@arm.com>

On Tue, Apr 21, 2020 at 03:26:01PM +0100, Catalin Marinas wrote:
> Even if the ID_AA64PFR1_EL1 register advertises the presence of MTE, it
> is not guaranteed that the memory system on the SoC supports the
> feature. In the absence of system-wide MTE support, the behaviour is
> undefined and the kernel should not enable the MTE memory type in
> MAIR_EL1.
> 
> For FDT, add an 'arm,armv8.5-memtag' property to the /memory nodes and
> check for its presence during MTE probing. For example:
> 
> 	memory@80000000 {
> 		device_type = "memory";
> 		arm,armv8.5-memtag;
> 		reg = <0x00000000 0x80000000 0 0x80000000>,
> 		      <0x00000008 0x80000000 0 0x80000000>;
> 	};
> 
> If the /memory nodes are not present in DT or if at least one node does
> not support MTE, the feature will be disabled. On EFI systems, it is
> assumed that the memory description matches the EFI memory map (if not,
> it is considered a firmware bug).
> 
> MTE is not currently supported on ACPI systems.
> 
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Rob Herring <Rob.Herring@arm.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Suzuki K Poulose <Suzuki.Poulose@arm.com>

This patch turns out to be incomplete. While it does not expose the
HWCAP2_MTE to user when the above DT property is not present, it still
allows user access to the ID_AA64PFR1_EL1.MTE field (via MRS emulations)
since it is marked as visible.

-- 
Catalin