subkernels: support exceptions from subkernels

src/libboard_artiq/src/drtioaux_proto.rs

@@ -267,12 +267,14 @@ pub enum Packet {
         exception_src: u8,
     },
     SubkernelExceptionRequest {
+        source: u8,
         destination: u8,
     },
     SubkernelException {
+        destination: u8,
         last: bool,
         length: u16,
-        data: [u8; SAT_PAYLOAD_MAX_SIZE],
+        data: [u8; MASTER_PAYLOAD_MAX_SIZE],
     },
     SubkernelMessage {
         source: u8,
@@ -524,14 +526,17 @@ impl Packet {
                 exception_src: reader.read_u8()?,
             },
             0xc9 => Packet::SubkernelExceptionRequest {
+                source: reader.read_u8()?,
                 destination: reader.read_u8()?,
             },
             0xca => {
+                let destination = reader.read_u8()?;
                 let last = reader.read_bool()?;
                 let length = reader.read_u16()?;
-                let mut data: [u8; SAT_PAYLOAD_MAX_SIZE] = [0; SAT_PAYLOAD_MAX_SIZE];
+                let mut data: [u8; MASTER_PAYLOAD_MAX_SIZE] = [0; MASTER_PAYLOAD_MAX_SIZE];
                 reader.read_exact(&mut data[0..length as usize])?;
                 Packet::SubkernelException {
+                    destination: destination,
                     last: last,
                     length: length,
                     data: data,
@@ -896,12 +901,19 @@ impl Packet {
                 writer.write_bool(with_exception)?;
                 writer.write_u8(exception_src)?;
             }
-            Packet::SubkernelExceptionRequest { destination } => {
+            Packet::SubkernelExceptionRequest { source, destination } => {
                 writer.write_u8(0xc9)?;
+                writer.write_u8(source)?;
                 writer.write_u8(destination)?;
             }
-            Packet::SubkernelException { last, length, data } => {
+            Packet::SubkernelException {
+                destination,
+                last,
+                length,
+                data,
+            } => {
                 writer.write_u8(0xca)?;
+                writer.write_u8(destination)?;
                 writer.write_bool(last)?;
                 writer.write_u16(length)?;
                 writer.write_all(&data[0..length as usize])?;
@@ -943,6 +955,8 @@ impl Packet {
             Packet::SubkernelLoadRunReply { destination, .. } => Some(*destination),
             Packet::SubkernelMessage { destination, .. } => Some(*destination),
             Packet::SubkernelMessageAck { destination } => Some(*destination),
+            Packet::SubkernelExceptionRequest { destination, .. } => Some(*destination),
+            Packet::SubkernelException { destination, .. } => Some(*destination),
             Packet::DmaPlaybackStatus { destination, .. } => Some(*destination),
             Packet::SubkernelFinished { destination, .. } => Some(*destination),
             _ => None,

src/libksupport/src/eh_artiq.rs

@@ -14,8 +14,10 @@
 
 use core::mem;
 
+use core_io::Error as ReadError;
 use cslice::CSlice;
 use dwarf::eh::{self, EHAction, EHContext};
+use io::{Cursor, ProtoRead};
 use libc::{c_int, c_void, uintptr_t};
 use log::{error, trace};
 use unwind as uw;
@@ -295,6 +297,60 @@ pub unsafe extern "C" fn raise(exception: *const Exception) -> ! {
     unreachable!();
 }
 
+fn read_exception_string<'a>(reader: &mut Cursor<&[u8]>) -> Result<CSlice<'a, u8>, ReadError> {
+    let len = reader.read_u32()? as usize;
+    if len == usize::MAX {
+        let data = reader.read_u32()?;
+        Ok(unsafe { CSlice::new(data as *const u8, len) })
+    } else {
+        let pos = reader.position();
+        let slice = unsafe {
+            let ptr = reader.get_ref().as_ptr().offset(pos as isize);
+            CSlice::new(ptr, len)
+        };
+        reader.set_position(pos + len);
+        Ok(slice)
+    }
+}
+
+fn read_exception(raw_exception: &[u8]) -> Result<Exception, ReadError> {
+    let mut reader = Cursor::new(raw_exception);
+
+    let mut byte = reader.read_u8()?;
+    // to sync
+    while byte != 0x5a {
+        byte = reader.read_u8()?;
+    }
+    // skip sync bytes, 0x09 indicates exception
+    while byte != 0x09 {
+        byte = reader.read_u8()?;
+    }
+    let _len = reader.read_u32()?;
+    // ignore the remaining exceptions, stack traces etc. - unwinding from another device would be unwise anyway
+    Ok(Exception {
+        id: reader.read_u32()?,
+        message: read_exception_string(&mut reader)?,
+        param: [
+            reader.read_u64()? as i64,
+            reader.read_u64()? as i64,
+            reader.read_u64()? as i64,
+        ],
+        file: read_exception_string(&mut reader)?,
+        line: reader.read_u32()?,
+        column: reader.read_u32()?,
+        function: read_exception_string(&mut reader)?,
+    })
+}
+
+pub fn raise_raw(raw_exception: &[u8]) -> ! {
+    use crate::artiq_raise;
+    if let Ok(exception) = read_exception(raw_exception) {
+        unsafe { raise(&exception) };
+    } else {
+        artiq_raise!("SubkernelError", "Error passing exception");
+    }
+}
+
 pub unsafe extern "C" fn resume() -> ! {
     trace!("resume");
     assert!(EXCEPTION_BUFFER.exception_count != 0);

src/libksupport/src/kernel/mod.rs

@@ -23,6 +23,7 @@ pub enum SubkernelStatus {
     Timeout,
     IncorrectState,
     CommLost,
+    Exception(Vec<u8>),
     OtherError,
 }
 
@@ -90,9 +91,7 @@ pub enum Message {
         timeout: i64,
     },
     #[cfg(has_drtio)]
-    SubkernelAwaitFinishReply {
-        status: SubkernelStatus,
-    },
+    SubkernelAwaitFinishReply,
     #[cfg(has_drtio)]
     SubkernelMsgSend {
         id: u32,
@@ -109,9 +108,10 @@ pub enum Message {
     },
     #[cfg(has_drtio)]
     SubkernelMsgRecvReply {
-        status: SubkernelStatus,
         count: u8,
     },
+    #[cfg(has_drtio)]
+    SubkernelError(SubkernelStatus),
 }
 
 static CHANNEL_0TO1: Mutex<Option<sync_channel::Sender<'static, Message>>> = Mutex::new(None);

src/libksupport/src/kernel/subkernel.rs

@@ -3,7 +3,7 @@ use alloc::vec::Vec;
 use cslice::CSlice;
 
 use super::{Message, SubkernelStatus, KERNEL_CHANNEL_0TO1, KERNEL_CHANNEL_1TO0};
-use crate::{artiq_raise, rpc::send_args};
+use crate::{artiq_raise, eh_artiq, rpc::send_args};
 
 pub extern "C" fn load_run(id: u32, destination: u8, run: bool) {
     unsafe {
@@ -36,21 +36,18 @@ pub extern "C" fn await_finish(id: u32, timeout: i64) {
             });
     }
     match unsafe { KERNEL_CHANNEL_0TO1.as_mut().unwrap() }.recv() {
-        Message::SubkernelAwaitFinishReply {
-            status: SubkernelStatus::NoError,
-        } => (),
-        Message::SubkernelAwaitFinishReply {
-            status: SubkernelStatus::IncorrectState,
-        } => artiq_raise!("SubkernelError", "Subkernel not running"),
-        Message::SubkernelAwaitFinishReply {
-            status: SubkernelStatus::Timeout,
-        } => artiq_raise!("SubkernelError", "Subkernel timed out"),
-        Message::SubkernelAwaitFinishReply {
-            status: SubkernelStatus::CommLost,
-        } => artiq_raise!("SubkernelError", "Lost communication with satellite"),
-        Message::SubkernelAwaitFinishReply {
-            status: SubkernelStatus::OtherError,
-        } => artiq_raise!("SubkernelError", "An error occurred during subkernel operation"),
+        Message::SubkernelAwaitFinishReply => (),
+        Message::SubkernelError(SubkernelStatus::IncorrectState) => {
+            artiq_raise!("SubkernelError", "Subkernel not running")
+        }
+        Message::SubkernelError(SubkernelStatus::Timeout) => artiq_raise!("SubkernelError", "Subkernel timed out"),
+        Message::SubkernelError(SubkernelStatus::CommLost) => {
+            artiq_raise!("SubkernelError", "Lost communication with satellite")
+        }
+        Message::SubkernelError(SubkernelStatus::OtherError) => {
+            artiq_raise!("SubkernelError", "An error occurred during subkernel operation")
+        }
+        Message::SubkernelError(SubkernelStatus::Exception(raw_exception)) => eh_artiq::raise_raw(&raw_exception),
         _ => panic!("expected SubkernelAwaitFinishReply after SubkernelAwaitFinishRequest"),
     }
 }
@@ -92,30 +89,22 @@ pub extern "C" fn await_message(id: i32, timeout: i64, tags: &CSlice<u8>, min: u
             });
     }
     match unsafe { KERNEL_CHANNEL_0TO1.as_mut().unwrap() }.recv() {
-        Message::SubkernelMsgRecvReply {
-            status: SubkernelStatus::NoError,
-            count,
-        } => {
+        Message::SubkernelMsgRecvReply { count } => {
             if min > count || count > max {
                 artiq_raise!("SubkernelError", "Received more or less arguments than required")
             }
         }
-        Message::SubkernelMsgRecvReply {
-            status: SubkernelStatus::IncorrectState,
-            ..
-        } => artiq_raise!("SubkernelError", "Subkernel not running"),
-        Message::SubkernelMsgRecvReply {
-            status: SubkernelStatus::Timeout,
-            ..
-        } => artiq_raise!("SubkernelError", "Subkernel timed out"),
-        Message::SubkernelMsgRecvReply {
-            status: SubkernelStatus::CommLost,
-            ..
-        } => artiq_raise!("SubkernelError", "Lost communication with satellite"),
-        Message::SubkernelMsgRecvReply {
-            status: SubkernelStatus::OtherError,
-            ..
-        } => artiq_raise!("SubkernelError", "An error occurred during subkernel operation"),
+        Message::SubkernelError(SubkernelStatus::IncorrectState) => {
+            artiq_raise!("SubkernelError", "Subkernel not running")
+        }
+        Message::SubkernelError(SubkernelStatus::Timeout) => artiq_raise!("SubkernelError", "Subkernel timed out"),
+        Message::SubkernelError(SubkernelStatus::CommLost) => {
+            artiq_raise!("SubkernelError", "Lost communication with satellite")
+        }
+        Message::SubkernelError(SubkernelStatus::OtherError) => {
+            artiq_raise!("SubkernelError", "An error occurred during subkernel operation")
+        }
+        Message::SubkernelError(SubkernelStatus::Exception(raw_exception)) => eh_artiq::raise_raw(&raw_exception),
         _ => panic!("expected SubkernelMsgRecvReply after SubkernelMsgRecvRequest"),
     }
     // RpcRecvRequest should be called after this to receive message data

src/runtime/src/comms.rs

@@ -422,33 +422,23 @@ async fn handle_run_kernel(
             #[cfg(has_drtio)]
             kernel::Message::SubkernelAwaitFinishRequest { id, timeout } => {
                 let res = subkernel::await_finish(aux_mutex, routing_table, timer, id, timeout).await;
-                let status = match res {
-                    Ok(ref res) => {
+                let response = match res {
+                    Ok(res) => {
                         if res.status == subkernel::FinishStatus::CommLost {
-                            kernel::SubkernelStatus::CommLost
-                        } else if let Some(exception) = &res.exception {
-                            error!("Exception in subkernel");
-                            match stream {
-                                None => (),
-                                Some(stream) => {
-                                    write_chunk(stream, exception).await?;
-                                }
-                            }
-                            // will not be called after exception is served
-                            kernel::SubkernelStatus::OtherError
+                            kernel::Message::SubkernelError(kernel::SubkernelStatus::CommLost)
+                        } else if let Some(exception) = res.exception {
+                            kernel::Message::SubkernelError(kernel::SubkernelStatus::Exception(exception))
                         } else {
-                            kernel::SubkernelStatus::NoError
+                            kernel::Message::SubkernelAwaitFinishReply
                         }
                     }
-                    Err(SubkernelError::Timeout) => kernel::SubkernelStatus::Timeout,
-                    Err(SubkernelError::IncorrectState) => kernel::SubkernelStatus::IncorrectState,
-                    Err(_) => kernel::SubkernelStatus::OtherError,
+                    Err(SubkernelError::Timeout) => kernel::Message::SubkernelError(kernel::SubkernelStatus::Timeout),
+                    Err(SubkernelError::IncorrectState) => {
+                        kernel::Message::SubkernelError(kernel::SubkernelStatus::IncorrectState)
+                    }
+                    Err(_) => kernel::Message::SubkernelError(kernel::SubkernelStatus::OtherError),
                 };
-                control
-                    .borrow_mut()
-                    .tx
-                    .async_send(kernel::Message::SubkernelAwaitFinishReply { status: status })
-                    .await;
+                control.borrow_mut().tx.async_send(response).await;
             }
             #[cfg(has_drtio)]
             kernel::Message::SubkernelMsgSend { id, destination, data } => {
@@ -469,35 +459,23 @@ async fn handle_run_kernel(
             #[cfg(has_drtio)]
             kernel::Message::SubkernelMsgRecvRequest { id, timeout, tags } => {
                 let message_received = subkernel::message_await(id as u32, timeout, timer).await;
-                let (status, count) = match message_received {
-                    Ok(ref message) => (kernel::SubkernelStatus::NoError, message.count),
-                    Err(SubkernelError::Timeout) => (kernel::SubkernelStatus::Timeout, 0),
-                    Err(SubkernelError::IncorrectState) => (kernel::SubkernelStatus::IncorrectState, 0),
-                    Err(SubkernelError::CommLost) => (kernel::SubkernelStatus::CommLost, 0),
+                let response = match message_received {
+                    Ok(ref message) => kernel::Message::SubkernelMsgRecvReply { count: message.count },
+                    Err(SubkernelError::Timeout) => kernel::Message::SubkernelError(kernel::SubkernelStatus::Timeout),
+                    Err(SubkernelError::IncorrectState) => {
+                        kernel::Message::SubkernelError(kernel::SubkernelStatus::IncorrectState)
+                    }
+                    Err(SubkernelError::CommLost) => kernel::Message::SubkernelError(kernel::SubkernelStatus::CommLost),
                     Err(SubkernelError::SubkernelException) => {
-                        error!("Exception in subkernel");
                         // just retrieve the exception
                         let status = subkernel::await_finish(aux_mutex, routing_table, timer, id as u32, timeout)
                             .await
                             .unwrap();
-                        match stream {
-                            None => (),
-                            Some(stream) => {
-                                write_chunk(stream, &status.exception.unwrap()).await?;
+                        kernel::Message::SubkernelError(kernel::SubkernelStatus::Exception(status.exception.unwrap()))
                     }
-                        }
-                        (kernel::SubkernelStatus::OtherError, 0)
-                    }
-                    Err(_) => (kernel::SubkernelStatus::OtherError, 0),
+                    Err(_) => kernel::Message::SubkernelError(kernel::SubkernelStatus::OtherError),
                 };
-                control
-                    .borrow_mut()
-                    .tx
-                    .async_send(kernel::Message::SubkernelMsgRecvReply {
-                        status: status,
-                        count: count,
-                    })
-                    .await;
+                control.borrow_mut().tx.async_send(response).await;
                 if let Ok(message) = message_received {
                     // receive code almost identical to RPC recv, except we are not reading from a stream
                     let mut reader = Cursor::new(message.data);
@@ -529,7 +507,7 @@ async fn handle_run_kernel(
                             .async_send(kernel::Message::RpcRecvReply(Ok(0)))
                             .await;
                         i += 1;
-                        if i < count {
+                        if i < message.count {
                             current_tags = remaining_tags;
                         } else {
                             break;

src/runtime/src/rtio_mgt.rs

@@ -129,6 +129,8 @@ pub mod drtio {
             | Packet::SubkernelLoadRunReply { destination, .. }
             | Packet::SubkernelMessage { destination, .. }
             | Packet::SubkernelMessageAck { destination, .. }
+            | Packet::SubkernelException { destination, .. }
+            | Packet::SubkernelExceptionRequest { destination, .. }
             | Packet::DmaPlaybackStatus { destination, .. }
             | Packet::SubkernelFinished { destination, .. } => {
                 if destination == 0 {
@@ -183,10 +185,7 @@ pub mod drtio {
 
     async fn drain_buffer(linkno: u8, draining_time: Milliseconds, timer: GlobalTimer) {
         let max_time = timer.get_time() + draining_time;
-        loop {
-            if timer.get_time() > max_time {
-                return;
-            }
+        while timer.get_time() < max_time {
             let _ = drtioaux_async::recv(linkno).await;
         }
     }
@@ -835,13 +834,19 @@ pub mod drtio {
                 linkno,
                 routing_table,
                 &Packet::SubkernelExceptionRequest {
+                    source: 0,
                     destination: destination,
                 },
                 timer,
             )
             .await?;
             match reply {
-                Packet::SubkernelException { last, length, data } => {
+                Packet::SubkernelException {
+                    destination: 0,
+                    last,
+                    length,
+                    data,
+                } => {
                     remote_data.extend(&data[0..length as usize]);
                     if last {
                         return Ok(remote_data);

src/satman/src/main.rs

@@ -895,6 +895,7 @@ fn process_aux_packet(
             Ok(())
         }
         drtioaux::Packet::SubkernelExceptionRequest {
+            source,
             destination: _destination,
         } => {
             forward!(
@@ -907,17 +908,46 @@ fn process_aux_packet(
                 &packet,
                 timer
             );
-            let mut data_slice: [u8; SAT_PAYLOAD_MAX_SIZE] = [0; SAT_PAYLOAD_MAX_SIZE];
+            let mut data_slice: [u8; MASTER_PAYLOAD_MAX_SIZE] = [0; MASTER_PAYLOAD_MAX_SIZE];
             let meta = kernel_manager.exception_get_slice(&mut data_slice);
-            drtioaux::send(
-                0,
-                &drtioaux::Packet::SubkernelException {
+            router.send(
+                drtioaux::Packet::SubkernelException {
+                    destination: source,
                     last: meta.status.is_last(),
                     length: meta.len,
                     data: data_slice,
                 },
+                _routing_table,
+                *rank,
+                *self_destination,
             )
         }
+        drtioaux::Packet::SubkernelException {
+            destination: _destination,
+            last,
+            length,
+            data,
+        } => {
+            forward!(
+                router,
+                _routing_table,
+                _destination,
+                *rank,
+                *self_destination,
+                _repeaters,
+                &packet,
+                timer
+            );
+            kernel_manager.received_exception(
+                &data[..length as usize],
+                last,
+                router,
+                _routing_table,
+                *rank,
+                *self_destination,
+            );
+            Ok(())
+        }
         drtioaux::Packet::SubkernelMessage {
             source,
             destination: _destination,

src/satman/src/routing.rs

@@ -4,7 +4,7 @@ use core::cmp::min;
 #[cfg(has_drtio_routing)]
 use libboard_artiq::pl::csr;
 use libboard_artiq::{drtio_routing, drtioaux,
-                     drtioaux_proto::{PayloadStatus, MASTER_PAYLOAD_MAX_SIZE, SAT_PAYLOAD_MAX_SIZE}};
+                     drtioaux_proto::{PayloadStatus, MASTER_PAYLOAD_MAX_SIZE}};
 
 pub struct SliceMeta {
     pub destination: u8,
@@ -57,7 +57,6 @@ impl Sliceable {
         self.data.extend(data);
     }
 
-    get_slice_fn!(get_slice_sat, SAT_PAYLOAD_MAX_SIZE);
     get_slice_fn!(get_slice_master, MASTER_PAYLOAD_MAX_SIZE);
 }
 

src/satman/src/subkernel.rs

@@ -11,7 +11,7 @@ use io::{Cursor, ProtoWrite};
 use ksupport::{eh_artiq, kernel, rpc};
 use libboard_artiq::{drtio_routing::RoutingTable,
                      drtioaux,
-                     drtioaux_proto::{PayloadStatus, MASTER_PAYLOAD_MAX_SIZE, SAT_PAYLOAD_MAX_SIZE},
+                     drtioaux_proto::{PayloadStatus, MASTER_PAYLOAD_MAX_SIZE},
                      pl::csr};
 use libboard_zynq::{time::Milliseconds, timer::GlobalTimer};
 use libcortex_a9::sync_channel::Receiver;
@@ -47,6 +47,9 @@ enum KernelState {
     DmaAwait {
         max_time: Milliseconds,
     },
+    SubkernelRetrievingException {
+        destination: u8,
+    },
 }
 
 #[derive(Debug)]
@@ -123,10 +126,11 @@ struct MessageManager {
 struct Session {
     id: u32,
     kernel_state: KernelState,
-    last_exception: Option<Sliceable>,
+    last_exception: Option<Sliceable>,   // exceptions raised locally
+    external_exception: Option<Vec<u8>>, // exceptions from sub-subkernels
     messages: MessageManager,
     source: u8, // which destination requested running the kernel
-    subkernels_finished: Vec<u32>,
+    subkernels_finished: Vec<(u32, Option<u8>)>,
 }
 
 impl Session {
@@ -135,6 +139,7 @@ impl Session {
             id: id,
             kernel_state: KernelState::Absent,
             last_exception: None,
+            external_exception: None,
             messages: MessageManager::new(),
             source: 0,
             subkernels_finished: Vec::new(),
@@ -410,9 +415,9 @@ impl<'a> Manager<'_> {
         }
     }
 
-    pub fn exception_get_slice(&mut self, data_slice: &mut [u8; SAT_PAYLOAD_MAX_SIZE]) -> SliceMeta {
+    pub fn exception_get_slice(&mut self, data_slice: &mut [u8; MASTER_PAYLOAD_MAX_SIZE]) -> SliceMeta {
         match self.session.last_exception.as_mut() {
-            Some(exception) => exception.get_slice_sat(data_slice),
+            Some(exception) => exception.get_slice_master(data_slice),
             None => SliceMeta {
                 destination: 0,
                 len: 0,
@@ -540,7 +545,7 @@ impl<'a> Manager<'_> {
             return;
         }
 
-        match self.process_external_messages(timer) {
+        match self.process_external_messages(router, routing_table, rank, destination, timer) {
             Ok(()) => (),
             Err(Error::AwaitingMessage) => return, // kernel still waiting, do not process kernel messages
             Err(Error::KernelException(exception)) => {
@@ -596,6 +601,41 @@ impl<'a> Manager<'_> {
         }
     }
 
+    fn check_finished_kernels(
+        &mut self,
+        id: u32,
+        router: &mut Router,
+        routing_table: &RoutingTable,
+        rank: u8,
+        self_destination: u8,
+    ) {
+        for (i, (status, exception_source)) in self.session.subkernels_finished.iter().enumerate() {
+            if *status == id {
+                if exception_source.is_none() {
+                    self.control.tx.send(kernel::Message::SubkernelAwaitFinishReply);
+                    self.session.kernel_state = KernelState::Running;
+                    self.session.subkernels_finished.swap_remove(i);
+                } else {
+                    let destination = exception_source.unwrap();
+                    self.session.external_exception = Some(Vec::new());
+                    self.session.kernel_state = KernelState::SubkernelRetrievingException {
+                        destination: destination,
+                    };
+                    router.route(
+                        drtioaux::Packet::SubkernelExceptionRequest {
+                            source: self_destination,
+                            destination: destination,
+                        },
+                        &routing_table,
+                        rank,
+                        self_destination,
+                    );
+                }
+                break;
+            }
+        }
+    }
+
     pub fn subkernel_load_run_reply(&mut self, succeeded: bool) {
         if self.session.kernel_state == KernelState::SubkernelAwaitLoad {
             self.control
@@ -608,16 +648,46 @@ impl<'a> Manager<'_> {
     }
 
     pub fn remote_subkernel_finished(&mut self, id: u32, with_exception: bool, exception_source: u8) {
-        if with_exception {
-            self.kernel_stop();
-            self.last_finished = Some(SubkernelFinished {
-                source: self.session.source,
-                id: self.session.id,
-                with_exception: true,
-                exception_source: exception_source,
-            })
+        let exception_src = if with_exception { Some(exception_source) } else { None };
+        self.session.subkernels_finished.push((id, exception_src));
+    }
+
+    pub fn received_exception(
+        &mut self,
+        exception_data: &[u8],
+        last: bool,
+        router: &mut Router,
+        routing_table: &RoutingTable,
+        rank: u8,
+        self_destination: u8,
+    ) {
+        if let KernelState::SubkernelRetrievingException { destination } = self.session.kernel_state {
+            self.session
+                .external_exception
+                .as_mut()
+                .unwrap()
+                .extend_from_slice(exception_data);
+            if last {
+                self.control
+                    .tx
+                    .send(kernel::Message::SubkernelError(kernel::SubkernelStatus::Exception(
+                        self.session.external_exception.take().unwrap(),
+                    )));
+                self.session.kernel_state = KernelState::Running;
             } else {
-            self.session.subkernels_finished.push(id);
+                /* fetch another slice */
+                router.route(
+                    drtioaux::Packet::SubkernelExceptionRequest {
+                        source: self_destination,
+                        destination: destination,
+                    },
+                    routing_table,
+                    rank,
+                    self_destination,
+                );
+            }
+        } else {
+            warn!("Received unsolicited exception data");
         }
     }
 
@@ -780,28 +850,35 @@ impl<'a> Manager<'_> {
         Ok(false)
     }
 
-    fn process_external_messages(&mut self, timer: &GlobalTimer) -> Result<(), Error> {
+    fn process_external_messages(
+        &mut self,
+        router: &mut Router,
+        routing_table: &RoutingTable,
+        rank: u8,
+        self_destination: u8,
+        timer: &GlobalTimer,
+    ) -> Result<(), Error> {
         match &self.session.kernel_state {
             KernelState::MsgAwait { max_time, id, tags } => {
                 if let Some(max_time) = *max_time {
                     if timer.get_time() > max_time {
-                        self.control.tx.send(kernel::Message::SubkernelMsgRecvReply {
-                            status: kernel::SubkernelStatus::Timeout,
-                            count: 0,
-                        });
+                        self.control
+                            .tx
+                            .send(kernel::Message::SubkernelError(kernel::SubkernelStatus::Timeout));
                         self.session.kernel_state = KernelState::Running;
                         return Ok(());
                     }
                 }
                 if let Some(message) = self.session.messages.get_incoming(*id) {
-                    self.control.tx.send(kernel::Message::SubkernelMsgRecvReply {
-                        status: kernel::SubkernelStatus::NoError,
-                        count: message.count,
-                    });
+                    self.control
+                        .tx
+                        .send(kernel::Message::SubkernelMsgRecvReply { count: message.count });
                     let tags = tags.clone();
                     self.session.kernel_state = KernelState::Running;
                     self.pass_message_to_kernel(&message, tags, timer)
                 } else {
+                    let id = *id;
+                    self.check_finished_kernels(id, router, routing_table, rank, self_destination);
                     Err(Error::AwaitingMessage)
                 }
             }
@@ -817,27 +894,18 @@ impl<'a> Manager<'_> {
             KernelState::SubkernelAwaitFinish { max_time, id } => {
                 if let Some(max_time) = *max_time {
                     if timer.get_time() > max_time {
-                        self.control.tx.send(kernel::Message::SubkernelAwaitFinishReply {
-                            status: kernel::SubkernelStatus::Timeout,
-                        });
+                        self.control
+                            .tx
+                            .send(kernel::Message::SubkernelError(kernel::SubkernelStatus::Timeout));
                         self.session.kernel_state = KernelState::Running;
                         return Ok(());
                     }
                 }
-                let mut i = 0;
-                for status in &self.session.subkernels_finished {
-                    if *status == *id {
-                        self.control.tx.send(kernel::Message::SubkernelAwaitFinishReply {
-                            status: kernel::SubkernelStatus::NoError,
-                        });
-                        self.session.kernel_state = KernelState::Running;
-                        self.session.subkernels_finished.swap_remove(i);
-                        break;
-                    }
-                    i += 1;
-                }
+                let id = *id;
+                self.check_finished_kernels(id, router, routing_table, rank, self_destination);
                 Ok(())
             }
+            KernelState::SubkernelRetrievingException { .. } => Err(Error::AwaitingMessage),
             KernelState::DmaAwait { max_time } | KernelState::DmaPendingAwait { max_time, .. } => {
                 if timer.get_time() > *max_time {
                     self.control.tx.send(kernel::Message::DmaAwaitRemoteReply {