[RFC PATCH v1 1/5] arm64: cpufeature: Detect the ARMv8.4 TTRem feature

[Yanan Wang]

The ARMv8.4 TTRem feature offers 3 levels of support when changing block
size without changing any other parameters that are listed as requiring
use of break-before-make.

With level 0 supported, software must use break-before-make to avoid the
possible hardware problems. With level 1 supported, besides use of BBM,
software can also make use of the nT block translation entry. With level
2 supported, besides approaches of BBM and nT, software can also directly
change block size, but TLB conflicts possibly occur as a result.

We have found a place where TTRem can be used to improve the performance
in guest stage-2 translation. So detact the TTRem feature here.

Signed-off-by: Yanan Wang <wangyanan55@xxxxxxxxxx>
---
 arch/arm64/include/asm/cpucaps.h |  3 ++-
 arch/arm64/kernel/cpufeature.c   | 10 ++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
index b77d997b173b..e24570ea7444 100644
--- a/arch/arm64/include/asm/cpucaps.h
+++ b/arch/arm64/include/asm/cpucaps.h
@@ -66,7 +66,8 @@
 #define ARM64_WORKAROUND_1508412		58
 #define ARM64_HAS_LDAPR				59
 #define ARM64_KVM_PROTECTED_MODE		60
+#define ARM64_HAS_ARMv8_4_TTREM			61
 
-#define ARM64_NCAPS				61
+#define ARM64_NCAPS				62
 
 #endif /* __ASM_CPUCAPS_H */
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index e99eddec0a46..8295dd1d450b 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -1960,6 +1960,16 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.sign = FTR_UNSIGNED,
 		.min_field_value = ID_AA64ISAR0_TLB_RANGE,
 	},
+	{
+		.desc = "ARMv8.4 TTRem",
+		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
+		.capability = ARM64_HAS_ARMv8_4_TTREM,
+		.sys_reg = SYS_ID_AA64MMFR2_EL1,
+		.sign = FTR_UNSIGNED,
+		.field_pos = ID_AA64MMFR2_BBM_SHIFT,
+		.min_field_value = 1,
+		.matches = has_cpuid_feature,
+	},
 #ifdef CONFIG_ARM64_HW_AFDBM
 	{
 		/*
-- 
2.19.1