Fix RDTSC emulation's changing app's TLS by mistake

This commit switches from user mode to kernel mode before handling
the exception caused by RDTSC instruction.

src/libos/src/exception/rdtsc.rs

@@ -1,16 +1,9 @@
 use super::*;
-use process::Task;
+use crate::syscall::SyscallNum;
 use sgx_types::*;
 
 const RDTSC_OPCODE: u16 = 0x310F;
 
-extern "C" {
-    fn occlum_ocall_rdtsc(low: *mut u32, high: *mut u32) -> sgx_status_t;
-    fn __get_current_task() -> *const Task;
-    fn switch_td_to_kernel(task: *const Task);
-    fn switch_td_to_user(task: *const Task);
-}
-
 #[no_mangle]
 pub extern "C" fn handle_rdtsc_exception(info: *mut sgx_exception_info_t) -> u32 {
     let info = unsafe { &mut *info };
@@ -21,21 +14,21 @@ pub extern "C" fn handle_rdtsc_exception(info: *mut sgx_exception_info_t) -> u32
     {
         return EXCEPTION_CONTINUE_SEARCH;
     }
-    unsafe {
-        let task = __get_current_task();
-        switch_td_to_kernel(task);
+
     let (low, high) = {
         let mut low = 0;
         let mut high = 0;
-            let sgx_status = occlum_ocall_rdtsc(&mut low, &mut high);
-            assert!(sgx_status == sgx_status_t::SGX_SUCCESS);
+        let ret = unsafe { __occlum_syscall(SyscallNum::Rdtsc as u32, &mut low, &mut high) };
+        assert!(ret == 0);
         (low, high)
     };
     info.cpu_context.rax = low as u64;
     info.cpu_context.rdx = high as u64;
-        switch_td_to_user(task);
-    }
     info.cpu_context.rip += 2;
 
     EXCEPTION_CONTINUE_EXECUTION
 }
+
+extern "C" {
+    fn __occlum_syscall(num: u32, arg0: *mut u32, arg1: *mut u32) -> i64;
+}

src/libos/src/lib.rs

@@ -67,7 +67,5 @@ mod untrusted;
 mod util;
 mod vm;
 
-// Export system calls
-pub use syscall::*;
 // Export ECalls
 pub use entry::*;

src/libos/src/syscall/mod.rs

@@ -393,6 +393,8 @@ macro_rules! process_syscall_table_with_callback {
 
                 // Occlum-specific sytem calls
                 (Spawn = 360) => do_spawn(child_pid_ptr: *mut u32, path: *const i8, argv: *const *const i8, envp: *const *const i8, fdop_list: *const FdOp),
+                // Exception handling
+                (Rdtsc = 361) => do_rdtsc(low_ptr: *mut u32, high_ptr: *mut u32),
             }
     };
 }
@@ -932,6 +934,18 @@ fn do_clock_gettime(clockid: clockid_t, ts_u: *mut timespec_t) -> Result<isize>
     Ok(0)
 }
 
+fn do_rdtsc(low_ptr: *mut u32, high_ptr: *mut u32) -> Result<isize> {
+    check_mut_ptr(low_ptr)?;
+    check_mut_ptr(high_ptr)?;
+    let (low, high) = time::do_rdtsc()?;
+    debug!("do_rdtsc result {{ low: {:#x} high: {:#x}}}", low, high);
+    unsafe {
+        *low_ptr = low;
+        *high_ptr = high;
+    }
+    Ok(0)
+}
+
 // TODO: handle remainder
 fn do_nanosleep(req_u: *const timespec_t, rem_u: *mut timespec_t) -> Result<isize> {
     check_ptr(req_u)?;

src/libos/src/time/mod.rs

@@ -160,6 +160,17 @@ pub fn do_thread_getcpuclock() -> Result<timespec_t> {
     Ok(tv)
 }
 
+pub fn do_rdtsc() -> Result<(u32, u32)> {
+    extern "C" {
+        fn occlum_ocall_rdtsc(low: *mut u32, high: *mut u32) -> sgx_status_t;
+    }
+    let mut low = 0;
+    let mut high = 0;
+    let sgx_status = unsafe { occlum_ocall_rdtsc(&mut low, &mut high) };
+    assert!(sgx_status == sgx_status_t::SGX_SUCCESS);
+    Ok((low, high))
+}
+
 // For SEFS
 pub struct OcclumTimeProvider;