cpu: add support for zos/s390x

This adds feature detection that works on the s390x running zos.
There are some differences with Linux (no hwcap, facilities vector available in-memory).
These changes should not affect other platforms.

Fixes golang/go#41984.

Change-Id: Ieabbfafd07367a346a4d279dde0f00aa3fc4321b
Reviewed-on: https://go-review.googlesource.com/c/sys/+/262477
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>
Trust: Martin Möhrmann <moehrmann@google.com>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>

cpu/cpu_s390x_test.go

@@ -0,0 +1,75 @@
+// Copyright 2020 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_test
+
+import (
+	"runtime"
+	"testing"
+	"unsafe"
+
+	"golang.org/x/sys/cpu"
+)
+
+var s390xTests = []struct {
+	name      string
+	feature   bool
+	facility  uint
+	mandatory bool
+}{
+	{"ZARCH", cpu.S390X.HasZARCH, 1, true},
+	{"STFLE", cpu.S390X.HasSTFLE, 7, true},
+	{"LDISP", cpu.S390X.HasLDISP, 18, true},
+	{"EIMM", cpu.S390X.HasEIMM, 21, true},
+	{"DFP", cpu.S390X.HasDFP, 42, false},
+	{"MSA", cpu.S390X.HasMSA, 17, false},
+	{"VX", cpu.S390X.HasVX, 129, false},
+	{"VXE", cpu.S390X.HasVXE, 135, false},
+}
+
+// bitIsSet reports whether the bit at index is set. The bit index
+// is in big endian order, so bit index 0 is the leftmost bit.
+func bitIsSet(bits [4]uint64, i uint) bool {
+	return bits[i/64]&((1<<63)>>(i%64)) != 0
+}
+
+// facilityList contains the contents of location 200 on zos.
+// Bits are numbered in big endian order so the
+// leftmost bit (the MSB) is at index 0.
+type facilityList struct {
+	bits [4]uint64
+}
+
+func TestS390XVectorFacilityFeatures(t *testing.T) {
+	// vector-enhancements require vector facility to be enabled
+	if cpu.S390X.HasVXE && !cpu.S390X.HasVX {
+		t.Error("HasVX expected true, got false (VXE is true)")
+	}
+}
+
+func TestS390XMandatoryFeatures(t *testing.T) {
+	for _, tc := range s390xTests {
+		if tc.mandatory && !tc.feature {
+			t.Errorf("Feature %s is mandatory but is not present", tc.name)
+		}
+	}
+}
+
+func TestS390XFeatures(t *testing.T) {
+	if runtime.GOOS != "zos" {
+		return
+	}
+	// Read available facilities from address 200.
+	facilitiesAddress := uintptr(200)
+	var facilities facilityList
+	for i := 0; i < 4; i++ {
+		facilities.bits[i] = *(*uint64)(unsafe.Pointer(facilitiesAddress + uintptr(8*i)))
+	}
+
+	for _, tc := range s390xTests {
+		if want := bitIsSet(facilities.bits, tc.facility); want != tc.feature {
+			t.Errorf("Feature %s expected %v, got %v", tc.name, want, tc.feature)
+		}
+	}
+}