[PATCH v5 19/21] arm64: cpufeatures: Allow disabling of BTI from the command-line

Marc Zyngier maz at kernel.org
Mon Jan 25 05:50:17 EST 2021


In order to be able to disable BTI at runtime, whether it is
for testing purposes, or to work around HW issues, let's add
support for overriding the ID_AA64PFR1_EL1.BTI field.

This is further mapped on the arm64.nobti command-line alias.

Signed-off-by: Marc Zyngier <maz at kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>
Acked-by: David Brazdil <dbrazdil at google.com>
---
 Documentation/admin-guide/kernel-parameters.txt |  3 +++
 arch/arm64/include/asm/cpufeature.h             |  1 +
 arch/arm64/kernel/cpufeature.c                  |  4 +++-
 arch/arm64/kernel/idreg-override.c              | 11 +++++++++++
 arch/arm64/mm/mmu.c                             |  2 +-
 5 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 2786fd39a047..7599fd0f1ad7 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -373,6 +373,9 @@
 	arcrimi=	[HW,NET] ARCnet - "RIM I" (entirely mem-mapped) cards
 			Format: <io>,<irq>,<nodeID>
 
+	arm64.nobti	[ARM64] Unconditionally disable Branch Target
+			Identification support
+
 	ataflop=	[HW,M68k]
 
 	atarimouse=	[HW,MOUSE] Atari Mouse
diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index b0ed37da4067..4e2f2de9d0d7 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -819,6 +819,7 @@ static inline unsigned int get_vmid_bits(u64 mmfr1)
 }
 
 extern struct arm64_ftr_override id_aa64mmfr1_override;
+extern struct arm64_ftr_override id_aa64pfr1_override;
 
 u32 get_kvm_ipa_limit(void);
 void dump_cpu_features(void);
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index c1d6712c4249..bb99ddb212b5 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -558,6 +558,7 @@ static const struct arm64_ftr_bits ftr_raz[] = {
 #define ARM64_FTR_REG(id, table) ARM64_FTR_REG_OVERRIDE(id, table, &no_override)
 
 struct arm64_ftr_override id_aa64mmfr1_override;
+struct arm64_ftr_override id_aa64pfr1_override;
 
 static const struct __ftr_reg_entry {
 	u32			sys_id;
@@ -593,7 +594,8 @@ static const struct __ftr_reg_entry {
 
 	/* Op1 = 0, CRn = 0, CRm = 4 */
 	ARM64_FTR_REG(SYS_ID_AA64PFR0_EL1, ftr_id_aa64pfr0),
-	ARM64_FTR_REG(SYS_ID_AA64PFR1_EL1, ftr_id_aa64pfr1),
+	ARM64_FTR_REG_OVERRIDE(SYS_ID_AA64PFR1_EL1, ftr_id_aa64pfr1,
+			       &id_aa64pfr1_override),
 	ARM64_FTR_REG(SYS_ID_AA64ZFR0_EL1, ftr_id_aa64zfr0),
 
 	/* Op1 = 0, CRn = 0, CRm = 5 */
diff --git a/arch/arm64/kernel/idreg-override.c b/arch/arm64/kernel/idreg-override.c
index 3ccf51b84ba4..71349b644246 100644
--- a/arch/arm64/kernel/idreg-override.c
+++ b/arch/arm64/kernel/idreg-override.c
@@ -31,6 +31,15 @@ static const struct ftr_set_desc mmfr1 __initdata = {
 	},
 };
 
+static const struct ftr_set_desc pfr1 __initdata = {
+	.name		= "id_aa64pfr1",
+	.override	= &id_aa64pfr1_override,
+	.fields		= {
+	        { "bt", ID_AA64PFR1_BT_SHIFT },
+		{}
+	},
+};
+
 extern struct arm64_ftr_override kaslr_feature_override;
 
 static const struct ftr_set_desc kaslr __initdata = {
@@ -46,6 +55,7 @@ static const struct ftr_set_desc kaslr __initdata = {
 
 static const struct ftr_set_desc * const regs[] __initdata = {
 	&mmfr1,
+	&pfr1,
 	&kaslr,
 };
 
@@ -55,6 +65,7 @@ static const struct {
 } aliases[] __initdata = {
 	{ "kvm-arm.mode=nvhe",		"id_aa64mmfr1.vh=0" },
 	{ "kvm-arm.mode=protected",	"id_aa64mmfr1.vh=0" },
+	{ "arm64.nobti",		"id_aa64pfr1.bt=0" },
 	{ "nokaslr",			"kaslr.disabled=1" },
 };
 
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index ae0c3d023824..617e704c980b 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -628,7 +628,7 @@ static bool arm64_early_this_cpu_has_bti(void)
 	if (!IS_ENABLED(CONFIG_ARM64_BTI_KERNEL))
 		return false;
 
-	pfr1 = read_sysreg_s(SYS_ID_AA64PFR1_EL1);
+	pfr1 = __read_sysreg_by_encoding(SYS_ID_AA64PFR1_EL1);
 	return cpuid_feature_extract_unsigned_field(pfr1,
 						    ID_AA64PFR1_BT_SHIFT);
 }
-- 
2.29.2




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