internal/cpu: add DIT detection on arm64

Add support for detecting the DIT feature on ARM64 processors. This
mirrors https://go.dev/cl/597377, but using the platform specific
semantics.

Updates #66450

Change-Id: Ia107e3e3369de7825af70823b485afe2f587358e
Reviewed-on: https://go-review.googlesource.com/c/go/+/598335
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/internal/cpu/cpu.go

@@ -73,6 +73,7 @@ var ARM64 struct {
 	HasCRC32   bool
 	HasATOMICS bool
 	HasCPUID   bool
+	HasDIT     bool
 	IsNeoverse bool
 	_          CacheLinePad
 }

src/internal/cpu/cpu_arm64.go

@@ -28,13 +28,15 @@ func doinit() {
 
 func getisar0() uint64
 
+func getpfr0() uint64
+
 func getMIDR() uint64
 
 func extractBits(data uint64, start, end uint) uint {
 	return (uint)(data>>start) & ((1 << (end - start + 1)) - 1)
 }
 
-func parseARM64SystemRegisters(isar0 uint64) {
+func parseARM64SystemRegisters(isar0, pfr0 uint64) {
 	// ID_AA64ISAR0_EL1
 	switch extractBits(isar0, 4, 7) {
 	case 1:
@@ -66,4 +68,9 @@ func parseARM64SystemRegisters(isar0 uint64) {
 	case 2:
 		ARM64.HasATOMICS = true
 	}
+
+	switch extractBits(pfr0, 48, 51) {
+	case 1:
+		ARM64.HasDIT = true
+	}
 }

src/internal/cpu/cpu_arm64.s

@@ -11,6 +11,13 @@ TEXT ·getisar0(SB),NOSPLIT,$0
 	MOVD	R0, ret+0(FP)
 	RET
 
+// func getpfr0() uint64
+TEXT ·getpfr0(SB),NOSPLIT,$0-8
+	// get Processor Feature Register 0 into R0
+	MRS ID_AA64PFR0_EL1, R0
+	MOVD R0, ret+0(FP)
+	RET
+
 // func getMIDR() uint64
 TEXT ·getMIDR(SB), NOSPLIT, $0-8
 	MRS	MIDR_EL1, R0

src/internal/cpu/cpu_arm64_darwin.go

@@ -12,6 +12,7 @@ func osInit() {
 	ARM64.HasATOMICS = sysctlEnabled([]byte("hw.optional.armv8_1_atomics\x00"))
 	ARM64.HasCRC32 = sysctlEnabled([]byte("hw.optional.armv8_crc32\x00"))
 	ARM64.HasSHA512 = sysctlEnabled([]byte("hw.optional.armv8_2_sha512\x00"))
+	ARM64.HasDIT = sysctlEnabled([]byte("hw.optional.arm.FEAT_DIT\x00"))
 
 	// There are no hw.optional sysctl values for the below features on Mac OS 11.0
 	// to detect their supported state dynamically. Assume the CPU features that

src/internal/cpu/cpu_arm64_freebsd.go

@@ -9,6 +9,7 @@ package cpu
 func osInit() {
 	// Retrieve info from system register ID_AA64ISAR0_EL1.
 	isar0 := getisar0()
+	prf0 := getpfr0()
 
-	parseARM64SystemRegisters(isar0)
+	parseARM64SystemRegisters(isar0, prf0)
 }

src/internal/cpu/cpu_arm64_hwcap.go

@@ -31,6 +31,7 @@ const (
 	hwcap_ATOMICS = 1 << 8
 	hwcap_CPUID   = 1 << 11
 	hwcap_SHA512  = 1 << 21
+	hwcap_DIT     = 1 << 24
 )
 
 func hwcapInit(os string) {
@@ -44,6 +45,7 @@ func hwcapInit(os string) {
 	ARM64.HasCRC32 = isSet(HWCap, hwcap_CRC32)
 	ARM64.HasCPUID = isSet(HWCap, hwcap_CPUID)
 	ARM64.HasSHA512 = isSet(HWCap, hwcap_SHA512)
+	ARM64.HasDIT = isSet(HWCap, hwcap_DIT)
 
 	// The Samsung S9+ kernel reports support for atomics, but not all cores
 	// actually support them, resulting in SIGILL. See issue #28431.

src/internal/cpu/cpu_arm64_openbsd.go

@@ -13,6 +13,7 @@ const (
 	// From OpenBSD's machine/cpu.h.
 	_CPU_ID_AA64ISAR0 = 2
 	_CPU_ID_AA64ISAR1 = 3
+	_CPU_ID_AA64PFR0  = 8
 )
 
 //go:noescape
@@ -24,5 +25,11 @@ func osInit() {
 	if !ok {
 		return
 	}
-	parseARM64SystemRegisters(isar0)
+	// Get ID_AA64PFR0 from sysctl.
+	pfr0, ok := sysctlUint64([]uint32{_CTL_MACHDEP, _CPU_ID_AA64PFR0})
+	if !ok {
+		return
+	}
+
+	parseARM64SystemRegisters(isar0, pfr0)
 }