virtio-serial: Add support for flow control

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>

1 files changed, 17 insertions, 0 deletions

diff --git a/hw/virtio-serial.h b/hw/virtio-serial.h
index 9cc0fb3543..a308196786 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;