arm64: Add HAS_ECV_CNTPOFF capability

Add the probing code for the FEAT_ECV variant that implements CNTPOFF_EL2.
Why it is optional is a mystery, but let's try and detect it.
Reviewed-by: Reiji Watanabe <reijiw@google.com>
Reviewed-by: Colton Lewis <coltonlewis@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230330174800.2677007-4-maz@kernel.org

arch/arm64/kernel/cpufeature.c

@@ -2223,6 +2223,17 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.sign = FTR_UNSIGNED,
 		.min_field_value = 1,
 	},
+	{
+		.desc = "Enhanced Counter Virtualization (CNTPOFF)",
+		.capability = ARM64_HAS_ECV_CNTPOFF,
+		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
+		.matches = has_cpuid_feature,
+		.sys_reg = SYS_ID_AA64MMFR0_EL1,
+		.field_pos = ID_AA64MMFR0_EL1_ECV_SHIFT,
+		.field_width = 4,
+		.sign = FTR_UNSIGNED,
+		.min_field_value = ID_AA64MMFR0_EL1_ECV_CNTPOFF,
+	},
 #ifdef CONFIG_ARM64_PAN
 	{
 		.desc = "Privileged Access Never",

arch/arm64/tools/cpucaps

@@ -23,6 +23,7 @@ HAS_DCPOP
 HAS_DIT
 HAS_E0PD
 HAS_ECV
+HAS_ECV_CNTPOFF
 HAS_EPAN
 HAS_GENERIC_AUTH
 HAS_GENERIC_AUTH_ARCH_QARMA3