[PATCH v4 2/4] arm64: cpufeatures: Add capability for LDAPR instruction

Will Deacon
2020-11-03T12:19:30Z

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>
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 1d466addb078..356c50b0447f 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1388,6 +1388,9 @@ 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 e7d98997c09c..64ea0bb9f420 100644
--- a/arch/arm64/include/asm/cpucaps.h
+++ b/arch/arm64/include/asm/cpucaps.h
@@ -66,7 +66,8 @@
#define ARM64_HAS_TLB_RANGE 56
#define ARM64_MTE 57
#define ARM64_WORKAROUND_1508412 58
+#define ARM64_HAS_LDAPR 59

-#define ARM64_NCAPS 59
+#define ARM64_NCAPS 60

#endif /* __ASM_CPUCAPS_H */
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,
+ },
{},
};

--
2.29.1.341.ge80a0c044ae-goog