[Qemu-devel] [PATCH 20/20] virtio-serial: make flow control explicit in virtio-console

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

virtio-console.c used to return a value less than the number of bytes
asked to be written out to a chardev backend in case the backend is not
writable.  virtio-serial-bus.c then implicitly enabled flow control for
that port.

Make this explicit instead.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
---
 hw/virtio-console.c    | 17 +++++++++--------
 hw/virtio-serial-bus.c | 19 +------------------
 2 files changed, 10 insertions(+), 26 deletions(-)

diff --git a/hw/virtio-console.c b/hw/virtio-console.c
index b3ee074..194de64 100644
--- a/hw/virtio-console.c
+++ b/hw/virtio-console.c
@@ -47,20 +47,21 @@ static ssize_t flush_buf(VirtIOSerialPort *port, const uint8_t *buf, size_t len)
     ret = qemu_chr_fe_write(vcon->chr, buf, len);
     trace_virtio_console_flush_buf(port->id, len, ret);
 
-    if (ret < 0) {
+    if (ret <= 0) {
+        VirtIOSerialPortClass *k = VIRTIO_SERIAL_PORT_GET_CLASS(port);
+
         /*
          * Ideally we'd get a better error code than just -1, but
          * that's what the chardev interface gives us right now.  If
          * we had a finer-grained message, like -EPIPE, we could close
-         * this connection.  Absent such error messages, the most we
-         * can do is to return 0 here.
-         *
-         * This will prevent stray -1 values to go to
-         * virtio-serial-bus.c and cause abort()s in
-         * do_flush_queued_data().
+         * this connection.
          */
         ret = 0;
-        qemu_chr_fe_add_watch(vcon->chr, G_IO_OUT, chr_write_unblocked, vcon);
+        if (!k->is_console) {
+            virtio_serial_throttle_port(port, true);
+            qemu_chr_fe_add_watch(vcon->chr, G_IO_OUT, chr_write_unblocked,
+                                  vcon);
+        }
     }
     return ret;
 }
diff --git a/hw/virtio-serial-bus.c b/hw/virtio-serial-bus.c
index aa7d0d7..f76c505 100644
--- a/hw/virtio-serial-bus.c
+++ b/hw/virtio-serial-bus.c
@@ -172,24 +172,7 @@ static void do_flush_queued_data(VirtIOSerialPort *port, VirtQueue *vq,
                                   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)) {
-		/*
-                 * this is a temporary check until chardevs can signal to
-                 * frontends that they are writable again. This prevents
-                 * the console from going into throttled mode (forever)
-                 * if virtio-console is connected to a pty without a
-                 * listener. Otherwise the guest spins forever.
-                 * We can revert this if
-                 * 1: chardevs can notify frondends
-                 * 2: the guest driver does not spin in these cases
-                 */
-                if (!vsc->is_console) {
-                    virtio_serial_throttle_port(port, true);
-                }
+            if (port->throttled) {
                 port->iov_idx = i;
                 if (ret > 0) {
                     port->iov_offset += ret;
-- 
1.8.1.2