On (Thu) 29 Dec 2011 [13:47:43], Christian Borntraeger wrote:
> From: Christian Borntraeger <borntrae...@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 <borntrae...@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);
I'm surprised: did you test this with upstream qemu? That codebase
doesn't yet throttle writes, and this code path won't execute. Does
it really not reproduce with this patch?
> + /*
> + * 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;
>
Amit
