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

crates/adapter-riscv/src/mmu/allocator.rs

@@ -8,12 +8,19 @@ pub fn init_allocator(start: usize) {
     unsafe { NEXT_FREE = start; }
 }
 
-pub fn alloc_page_table() -> &'static mut PageTable {
+pub fn alloc_zeroed_page() -> usize {
     unsafe {
         let addr = NEXT_FREE;
         NEXT_FREE += PAGE_SIZE;
+        core::ptr::write_bytes(addr as *mut u8, 0, PAGE_SIZE);
+        addr
+    }
+}
+
+pub fn alloc_page_table() -> &'static mut PageTable {
+    unsafe {
+        let addr = alloc_zeroed_page();
         let table = &mut *(addr as *mut PageTable);
-        table.zero();
         table
     }
 }