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>
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@ -163,7 +163,19 @@ static void do_flush_queued_data(VirtIOSerialPort *port, VirtQueue *vq,
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abort();
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}
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if (ret == -EAGAIN || (ret >= 0 && ret < buf_size)) {
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/*
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* this is a temporary check until chardevs can signal to
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* frontends that they are writable again. This prevents
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* the console from going into throttled mode (forever)
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* if virtio-console is connected to a pty without a
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* listener. Otherwise the guest spins forever.
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* We can revert this if
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* 1: chardevs can notify frondends
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* 2: the guest driver does not spin in these cases
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*/
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if (!info->is_console) {
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virtio_serial_throttle_port(port, true);
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}
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port->iov_idx = i;
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if (ret > 0) {
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port->iov_offset += ret;
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