[Qemu-devel] [PATCH 6/7] virtio-serial: Add support for flow control

[Amit Shah <amit.shah@redhat.com>]

This commit lets apps signal an incomplete write.  When that happens,
stop sending out any more data to the app and wait for it to unthrottle
the port.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
---
 hw/virtio-serial-bus.c |   49 +++++++++++++++++++++++++++++++++++++----------
 hw/virtio-serial.h     |   17 ++++++++++++++++
 2 files changed, 55 insertions(+), 11 deletions(-)

diff --git a/hw/virtio-serial-bus.c b/hw/virtio-serial-bus.c
index 6726f72..32938b5 100644
--- a/hw/virtio-serial-bus.c
+++ b/hw/virtio-serial-bus.c
@@ -126,24 +126,49 @@ static void discard_vq_data(VirtQueue *vq, VirtIODevice *vdev)
 static void do_flush_queued_data(VirtIOSerialPort *port, VirtQueue *vq,
                                  VirtIODevice *vdev)
 {
-    VirtQueueElement elem;
-
     assert(port);
     assert(virtio_queue_ready(vq));
 
-    while (!port->throttled && virtqueue_pop(vq, &elem)) {
+    while (!port->throttled) {
         unsigned int i;
 
-        for (i = 0; i < elem.out_num; i++) {
-            size_t buf_size;
-
-            buf_size = elem.out_sg[i].iov_len;
+        /* Pop an elem only if we haven't left off a previous one mid-way */
+        if (!port->elem.out_num) {
+            if (!virtqueue_pop(vq, &port->elem)) {
+                break;
+            }
+            port->iov_idx = 0;
+            port->iov_offset = 0;
+        }
 
-            port->info->have_data(port,
-                                  elem.out_sg[i].iov_base,
-                                  buf_size);
+        for (i = port->iov_idx; i < port->elem.out_num; i++) {
+            size_t buf_size;
+            ssize_t ret;
+
+            buf_size = port->elem.out_sg[i].iov_len - port->iov_offset;
+            ret = port->info->have_data(port,
+                                        port->elem.out_sg[i].iov_base
+                                          + port->iov_offset,
+                                        buf_size);
+            if (ret < 0 && ret != -EAGAIN) {
+                /* We don't handle any other type of errors here */
+                abort();
+            }
+            if (ret == -EAGAIN || (ret >= 0 && ret < buf_size)) {
+                virtio_serial_throttle_port(port, true);
+                port->iov_idx = i;
+                if (ret > 0) {
+                    port->iov_offset += ret;
+                }
+                break;
+            }
+            port->iov_offset = 0;
         }
-        virtqueue_push(vq, &elem, 0);
+        if (port->throttled) {
+            break;
+        }
+        virtqueue_push(vq, &port->elem, 0);
+        port->elem.out_num = 0;
     }
     virtio_notify(vdev, vq);
 }
@@ -709,6 +734,8 @@ static int virtser_port_qdev_init(DeviceState *qdev, DeviceInfo *base)
         port->guest_connected = true;
     }
 
+    port->elem.out_num = 0;
+
     QTAILQ_INSERT_TAIL(&port->vser->ports, port, next);
     port->ivq = port->vser->ivqs[port->id];
     port->ovq = port->vser->ovqs[port->id];
diff --git a/hw/virtio-serial.h b/hw/virtio-serial.h
index 9cc0fb3..a308196 100644
--- a/hw/virtio-serial.h
+++ b/hw/virtio-serial.h
@@ -102,6 +102,23 @@ struct VirtIOSerialPort {
      */
     uint32_t id;
 
+    /*
+     * This is the elem that we pop from the virtqueue.  A slow
+     * backend that consumes guest data (e.g. the file backend for
+     * qemu chardevs) can cause the guest to block till all the output
+     * is flushed.  This isn't desired, so we keep a note of the last
+     * element popped and continue consuming it once the backend
+     * becomes writable again.
+     */
+    VirtQueueElement elem;
+
+    /*
+     * The index and the offset into the iov buffer that was popped in
+     * elem above.
+     */
+    uint32_t iov_idx;
+    uint64_t iov_offset;
+
     /* Identify if this is a port that binds with hvc in the guest */
     uint8_t is_console;
 
-- 
1.7.3.4