[PATCH v3 17/19] arm64: cpufeatures: Add capability for LDAPR instruction

[Will Deacon]

Armv8.3 introduced the LDAPR instruction, which provides weaker memory
ordering semantics than LDARi (RCpc vs RCsc). Generally, we provide an
RCsc implementation when implementing the Linux memory model, but LDAPR
can be used as a useful alternative to dependency ordering, particularly
when the compiler is capable of breaking the dependencies.

Since LDAPR is not available on all CPUs, add a cpufeature to detect it at
runtime and allow the instruction to be used with alternative code
patching.

Signed-off-by: Will Deacon <will at kernel.org>
---
 arch/arm64/Kconfig               |  3 +++
 arch/arm64/include/asm/cpucaps.h |  3 ++-
 arch/arm64/kernel/cpufeature.c   | 10 ++++++++++
 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 66dc41fd49f2..e1073210e70b 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1409,6 +1409,9 @@ config ARM64_PAN
 	 The feature is detected at runtime, and will remain as a 'nop'
 	 instruction if the cpu does not implement the feature.
 
+config AS_HAS_LDAPR
+	def_bool $(as-instr,.arch_extension rcpc)
+
 config ARM64_LSE_ATOMICS
 	bool
 	default ARM64_USE_LSE_ATOMICS
diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
index d7b3bb0cb180..3ff0103d4dfd 100644
--- a/arch/arm64/include/asm/cpucaps.h
+++ b/arch/arm64/include/asm/cpucaps.h
@@ -62,7 +62,8 @@
 #define ARM64_HAS_GENERIC_AUTH			52
 #define ARM64_HAS_32BIT_EL1			53
 #define ARM64_BTI				54
+#define ARM64_HAS_LDAPR				55
 
-#define ARM64_NCAPS				55
+#define ARM64_NCAPS				56
 
 #endif /* __ASM_CPUCAPS_H */
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 9fae0efc80c1..498bd9a7f1bc 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -2058,6 +2058,16 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.sign = FTR_UNSIGNED,
 	},
 #endif
+	{
+		.desc = "RCpc load-acquire (LDAPR)",
+		.capability = ARM64_HAS_LDAPR,
+		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
+		.sys_reg = SYS_ID_AA64ISAR1_EL1,
+		.sign = FTR_UNSIGNED,
+		.field_pos = ID_AA64ISAR1_LRCPC_SHIFT,
+		.matches = has_cpuid_feature,
+		.min_field_value = 1,
+	},
 	{},
 };
 
-- 
2.27.0.383.g050319c2ae-goog