Flush icache before returning to user.

os/src/trap/mod.rs

@@ -86,6 +86,7 @@ pub fn trap_return() -> ! {
     }
     let restore_va = __restore as usize - __alltraps as usize + TRAMPOLINE;
     unsafe {
+        llvm_asm!("fence.i" :::: "volatile");
         llvm_asm!("jr $0" :: "r"(restore_va), "{a0}"(trap_cx_ptr), "{a1}"(user_satp) :: "volatile");
     }
     panic!("Unreachable in back_to_user!");