arm64: Add helpers to probe local CPU for PAC and BTI support

Add some helpers that will be used by the early kernel mapping code to
check feature support on the local CPU. This permits the early kernel
mapping to be created with the right attributes, removing the need for
tearing it down and recreating it.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240214122845.2033971-58-ardb+git@google.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/cpufeature.h

@@ -963,6 +963,38 @@ static inline bool kaslr_disabled_cmdline(void)
 u32 get_kvm_ipa_limit(void);
 void dump_cpu_features(void);
 
+static inline bool cpu_has_bti(void)
+{
+	if (!IS_ENABLED(CONFIG_ARM64_BTI))
+		return false;
+
+	return arm64_apply_feature_override(read_cpuid(ID_AA64PFR1_EL1),
+					    ID_AA64PFR1_EL1_BT_SHIFT, 4,
+					    &id_aa64pfr1_override);
+}
+
+static inline bool cpu_has_pac(void)
+{
+	u64 isar1, isar2;
+
+	if (!IS_ENABLED(CONFIG_ARM64_PTR_AUTH))
+		return false;
+
+	isar1 = read_cpuid(ID_AA64ISAR1_EL1);
+	isar2 = read_cpuid(ID_AA64ISAR2_EL1);
+
+	if (arm64_apply_feature_override(isar1, ID_AA64ISAR1_EL1_APA_SHIFT, 4,
+					 &id_aa64isar1_override))
+		return true;
+
+	if (arm64_apply_feature_override(isar1, ID_AA64ISAR1_EL1_API_SHIFT, 4,
+					 &id_aa64isar1_override))
+		return true;
+
+	return arm64_apply_feature_override(isar2, ID_AA64ISAR2_EL1_APA3_SHIFT, 4,
+					    &id_aa64isar2_override);
+}
+
 #endif /* __ASSEMBLY__ */
 
 #endif