[Qemu-devel] [PATCH 17/17] virtio-serial: Implement flow control for individual ports

[Amit Shah]

Individual ports can now signal to the virtio-serial core to stop
sending data if the ports cannot immediately handle new data.  When a
port later unthrottles, any data queued up in the virtqueue are sent to
the port.

Signed-off-by: Amit Shah <address@hidden>
---
 hw/virtio-serial-bus.c |   24 ++++++++++++++++++++++++
 hw/virtio-serial.h     |    9 +++++++++
 2 files changed, 33 insertions(+), 0 deletions(-)

diff --git a/hw/virtio-serial-bus.c b/hw/virtio-serial-bus.c
index 757de7c..7b57ed8 100644
--- a/hw/virtio-serial-bus.c
+++ b/hw/virtio-serial-bus.c
@@ -187,6 +187,11 @@ int virtio_serial_close(VirtIOSerialPort *port)
     port->host_connected = false;
     send_control_event(port, VIRTIO_CONSOLE_PORT_OPEN, 0);
 
+    /*
+     * If there's any data the guest sent which the app didn't
+     * consume, discard it.
+     */
+    flush_queued_data(port, true);
     return 0;
 }
 
@@ -226,6 +231,21 @@ size_t virtio_serial_guest_ready(VirtIOSerialPort *port)
     return 0;
 }
 
+void virtio_serial_throttle_port(VirtIOSerialPort *port, bool throttle)
+{
+    if (!port) {
+        return;
+    }
+
+    if (throttle) {
+        port->throttled = true;
+        return;
+    }
+
+    port->throttled = false;
+    flush_queued_data(port, false);
+}
+
 /* Guest wants to notify us of some event */
 static void handle_control_message(VirtIOSerial *vser, void *buf, size_t len)
 {
@@ -382,6 +402,10 @@ static void handle_output(VirtIODevice *vdev, VirtQueue 
*vq)
         discard = true;
     }
 
+    if (!discard && port->throttled) {
+        return;
+    }
+
     flush_queued_data(port, discard);
 }
 
diff --git a/hw/virtio-serial.h b/hw/virtio-serial.h
index 62d76a2..a93b545 100644
--- a/hw/virtio-serial.h
+++ b/hw/virtio-serial.h
@@ -110,6 +110,8 @@ struct VirtIOSerialPort {
     bool guest_connected;
     /* Is this device open for IO on the host? */
     bool host_connected;
+    /* Do apps not want to receive data? */
+    bool throttled;
 };
 
 struct VirtIOSerialPortInfo {
@@ -173,4 +175,11 @@ ssize_t virtio_serial_write(VirtIOSerialPort *port, const 
uint8_t *buf,
  */
 size_t virtio_serial_guest_ready(VirtIOSerialPort *port);
 
+/*
+ * Flow control: Ports can signal to the virtio-serial core to stop
+ * sending data or re-start sending data, depending on the 'throttle'
+ * value here.
+ */
+void virtio_serial_throttle_port(VirtIOSerialPort *port, bool throttle);
+
 #endif
-- 
1.6.2.5