[Qemu-devel] [PATCHv2] Fix virtio-console failure on unconnected pty

[Christian Borntraeger <borntraeger@de.ibm.com>]

From: Christian Borntraeger <borntraeger@de.ibm.com>
    
when I tried qemu with -virtio-console pty the guest hangs and attaching on /dev/pts/<x> does
not return anything if the attachement is too late.
Turns out that the console is already throttled and the guest is heavily spinning but get_buf
never returns the buffer. There seems to be no way for the console to unthrottle the port.
    
For the virtio-serial use case we dont want to loose data but for the console case we better
drop  data instead of "killing" the guest console. If we get chardev->frontend notification
and a better behaving virtio-console we can revert this fix.
    
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

---
 hw/virtio-serial-bus.c |   14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

Index: b/hw/virtio-serial-bus.c
===================================================================
--- a/hw/virtio-serial-bus.c
+++ b/hw/virtio-serial-bus.c
@@ -163,7 +163,19 @@ static void do_flush_queued_data(VirtIOS
                 abort();
             }
             if (ret == -EAGAIN || (ret >= 0 && ret < buf_size)) {
-                virtio_serial_throttle_port(port, true);
+		/*
+                 * 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 (!info->is_console) {
+                    virtio_serial_throttle_port(port, true);
+                }
                 port->iov_idx = i;
                 if (ret > 0) {
                     port->iov_offset += ret;