[PATCH v4 19/21] arm64: cpufeatures: Allow disabling of BTI from the command-line

Marc Zyngier
2021-01-18T10:02:18Z

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@kernel.org>
---
Documentation/admin-guide/kernel-parameters.txt | 3 +++
arch/arm64/include/asm/cpufeature.h | 2 ++
arch/arm64/kernel/cpufeature.c | 5 ++++-
arch/arm64/kernel/idreg-override.c | 12 ++++++++++++
arch/arm64/mm/mmu.c | 2 +-
5 files changed, 22 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 80a5f423444e..d3e0f6dd43c4 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -816,6 +816,8 @@ static inline unsigned int get_vmid_bits(u64 mmfr1)

extern u64 id_aa64mmfr1_val;
extern u64 id_aa64mmfr1_mask;
+extern u64 id_aa64pfr1_val;
+extern u64 id_aa64pfr1_mask;

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 5b9343d2e9f0..f223171a7c34 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -557,6 +557,8 @@ static const struct arm64_ftr_bits ftr_raz[] = {

u64 id_aa64mmfr1_val;
u64 id_aa64mmfr1_mask;
+u64 id_aa64pfr1_val;
+u64 id_aa64pfr1_mask;

static const struct __ftr_reg_entry {
u32 sys_id;
@@ -592,7 +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_val, &id_aa64pfr1_mask),
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 143fe7b8e3ce..a9e3ed193fd4 100644
--- a/arch/arm64/kernel/idreg-override.c
+++ b/arch/arm64/kernel/idreg-override.c
@@ -33,6 +33,16 @@ static const struct reg_desc mmfr1 __initdata = {
},
};

+static const struct reg_desc pfr1 __initdata = {
+ .name = "id_aa64pfr1",
+ .val = &id_aa64pfr1_val,
+ .mask = &id_aa64pfr1_mask,
+ .fields = {
+ { "bt", ID_AA64PFR1_BT_SHIFT },
+ {}
+ },
+};
+
extern u64 kaslr_feature_val;
extern u64 kaslr_feature_mask;

@@ -50,6 +60,7 @@ static const struct reg_desc kaslr __initdata = {

static const struct reg_desc * const regs[] __initdata = {
&mmfr1,
+ &pfr1,
&kaslr,
};

@@ -59,6 +70,7 @@ 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