Add first user program hello_world.rs

user/src/syscall.rs

@@ -0,0 +1,20 @@
+pub const STDOUT: usize = 1;
+
+const SYSCALL_WRITE: usize = 64;
+
+fn syscall(id: usize, args: [usize; 3]) -> isize {
+    let mut ret: isize;
+    unsafe {
+        llvm_asm!("ecall"
+            : "={x10}" (ret)
+            : "{x10}" (args[0]), "{x11}" (args[1]), "{x12}" (args[2]), "{x17}" (id)
+            : "memory"
+            : "volatile"
+        );
+    }
+    ret
+}
+
+pub fn sys_write(fd: usize, buffer: &[u8]) -> isize {
+    syscall(SYSCALL_WRITE, [fd, buffer.as_ptr() as usize, buffer.len()])
+}