diff --git a/nac3artiq/src/lib.rs b/nac3artiq/src/lib.rs
index 5906320b..4d163278 100644
--- a/nac3artiq/src/lib.rs
+++ b/nac3artiq/src/lib.rs
@@ -482,6 +482,9 @@ impl Nac3 {
                     if decorator_list.iter().any(|decorator| matches!(decorator.node, ExprKind::Name { id, .. } if id == "rpc".into())) {
                         store_fun.call1(py, (def_id.0.into_py(py), module.getattr(py, name.to_string().as_str()).unwrap())).unwrap();
                         rpc_ids.push((None, def_id));
+                    } else if decorator_list.iter().any(|decorator| matches!(decorator.node, ExprKind::Name { id, .. } if id == "subkernel".into())) {
+                        store_fun.call1(py, (def_id.0.into_py(py), module.getattr(py, name.to_string().as_str()).unwrap())).unwrap();
+                        //rpc_ids.push((None, def_id));
                     }
                 }
                 StmtKind::ClassDef { name, body, .. } => {
diff --git a/nac3artiq/src/subkernels.rs b/nac3artiq/src/subkernels.rs
index 3bbd1725..f2c777b4 100644
--- a/nac3artiq/src/subkernels.rs
+++ b/nac3artiq/src/subkernels.rs
@@ -58,10 +58,20 @@ impl Subkernels {
             ]
     }
 
-    fn gen_subkernel_await<'ctx>(ctx: &mut CodeGenContext<'ctx, '_>, awaited: BasicValueEnum<'ctx>, timeout: BasicValueEnum<'ctx>) {
+    fn gen_subkernel_await<'ctx>(
+        ctx: &mut CodeGenContext<'ctx, '_>,
+        obj: &Option<(Type, ValueEnum<'ctx>)>,
+        fun: (&FunSignature, DefinitionId),
+        args: &[(Option<StrRef>, ValueEnum<'ctx>)],
+        generator: &mut dyn CodeGenerator,
+    ) {
         let sid_type = ctx.ctx.i32_type();
-        // how to deal with optional arguments?
         let timeout_type = ctx.ctx.i64_type();
+        assert!(matches!(args.len(), 1..=2));
+        let timeout = if args.len() == 1 {
+            timeout_type.const_zero().to_basic_value_enum(context, generator, obj_ty)?; // ?
+        } else { args[0].1.clone().to_basic_value_enum(context, generator, obj_ty)?; } // ?
+
         let subkernel_await_finish = ctx.module.get_function("subkernel_await_finish").unwrap_or_else(|| {
             ctx.module.add_function(
                 "subkernel_await_finish",
@@ -74,18 +84,257 @@ impl Subkernels {
         // generate RPC for receiving return value depending on fun ret
     }
 
-    fn gen_subkernel_preload<'ctx>(ctx: &mut CodeGenContext<'ctx, '_>, preloaded: BasicValueEnum<'ctx>) {
+    fn gen_subkernel_preload<'ctx>(
+        ctx: &mut CodeGenContext<'ctx, '_>,
+        obj: &Option<(Type, ValueEnum<'ctx>)>,
+        fun: (&FunSignature, DefinitionId),
+        args: &[(Option<StrRef>, ValueEnum<'ctx>)],
+        generator: &mut dyn CodeGenerator,
+    ) {
+        assert_eq!(args.len(), 1);
         let sid_type = ctx.ctx.i32_type();
         let dest_type = ctx.ctx.i8_type();
-        let run_type = ctx.ctx.i1_type();
+        let run_type = ctx.ctx.bool_type();
         let subkernel_load_run = ctx.module.get_function("subkernel_load_run").unwrap_or_else(|| {
             ctx.module.add_function(
                 "subkernel_load_run",
-                ctx.ctx.void_type().fn_type(&[sid_type.into(), dest_type, run_type], false),
+                ctx.ctx.void_type().fn_type(&[sid_type.into(), dest_type.into(), run_type.into()], false),
                 None,
             )
         });
-        // retrieve destination and sid from the fn (?)
-        // call or invoke
+
+        let subkernel_id = int32.const_int(fun.1 .0 as u64, false);
+        let destination = int32.const_int(fun.? as u64, false); // TODO
+        ctx.builder
+            .build_call_or_invoke(subkernel_load_run, &[subkernel_id.into(), destination.into(), run_type.const_zero()], "subkernel.preload")
+            .unwrap();
+    }
+
+    fn subkernel_callback_fn<'ctx>(
+        ctx: &mut CodeGenContext<'ctx, '_>,
+        obj: Option<(Type, ValueEnum<'ctx>)>,
+        fun: (&FunSignature, DefinitionId),
+        args: Vec<(Option<StrRef>, ValueEnum<'ctx>)>,
+        generator: &mut dyn CodeGenerator,
+    ) -> Result<Option<BasicValueEnum<'ctx>>, String> {
+        let int8 = ctx.ctx.i8_type();
+        let int32 = ctx.ctx.i32_type();
+        let size_type = generator.get_size_type(ctx.ctx);
+        let ptr_type = int8.ptr_type(AddressSpace::default());
+        let tag_ptr_type = ctx.ctx.struct_type(&[ptr_type.into(), size_type.into()], false);
+    
+        let subkernel_id = int32.const_int(fun.1 .0 as u64, false);
+        let destination = int32.const_int(fun.? as u64, false); // TODO
+
+        // -- start the subkernel
+        let sid_type = ctx.ctx.i32_type();
+        let dest_type = ctx.ctx.i8_type();
+        let run_type = ctx.ctx.i1_type();
+        let subkernel_start = ctx.module.get_function("subkernel_load_run").unwrap_or_else(|| {
+            ctx.module.add_function(
+                "subkernel_load_run",
+                ctx.ctx.void_type().fn_type(
+                    &[
+                        ctx.ctx.void_type().fn_type(&[sid_type.into(), dest_type.into(), run_type.into()], false),
+                    ],
+                    false,
+                ),
+                None,
+            )
+        });
+        ctx.builder
+            .build_call_or_invoke(subkernel_start, &[subkernel_id.into(), destination.into(), run_type.const_int(1, false)], "subkernel.run")
+            .unwrap();
+        // -- setup rpc tags
+        let mut tag = Vec::new();
+        if obj.is_some() {
+            tag.push(b'O');
+        }
+        for arg in &fun.0.args {
+            gen_rpc_tag(ctx, arg.ty, &mut tag)?;
+        }
+        tag.push(b':');
+        gen_rpc_tag(ctx, fun.0.ret, &mut tag)?;
+    
+        let mut hasher = DefaultHasher::new();
+        tag.hash(&mut hasher);
+        let hash = format!("{}", hasher.finish());
+    
+        let tag_ptr = ctx
+            .module
+            .get_global(hash.as_str())
+            .unwrap_or_else(|| {
+                let tag_arr_ptr = ctx.module.add_global(
+                    int8.array_type(tag.len() as u32),
+                    None,
+                    format!("tagptr{}", fun.1 .0).as_str(),
+                );
+                tag_arr_ptr.set_initializer(&int8.const_array(
+                    &tag.iter().map(|v| int8.const_int(u64::from(*v), false)).collect::<Vec<_>>(),
+                ));
+                tag_arr_ptr.set_linkage(Linkage::Private);
+                let tag_ptr = ctx.module.add_global(tag_ptr_type, None, &hash);
+                tag_ptr.set_linkage(Linkage::Private);
+                tag_ptr.set_initializer(&ctx.ctx.const_struct(
+                    &[
+                        tag_arr_ptr.as_pointer_value().const_cast(ptr_type).into(),
+                        size_type.const_int(tag.len() as u64, false).into(),
+                    ],
+                    false,
+                ));
+                tag_ptr
+            })
+            .as_pointer_value();
+    
+        let arg_length = args.len() + usize::from(obj.is_some());
+    
+        let stackptr = call_stacksave(ctx, Some("rpc.stack"));
+        let args_ptr = ctx
+            .builder
+            .build_array_alloca(
+                ptr_type,
+                ctx.ctx.i32_type().const_int(arg_length as u64, false),
+                "argptr",
+            )
+            .unwrap();
+    
+        // -- rpc args handling
+        let mut keys = fun.0.args.clone();
+        let mut mapping = HashMap::new();
+        for (key, value) in args {
+            mapping.insert(key.unwrap_or_else(|| keys.remove(0).name), value);
+        }
+        // default value handling
+        for k in keys {
+            mapping
+                .insert(k.name, ctx.gen_symbol_val(generator, &k.default_value.unwrap(), k.ty).into());
+        }
+        // reorder the parameters
+        let mut real_params = fun
+            .0
+            .args
+            .iter()
+            .map(|arg| {
+                mapping
+                    .remove(&arg.name)
+                    .unwrap()
+                    .to_basic_value_enum(ctx, generator, arg.ty)
+                    .map(|llvm_val| (llvm_val, arg.ty))
+            })
+            .collect::<Result<Vec<(_, _)>, _>>()?;
+        if let Some(obj) = obj {
+            if let ValueEnum::Static(obj_val) = obj.1 {
+                real_params.insert(0, (obj_val.get_const_obj(ctx, generator), obj.0));
+            } else {
+                // should be an error here...
+                panic!("only host object is allowed");
+            }
+        }
+    
+        for (i, (arg, arg_ty)) in real_params.iter().enumerate() {
+            let arg_slot = format_rpc_arg(generator, ctx, (*arg, *arg_ty, i));
+            let arg_ptr = unsafe {
+                ctx.builder.build_gep(
+                    args_ptr,
+                    &[int32.const_int(i as u64, false)],
+                    &format!("rpc.arg{i}"),
+                )
+            }
+            .unwrap();
+            ctx.builder.build_store(arg_ptr, arg_slot).unwrap();
+        }
+    
+        // send the message
+        let subkernel_send = ctx.module.get_function("subkernel_send_message").unwrap_or_else(|| {
+            ctx.module.add_function(
+                "subkernel_send_message",
+                ctx.ctx.void_type().fn_type(
+                    &[
+                        int32.into(),
+                        tag_ptr_type.ptr_type(AddressSpace::default()).into(),
+                        ptr_type.ptr_type(AddressSpace::default()).into(),
+                    ],
+                    false,
+                ),
+                None,
+            )
+        });
+        ctx.builder
+            .build_call_or_invoke(subkernel_send, &[service_id.into(), tag_ptr.into(), args_ptr.into()], "rpc.send")
+            .unwrap();
+        // reclaim stack space used by arguments
+        call_stackrestore(ctx, stackptr);
+    }
+
+    pub fn subkernel_codegen_callback() -> Arc<GenCall> {
+        Arc::new(GenCall::new(Box::new(|ctx, obj, fun, args, generator| {
+            subkernel_codegen_callback_fn(ctx, obj, fun, args, generator)
+        })))
+    }
+
+    fn subkernel_recv_message<'ctx>(
+        ctx: &mut CodeGenContext<'ctx, '_>,
+        obj: Option<(Type, ValueEnum<'ctx>)>,
+        fun: (&FunSignature, DefinitionId),
+        args: Vec<(Option<StrRef>, ValueEnum<'ctx>)>,
+        generator: &mut dyn CodeGenerator,
+    ) -> Result<Option<BasicValueEnum<'ctx>>, String>  {
+        // -- receive value:
+        // T result = {
+        //   void *ret_ptr = alloca(sizeof(T));
+        //   void *ptr = ret_ptr;
+        //   loop: int size = rpc_recv(ptr);
+        //   // Non-zero: Provide `size` bytes of extra storage for variable-length data.
+        //   if(size) { ptr = alloca(size); goto loop; }
+        //   else *(T*)ret_ptr
+        // }
+        let rpc_recv = ctx.module.get_function("rpc_recv").unwrap_or_else(|| {
+            ctx.module.add_function("rpc_recv", int32.fn_type(&[ptr_type.into()], false), None)
+        });
+    
+        if ctx.unifier.unioned(fun.0.ret, ctx.primitives.none) {
+            ctx.build_call_or_invoke(rpc_recv, &[ptr_type.const_null().into()], "rpc_recv");
+            return Ok(None);
+        }
+    
+        let prehead_bb = ctx.builder.get_insert_block().unwrap();
+        let current_function = prehead_bb.get_parent().unwrap();
+        let head_bb = ctx.ctx.append_basic_block(current_function, "rpc.head");
+        let alloc_bb = ctx.ctx.append_basic_block(current_function, "rpc.continue");
+        let tail_bb = ctx.ctx.append_basic_block(current_function, "rpc.tail");
+    
+        let ret_ty = ctx.get_llvm_abi_type(generator, fun.0.ret);
+        let need_load = !ret_ty.is_pointer_type();
+        let slot = ctx.builder.build_alloca(ret_ty, "rpc.ret.slot").unwrap();
+        let slotgen = ctx.builder.build_bitcast(slot, ptr_type, "rpc.ret.ptr").unwrap();
+        ctx.builder.build_unconditional_branch(head_bb).unwrap();
+        ctx.builder.position_at_end(head_bb);
+    
+        let phi = ctx.builder.build_phi(ptr_type, "rpc.ptr").unwrap();
+        phi.add_incoming(&[(&slotgen, prehead_bb)]);
+        let alloc_size = ctx
+            .build_call_or_invoke(rpc_recv, &[phi.as_basic_value()], "rpc.size.next")
+            .unwrap()
+            .into_int_value();
+        let is_done = ctx
+            .builder
+            .build_int_compare(inkwell::IntPredicate::EQ, int32.const_zero(), alloc_size, "rpc.done")
+            .unwrap();
+    
+        ctx.builder.build_conditional_branch(is_done, tail_bb, alloc_bb).unwrap();
+        ctx.builder.position_at_end(alloc_bb);
+    
+        let alloc_ptr = ctx.builder.build_array_alloca(ptr_type, alloc_size, "rpc.alloc").unwrap();
+        let alloc_ptr = ctx.builder.build_bitcast(alloc_ptr, ptr_type, "rpc.alloc.ptr").unwrap();
+        phi.add_incoming(&[(&alloc_ptr, alloc_bb)]);
+        ctx.builder.build_unconditional_branch(head_bb).unwrap();
+    
+        ctx.builder.position_at_end(tail_bb);
+    
+        let result = ctx.builder.build_load(slot, "rpc.result").unwrap();
+        if need_load {
+            call_stackrestore(ctx, stackptr);
+        }
+        Ok(Some(result))
     }
 }