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

[Marc Zyngier]

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 <m...@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