From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

On a real serial device, the open can block if the handshake
lines are in a particular state.  If a QEMU is passing the serial
device to the guest, the QEMU startup is blocked opening the device
(with a symptom seen as a timeout from libvirt).

Open the serial port with O_NONBLOCK.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 chardev/char-serial.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/chardev/char-serial.c b/chardev/char-serial.c
index ae548d2..3299b46 100644
--- a/chardev/char-serial.c
+++ b/chardev/char-serial.c
@@ -265,7 +265,8 @@ static void qmp_chardev_open_serial(Chardev *chr,
     ChardevHostdev *serial = backend->u.serial.data;
     int fd;
 
-    fd = qmp_chardev_open_file_source(serial->device, O_RDWR, errp);
+    fd = qmp_chardev_open_file_source(serial->device, O_RDWR | O_NONBLOCK,
+                                      errp);
     if (fd < 0) {
         return;
     }
-- 
1.8.3.1