BACKPORT: arm64: cpufeatures: Add capability for LDAPR instruction

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.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
(cherry picked from commit 364a5a8ae8)
Signed-off-by: Will Deacon <willdeacon@google.com>
[will: Fixed trivial cpucaps conflict with ARM64_KVM_PROTECTED_MODE]
Bug: 145210207
Change-Id: Ie0c9515dae382c8de49f793b8d40fef2f40dfeda

arch/arm64/Kconfig

@@ -1395,6 +1395,9 @@ 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

arch/arm64/include/asm/cpucaps.h

@@ -66,8 +66,9 @@
 #define ARM64_HAS_TLB_RANGE			56
 #define ARM64_MTE				57
 #define ARM64_WORKAROUND_1508412		58
-#define ARM64_KVM_PROTECTED_MODE		59
+#define ARM64_HAS_LDAPR				59
+#define ARM64_KVM_PROTECTED_MODE		60
 
-#define ARM64_NCAPS				60
+#define ARM64_NCAPS				61
 
 #endif /* __ASM_CPUCAPS_H */

arch/arm64/kernel/cpufeature.c

@@ -2182,6 +2182,16 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.cpu_enable = cpu_enable_mte,
 	},
 #endif /* CONFIG_ARM64_MTE */
+	{
+		.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,
+	},
 	{},
 };