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virtio-console: Fix failure on unconnected pty 

when I tried qemu with -virtio-console pty the guest hangs and attaching
on /dev/pts/<x> does not return anything if the attachment is too late.

This results in pty_chr_write() returning 0, which causes the port to
get throttled. This results in the guest getting frozen as the
guest->host virtio_console writes don't return until the host releases
the vq element back to the guest.

For the virtio-serial use case we don't want to lose 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>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
This commit is contained in:
Christian Borntraeger 2011-12-29 13:47:43 +01:00 committed by Amit Shah
parent 8d3bc5178f
commit ed8e5a85a1
1 changed files with 13 additions and 1 deletions
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14
hw/virtio-serial-bus.c
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@ -163,7 +163,19 @@ 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);
/*
* 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;