[PATCH v3 1/1] arm64/cpufeature: Optionally disable MTE via command-line

[<yee.lee@mediatek.com>]

From: Yee Lee <yee.lee@mediatek.com>

For some low-end devices with limited resources,
MTE needs to be optionally disabled to save system
costs such as tag memory and firmware controls.

This allows ID_AA64PFR1_EL1.MTE to be overridden on 
its shadow value by giving "arm64.nomte" on cmdline,
and to suppress MTE feature.

Suggested-by: Marc Zyngier <maz@kernel.org>
Suggested-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Yee Lee <yee.lee@mediatek.com>
---
 Documentation/admin-guide/kernel-parameters.txt | 3 +++
 arch/arm64/kernel/idreg-override.c              | 2 ++
 2 files changed, 5 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index bdb22006f713..6f257e39d89e 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -380,6 +380,9 @@
 	arm64.nopauth	[ARM64] Unconditionally disable Pointer Authentication
 			support
 
+	arm64.nomte	[ARM64] Unconditionally disable Memory Tagging Extension
+			support
+
 	ataflop=	[HW,M68k]
 
 	atarimouse=	[HW,MOUSE] Atari Mouse
diff --git a/arch/arm64/kernel/idreg-override.c b/arch/arm64/kernel/idreg-override.c
index 53a381a7f65d..d8e606fe3c21 100644
--- a/arch/arm64/kernel/idreg-override.c
+++ b/arch/arm64/kernel/idreg-override.c
@@ -54,6 +54,7 @@ static const struct ftr_set_desc pfr1 __initconst = {
 	.override	= &id_aa64pfr1_override,
 	.fields		= {
 	        { "bt", ID_AA64PFR1_BT_SHIFT },
+		{ "mte", ID_AA64PFR1_MTE_SHIFT},
 		{}
 	},
 };
@@ -100,6 +101,7 @@ static const struct {
 	{ "arm64.nopauth",
 	  "id_aa64isar1.gpi=0 id_aa64isar1.gpa=0 "
 	  "id_aa64isar1.api=0 id_aa64isar1.apa=0"	   },
+	{ "arm64.nomte",		"id_aa64pfr1.mte=0" },
 	{ "nokaslr",			"kaslr.disabled=1" },
 };
 
-- 
2.18.0