bugfix: Putting fence.i after loading apps

os/src/loader.rs

@@ -70,10 +70,6 @@ pub fn load_apps() {
     let num_app_ptr = _num_app as usize as *const usize;
     let num_app = get_num_app();
     let app_start = unsafe { core::slice::from_raw_parts(num_app_ptr.add(1), num_app + 1) };
-    // clear i-cache first
-    unsafe {
-        asm!("fence.i");
-    }
     // load apps
     for i in 0..num_app {
         let base_i = get_base_i(i);
@@ -87,6 +83,15 @@ pub fn load_apps() {
         let dst = unsafe { core::slice::from_raw_parts_mut(base_i as *mut u8, src.len()) };
         dst.copy_from_slice(src);
     }
+    // Memory fence about fetching the instruction memory
+    // It is guaranteed that a subsequent instruction fetch must
+    // observes all previous writes to the instruction memory.
+    // Therefore, fence.i must be executed after we have loaded
+    // the code of the next app into the instruction memory.
+    // See also: riscv non-priv spec chapter 3, 'Zifencei' extension.
+    unsafe {
+        asm!("fence.i");
+    }
 }
 
 /// get app info with entry and sp and save `TrapContext` in kernel stack