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Subject[PATCH 16/18] arm64: cpufeatures: Add capability for LDAPR instruction
Date
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@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 9f63053a63a9..a29256a782e9 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -2056,6 +2056,16 @@ 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,
+ },
{},
};

--
2.27.0.212.ge8ba1cc988-goog
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