cpu: implement CPU feature detection for openbsd/arm64

OpenBSD 7.1 onwards expose the aarch64 ISAR0 and ISAR1 registers via sysctl:

  $ sysctl machdep
  machdep.compatible=apple,j274
  machdep.id_aa64isar0=153421459058925856
  machdep.id_aa64isar1=1172796674562

Implement CPU feature detection for openbsd/arm64 based on this information.

Updates golang/go#31746

Change-Id: I6dc473d93ccff720582c05b75456de51269bc3e5
Reviewed-on: https://go-review.googlesource.com/c/sys/+/421799
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>

cpu/cpu_arm64.go

@@ -41,13 +41,10 @@ func archInit() {
 	switch runtime.GOOS {
 	case "freebsd":
 		readARM64Registers()
-	case "linux", "netbsd":
+	case "linux", "netbsd", "openbsd":
 		doinit()
 	default:
-		// Most platforms don't seem to allow reading these registers.
-		//
-		// OpenBSD:
-		// See https://golang.org/issue/31746
+		// Many platforms don't seem to allow reading these registers.
 		setMinimalFeatures()
 	}
 }

cpu/cpu_openbsd_arm64.go

@@ -0,0 +1,65 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cpu
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+// Minimal copy of functionality from x/sys/unix so the cpu package can call
+// sysctl without depending on x/sys/unix.
+
+const (
+	// From OpenBSD's sys/sysctl.h.
+	_CTL_MACHDEP = 7
+
+	// From OpenBSD's machine/cpu.h.
+	_CPU_ID_AA64ISAR0 = 2
+	_CPU_ID_AA64ISAR1 = 3
+)
+
+// Implemented in the runtime package (runtime/sys_openbsd3.go)
+func syscall_syscall6(fn, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err syscall.Errno)
+
+//go:linkname syscall_syscall6 syscall.syscall6
+
+func sysctl(mib []uint32, old *byte, oldlen *uintptr, new *byte, newlen uintptr) (err error) {
+	_, _, errno := syscall_syscall6(libc_sysctl_trampoline_addr, uintptr(unsafe.Pointer(&mib[0])), uintptr(len(mib)), uintptr(unsafe.Pointer(old)), uintptr(unsafe.Pointer(oldlen)), uintptr(unsafe.Pointer(new)), uintptr(newlen))
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}
+
+var libc_sysctl_trampoline_addr uintptr
+
+//go:cgo_import_dynamic libc_sysctl sysctl "libc.so"
+
+func sysctlUint64(mib []uint32) (uint64, bool) {
+	var out uint64
+	nout := unsafe.Sizeof(out)
+	if err := sysctl(mib, (*byte)(unsafe.Pointer(&out)), &nout, nil, 0); err != nil {
+		return 0, false
+	}
+	return out, true
+}
+
+func doinit() {
+	setMinimalFeatures()
+
+	// Get ID_AA64ISAR0 and ID_AA64ISAR1 from sysctl.
+	isar0, ok := sysctlUint64([]uint32{_CTL_MACHDEP, _CPU_ID_AA64ISAR0})
+	if !ok {
+		return
+	}
+	isar1, ok := sysctlUint64([]uint32{_CTL_MACHDEP, _CPU_ID_AA64ISAR1})
+	if !ok {
+		return
+	}
+	parseARM64SystemRegisters(isar0, isar1, 0)
+
+	Initialized = true
+}

cpu/cpu_openbsd_arm64.s

@@ -0,0 +1,11 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "textflag.h"
+
+TEXT libc_sysctl_trampoline<>(SB),NOSPLIT,$0-0
+	JMP	libc_sysctl(SB)
+
+GLOBL	·libc_sysctl_trampoline_addr(SB), RODATA, $8
+DATA	·libc_sysctl_trampoline_addr(SB)/8, $libc_sysctl_trampoline<>(SB)

cpu/cpu_other_arm64.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !linux && !netbsd && arm64
-// +build !linux,!netbsd,arm64
+//go:build !linux && !netbsd && !openbsd && arm64
+// +build !linux,!netbsd,!openbsd,arm64
 
 package cpu