cpu: add support for detecting RISC-V extensions

Add a RISCV64 variable to cpu that indicates both the presence
of RISC-V extensions and performance information about the underlying
RISC-V cores.  The variable is only populated with non false values on
Linux.  The detection code first attempts to use the riscv_hwprobe
syscall introduced in Linux 6.4, falling back to HWCAP if riscv_hwprobe
is not supported.  The patch can detect the C, V, Zba, Zbb and Zbs
extensions.  V, Zba, Zbb and Zbs can only be detected on a 6.5 kernel
or later (without backports).

Updates golang/go#61416

Change-Id: I40f92724ee3d337c06bdc559ff0b18a8f6bfda9f
Reviewed-on: https://go-review.googlesource.com/c/sys/+/605815
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>

cpu/cpu.go

@@ -201,6 +201,25 @@ var S390X struct {
 	_         CacheLinePad
 }
 
+// RISCV64 contains the supported CPU features and performance characteristics for riscv64
+// platforms. The booleans in RISCV64, with the exception of HasFastMisaligned, indicate
+// the presence of RISC-V extensions.
+//
+// It is safe to assume that all the RV64G extensions are supported and so they are omitted from
+// this structure. As riscv64 Go programs require at least RV64G, the code that populates
+// this structure cannot run successfully if some of the RV64G extensions are missing.
+// The struct is padded to avoid false sharing.
+var RISCV64 struct {
+	_                 CacheLinePad
+	HasFastMisaligned bool // Fast misaligned accesses
+	HasC              bool // Compressed instruction-set extension
+	HasV              bool // Vector extension compatible with RVV 1.0
+	HasZba            bool // Address generation instructions extension
+	HasZbb            bool // Basic bit-manipulation extension
+	HasZbs            bool // Single-bit instructions extension
+	_                 CacheLinePad
+}
+
 func init() {
 	archInit()
 	initOptions()

cpu/cpu_linux_noinit.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build linux && !arm && !arm64 && !mips64 && !mips64le && !ppc64 && !ppc64le && !s390x
+//go:build linux && !arm && !arm64 && !mips64 && !mips64le && !ppc64 && !ppc64le && !s390x && !riscv64
 
 package cpu
 

cpu/cpu_linux_riscv64.go

@@ -0,0 +1,137 @@
+// Copyright 2024 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"
+)
+
+// RISC-V extension discovery code for Linux. The approach here is to first try the riscv_hwprobe
+// syscall falling back to HWCAP to check for the C extension if riscv_hwprobe is not available.
+//
+// A note on detection of the Vector extension using HWCAP.
+//
+// Support for the Vector extension version 1.0 was added to the Linux kernel in release 6.5.
+// Support for the riscv_hwprobe syscall was added in 6.4. It follows that if the riscv_hwprobe
+// syscall is not available then neither is the Vector extension (which needs kernel support).
+// The riscv_hwprobe syscall should then be all we need to detect the Vector extension.
+// However, some RISC-V board manufacturers ship boards with an older kernel on top of which
+// they have back-ported various versions of the Vector extension patches but not the riscv_hwprobe
+// patches. These kernels advertise support for the Vector extension using HWCAP. Falling
+// back to HWCAP to detect the Vector extension, if riscv_hwprobe is not available, or simply not
+// bothering with riscv_hwprobe at all and just using HWCAP may then seem like an attractive option.
+//
+// Unfortunately, simply checking the 'V' bit in AT_HWCAP will not work as this bit is used by
+// RISC-V board and cloud instance providers to mean different things. The Lichee Pi 4A board
+// and the Scaleway RV1 cloud instances use the 'V' bit to advertise their support for the unratified
+// 0.7.1 version of the Vector Specification. The Banana Pi BPI-F3 and the CanMV-K230 board use
+// it to advertise support for 1.0 of the Vector extension. Versions 0.7.1 and 1.0 of the Vector
+// extension are binary incompatible. HWCAP can then not be used in isolation to populate the
+// HasV field as this field indicates that the underlying CPU is compatible with RVV 1.0.
+//
+// There is a way at runtime to distinguish between versions 0.7.1 and 1.0 of the Vector
+// specification by issuing a RVV 1.0 vsetvli instruction and checking the vill bit of the vtype
+// register. This check would allow us to safely detect version 1.0 of the Vector extension
+// with HWCAP, if riscv_hwprobe were not available. However, the check cannot
+// be added until the assembler supports the Vector instructions.
+//
+// Note the riscv_hwprobe syscall does not suffer from these ambiguities by design as all of the
+// extensions it advertises support for are explicitly versioned. It's also worth noting that
+// the riscv_hwprobe syscall is the only way to detect multi-letter RISC-V extensions, e.g., Zba.
+// These cannot be detected using HWCAP and so riscv_hwprobe must be used to detect the majority
+// of RISC-V extensions.
+//
+// Please see https://docs.kernel.org/arch/riscv/hwprobe.html for more information.
+
+// golang.org/x/sys/cpu is not allowed to depend on golang.org/x/sys/unix so we must
+// reproduce the constants, types and functions needed to make the riscv_hwprobe syscall
+// here.
+
+const (
+	// Copied from golang.org/x/sys/unix/ztypes_linux_riscv64.go.
+	riscv_HWPROBE_KEY_IMA_EXT_0   = 0x4
+	riscv_HWPROBE_IMA_C           = 0x2
+	riscv_HWPROBE_IMA_V           = 0x4
+	riscv_HWPROBE_EXT_ZBA         = 0x8
+	riscv_HWPROBE_EXT_ZBB         = 0x10
+	riscv_HWPROBE_EXT_ZBS         = 0x20
+	riscv_HWPROBE_KEY_CPUPERF_0   = 0x5
+	riscv_HWPROBE_MISALIGNED_FAST = 0x3
+	riscv_HWPROBE_MISALIGNED_MASK = 0x7
+)
+
+const (
+	// sys_RISCV_HWPROBE is copied from golang.org/x/sys/unix/zsysnum_linux_riscv64.go.
+	sys_RISCV_HWPROBE = 258
+)
+
+// riscvHWProbePairs is copied from golang.org/x/sys/unix/ztypes_linux_riscv64.go.
+type riscvHWProbePairs struct {
+	key   int64
+	value uint64
+}
+
+const (
+	// CPU features
+	hwcap_RISCV_ISA_C = 1 << ('C' - 'A')
+)
+
+func doinit() {
+	// A slice of key/value pair structures is passed to the RISCVHWProbe syscall. The key
+	// field should be initialised with one of the key constants defined above, e.g.,
+	// RISCV_HWPROBE_KEY_IMA_EXT_0. The syscall will set the value field to the appropriate value.
+	// If the kernel does not recognise a key it will set the key field to -1 and the value field to 0.
+
+	pairs := []riscvHWProbePairs{
+		{riscv_HWPROBE_KEY_IMA_EXT_0, 0},
+		{riscv_HWPROBE_KEY_CPUPERF_0, 0},
+	}
+
+	// This call only indicates that extensions are supported if they are implemented on all cores.
+	if riscvHWProbe(pairs, 0) {
+		if pairs[0].key != -1 {
+			v := uint(pairs[0].value)
+			RISCV64.HasC = isSet(v, riscv_HWPROBE_IMA_C)
+			RISCV64.HasV = isSet(v, riscv_HWPROBE_IMA_V)
+			RISCV64.HasZba = isSet(v, riscv_HWPROBE_EXT_ZBA)
+			RISCV64.HasZbb = isSet(v, riscv_HWPROBE_EXT_ZBB)
+			RISCV64.HasZbs = isSet(v, riscv_HWPROBE_EXT_ZBS)
+		}
+		if pairs[1].key != -1 {
+			v := pairs[1].value & riscv_HWPROBE_MISALIGNED_MASK
+			RISCV64.HasFastMisaligned = v == riscv_HWPROBE_MISALIGNED_FAST
+		}
+	}
+
+	// Let's double check with HWCAP if the C extension does not appear to be supported.
+	// This may happen if we're running on a kernel older than 6.4.
+
+	if !RISCV64.HasC {
+		RISCV64.HasC = isSet(hwCap, hwcap_RISCV_ISA_C)
+	}
+}
+
+func isSet(hwc uint, value uint) bool {
+	return hwc&value != 0
+}
+
+// riscvHWProbe is a simplified version of the generated wrapper function found in
+// golang.org/x/sys/unix/zsyscall_linux_riscv64.go. We simplify it by removing the
+// cpuCount and cpus parameters which we do not need. We always want to pass 0 for
+// these parameters here so the kernel only reports the extensions that are present
+// on all cores.
+func riscvHWProbe(pairs []riscvHWProbePairs, flags uint) bool {
+	var _zero uintptr
+	var p0 unsafe.Pointer
+	if len(pairs) > 0 {
+		p0 = unsafe.Pointer(&pairs[0])
+	} else {
+		p0 = unsafe.Pointer(&_zero)
+	}
+
+	_, _, e1 := syscall.Syscall6(sys_RISCV_HWPROBE, uintptr(p0), uintptr(len(pairs)), uintptr(0), uintptr(0), uintptr(flags), 0)
+	return e1 == 0
+}

cpu/cpu_riscv64.go

@@ -8,4 +8,13 @@ package cpu
 
 const cacheLineSize = 64
 
-func initOptions() {}
+func initOptions() {
+	options = []option{
+		{Name: "fastmisaligned", Feature: &RISCV64.HasFastMisaligned},
+		{Name: "c", Feature: &RISCV64.HasC},
+		{Name: "v", Feature: &RISCV64.HasV},
+		{Name: "zba", Feature: &RISCV64.HasZba},
+		{Name: "zbb", Feature: &RISCV64.HasZbb},
+		{Name: "zbs", Feature: &RISCV64.HasZbs},
+	}
+}

cpu/cpu_test.go

@@ -61,6 +61,14 @@ func TestMIPS64Initialized(t *testing.T) {
 	}
 }
 
+func TestRISCV64Initialized(t *testing.T) {
+	if runtime.GOARCH == "riscv64" {
+		if !cpu.Initialized {
+			t.Fatal("Initialized expected true, got false")
+		}
+	}
+}
+
 // On ppc64x, the ISA bit for POWER8 should always be set on POWER8 and beyond.
 func TestPPC64minimalFeatures(t *testing.T) {
 	// Do not run this with gccgo on ppc64, as it doesn't have POWER8 as a minimum