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