rt: Add upcall_call_shim_on_rust_stack

Author: brson

src/rt/rust_task.cpp

@@ -90,7 +90,8 @@ rust_task::rust_task(rust_task_thread *thread, rust_task_list *state,
     cc_counter(0),
     total_stack_sz(0),
     c_stack(NULL),
-    next_c_sp(0)
+    next_c_sp(0),
+    next_rust_sp(0)
 {
     LOGPTR(thread, "new task", (uintptr_t)this);
     DLOG(thread, task, "sizeof(task) = %d (0x%x)", sizeof *this, sizeof *this);
@@ -658,6 +659,7 @@ rust_task::prev_stack() {
 
 void
 rust_task::record_stack_limit() {
+    I(thread, stk);
     // The function prolog compares the amount of stack needed to the end of
     // the stack. As an optimization, when the frame size is less than 256
     // bytes, it will simply compare %esp to to the stack limit instead of
@@ -732,18 +734,40 @@ rust_task::config_notify(chan_handle chan) {
 
 extern "C" void __morestack(void *args, void *fn_ptr, uintptr_t stack_ptr);
 
+static uintptr_t
+sanitize_next_sp(uintptr_t next_sp) {
+
+    // Since I'm not precisely sure where the next stack pointer sits in
+    // relation to where the context switch actually happened, nor in relation
+    // to the amount of stack needed for calling __morestack I've added some
+    // extra bytes here.
+
+    // FIXME: On the rust stack this potentially puts is quite far into the
+    // red zone. Might want to just allocate a new rust stack every time we
+    // switch back to rust.
+    const uintptr_t padding = 16;
+
+    return align_down(next_sp - padding);
+}
+
 void
 rust_task::call_on_c_stack(void *args, void *fn_ptr) {
     I(thread, on_rust_stack());
 
+    next_rust_sp = get_sp();
+
     bool borrowed_a_c_stack = false;
+    uintptr_t sp;
     if (c_stack == NULL) {
         c_stack = thread->borrow_c_stack();
         next_c_sp = align_down(c_stack->end);
+        sp = next_c_sp;
         borrowed_a_c_stack = true;
+    } else {
+        sp = sanitize_next_sp(next_c_sp);
     }
 
-    __morestack(args, fn_ptr, next_c_sp);
+    __morestack(args, fn_ptr, sp);
 
     // Note that we may not actually get here if we threw an exception,
     // in which case we will return the c stack when the exception is caught.
@@ -752,6 +776,18 @@ rust_task::call_on_c_stack(void *args, void *fn_ptr) {
     }
 }
 
+void
+rust_task::call_on_rust_stack(void *args, void *fn_ptr) {
+    I(thread, !on_rust_stack());
+    I(thread, next_rust_sp);
+
+    next_c_sp = get_sp();
+
+    uintptr_t sp = sanitize_next_sp(next_rust_sp);
+
+    __morestack(args, fn_ptr, sp);
+}
+
 void
 rust_task::return_c_stack() {
     I(thread, on_rust_stack());

src/rt/rust_task.h

@@ -110,6 +110,7 @@ rust_task : public kernel_owned<rust_task>, rust_cond
     // The stack used for running C code, borrowed from the scheduler thread
     stk_seg *c_stack;
     uintptr_t next_c_sp;
+    uintptr_t next_rust_sp;
 
     // Called when the atomic refcount reaches zero
     void delete_this();
@@ -194,6 +195,7 @@ rust_task : public kernel_owned<rust_task>, rust_cond
     void config_notify(chan_handle chan);
 
     void call_on_c_stack(void *args, void *fn_ptr);
+    void call_on_rust_stack(void *args, void *fn_ptr);
 };
 
 //

src/rt/rust_upcall.cpp

@@ -71,13 +71,38 @@ upcall_call_shim_on_c_stack(void *args, void *fn_ptr) {
     try {
         task->call_on_c_stack(args, fn_ptr);
     } catch (...) {
-        A(task->thread, false, "Native code threw an exception");
+        LOG_ERR(task, task, "Native code threw an exception");
+        abort();
     }
 
     task = rust_task_thread::get_task();
     task->record_stack_limit();
 }
 
+/*
+ * The opposite of above. Starts on a C stack and switches to the Rust
+ * stack. This is the only upcall that runs from the C stack.
+ */
+extern "C" CDECL void
+upcall_call_shim_on_rust_stack(void *args, void *fn_ptr) {
+    rust_task *task = rust_task_thread::get_task();
+
+    // FIXME: Because of the hack in the other function that disables the
+    // stack limit when entering the C stack, here we restore the stack limit
+    // again.
+    task->record_stack_limit();
+
+    try {
+        task->call_on_rust_stack(args, fn_ptr);
+    } catch (...) {
+        // We can't count on being able to unwind through arbitrary
+        // code. Our best option is to just fail hard.
+        LOG_ERR(task, task,
+                "Rust task failed after reentering the Rust stack");
+        abort();
+    }
+}
+
 /**********************************************************************/
 
 struct s_fail_args {

src/rt/rustrt.def.in

@@ -75,6 +75,7 @@ upcall_shared_free
 upcall_vec_grow
 upcall_vec_push
 upcall_call_shim_on_c_stack
+upcall_call_shim_on_rust_stack
 upcall_new_stack
 upcall_del_stack
 upcall_reset_stack_limit