virtio-serial: Implement flow control for individual ports

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.

Disable throttling once a port is closed (and we discard all the
unconsumed buffers in the vq).

The guest kernel can reclaim the buffers when it receives the port close
event or when a port is being removed. Ensure we free up the buffers
before we send out any events to the guest.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

1 files changed, 9 insertions, 0 deletions

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