syscall: support rawVforkSyscall on linux/riscv64

Updates #31936

Change-Id: I7dcb8987d4c306ccc97704b9c1b12313ba8bf242
Reviewed-on: https://go-review.googlesource.com/c/go/+/234960
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/syscall/asm_linux_riscv64.s

@@ -104,6 +104,28 @@ err:
 	MOV	A0, err+72(FP)	// errno
 	RET
 
+// func rawVforkSyscall(trap, a1 uintptr) (r1, err uintptr)
+TEXT ·rawVforkSyscall(SB),NOSPLIT|NOFRAME,$0-32
+	MOV	a1+8(FP), A0
+	MOV	ZERO, A1
+	MOV	ZERO, A2
+	MOV	ZERO, A3
+	MOV	ZERO, A4
+	MOV	ZERO, A5
+	MOV	trap+0(FP), A7	// syscall entry
+	ECALL
+	MOV	$-4096, T0
+	BLTU	T0, A0, err
+	MOV	A0, r1+16(FP)	// r1
+	MOV	ZERO, err+24(FP)	// errno
+	RET
+err:
+	MOV	$-1, T0
+	MOV	T0, r1+16(FP)	// r1
+	SUB	A0, ZERO, A0
+	MOV	A0, err+24(FP)	// errno
+	RET
+
 TEXT ·rawSyscallNoError(SB),NOSPLIT,$0-48
 	MOV	a1+8(FP), A0
 	MOV	a2+16(FP), A1

src/syscall/exec_linux.go

@@ -207,7 +207,11 @@ func forkAndExecInChild1(argv0 *byte, argv, envv []*byte, chroot, dir *byte, att
 		}
 	}
 
-	hasRawVforkSyscall := runtime.GOARCH == "amd64" || runtime.GOARCH == "ppc64" || runtime.GOARCH == "s390x" || runtime.GOARCH == "arm64"
+	var hasRawVforkSyscall bool
+	switch runtime.GOARCH {
+	case "amd64", "arm64", "ppc64", "riscv64", "s390x":
+		hasRawVforkSyscall = true
+	}
 
 	// About to call fork.
 	// No more allocation or calls of non-assembly functions.

src/syscall/syscall_linux_riscv64.go

@@ -199,6 +199,4 @@ func Pause() error {
 	return err
 }
 
-func rawVforkSyscall(trap, a1 uintptr) (r1 uintptr, err Errno) {
+func rawVforkSyscall(trap, a1 uintptr) (r1 uintptr, err Errno)
-	panic("not implemented")
-}