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>

1 files changed, 10 insertions, 0 deletions

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index dcc165b3fc04..b7b6804cb931 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -2136,6 +2136,16 @@ 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,
+	},
 	{},
 };