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Date: Thu, 15 Dec 2011 13:44:41 +0100
From: Christian Borntraeger <borntraeger@de.ibm.com>
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Subject: [Qemu-devel] [RFC]  virtio-console fails on unconnected pty
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To: Amit Shah <amit.shah@redhat.com>
Cc: "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Alexander Graf <agraf@suse.de>

Amit,

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 we better drop 
data instead of "killing" the guest console. The old serial console also drops data after a 
retry, so what about dropping data righ away if no listener is connected to the pty?

This experimental patch seems to help. Makes sense?

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

--- a/hw/virtio-serial-bus.c
+++ b/hw/virtio-serial-bus.c
@@ -163,7 +163,9 @@ static void do_flush_queued_data(VirtIOSerialPort *port, VirtQueue *vq,
                 abort();
             }
             if (ret == -EAGAIN || (ret >= 0 && ret < buf_size)) {
-                virtio_serial_throttle_port(port, true);
+                if (!info->is_console) {
+                    virtio_serial_throttle_port(port, true);
+                }
                 port->iov_idx = i;
                 if (ret > 0) {
                     port->iov_offset += ret;