feat: userspace round-trip — syscall stubs, ecall dispatch, U-mode entry and exit on QEMU

crates/apps/hello/src/main.rs

@@ -0,0 +1,24 @@
+#![no_std]
+#![no_main]
+
+use core::arch::global_asm;
+
+global_asm!(
+    r#"
+.section .text.entry
+.globl _start
+_start:
+    call main
+    j _start
+"#
+);
+
+#[unsafe(no_mangle)]
+extern "C" fn main() -> ! {
+    userspace::syscall::exit();
+}
+
+#[panic_handler]
+fn panic(_: &core::panic::PanicInfo) -> ! {
+    loop {}
+}