On Tue, Nov 03, 2020 at 12:17:19PM +0000, Will Deacon wrote: > 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) <[email protected]> > Signed-off-by: Will Deacon <[email protected]>
Acked-by: Mark Rutland <[email protected]> Mark. > --- > 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 >
