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

From: Christian Borntraeger
Subject: [Qemu-devel] [PATCHv2] Fix virtio-console failure on unconnected pty
Date: Thu, 29 Dec 2011 13:47:43 +0100
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv: Gecko/20111108 Lightning/1.0b2 Thunderbird/3.1.16

From: Christian Borntraeger <address@hidden>
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 
and a better behaving virtio-console we can revert this fix.
Signed-off-by: Christian Borntraeger <address@hidden>

 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
             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;

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