Amit Shah <amit.s...@redhat.com> writes:
> Users of virtio-serial may want to know when a port becomes writable. A
> port can stop accepting writes if the guest port is open but not being
> read from. In this case, data gets queued up in the virtqueue, and
> after the vq is full, writes to the port do not succeed.
>
> When the guest reads off a vq element, and adds a new one for the host
> to put data in, we can tell users the port is available for more writes,
> via the new ->guest_writable() callback.
>
> Signed-off-by: Amit Shah <amit.s...@redhat.com>
>
> ---
> v2: check for port != NULL (Peter Maydell)
> ---
> hw/char/virtio-serial-bus.c | 27 +++++++++++++++++++++++++++
> include/hw/virtio/virtio-serial.h | 3 +++
> 2 files changed, 30 insertions(+)
>
> diff --git a/hw/char/virtio-serial-bus.c b/hw/char/virtio-serial-bus.c
> index 3931085..1c7acbf 100644
> --- a/hw/char/virtio-serial-bus.c
> +++ b/hw/char/virtio-serial-bus.c
> @@ -465,6 +465,33 @@ static void handle_output(VirtIODevice *vdev, VirtQueue
> *vq)
>
> static void handle_input(VirtIODevice *vdev, VirtQueue *vq)
> {
> + /*
> + * Users of virtio-serial would like to know when guest becomes
> + * writable again -- i.e. if a vq had stuff queued up and the
> + * guest wasn't reading at all, the host would not be able to
> + * write to the vq anymore. Once the guest reads off something,
> + * we can start queueing things up again.
> + */
> + VirtIOSerial *vser;
> + VirtIOSerialPort *port;
> + VirtIOSerialPortClass *vsc;
> +
> + vser = VIRTIO_SERIAL(vdev);
> + port = find_port_by_vq(vser, vq);
> +
> + if (!port) {
> + return;
> + }
> + vsc = VIRTIO_SERIAL_PORT_GET_CLASS(port);
> +
> + /*
> + * If guest_connected is false, this call is being made by the
> + * early-boot queueing up of descriptors, which is just noise for
> + * the host apps -- don't disturb them in that case.
> + */
> + if (port->guest_connected && port->host_connected &&
> vsc->guest_writable) {
> + vsc->guest_writable(port);
> + }
> }
>
> static uint32_t get_features(VirtIODevice *vdev, uint32_t features)
> diff --git a/include/hw/virtio/virtio-serial.h
> b/include/hw/virtio/virtio-serial.h
> index a679e54..b434f78 100644
> --- a/include/hw/virtio/virtio-serial.h
> +++ b/include/hw/virtio/virtio-serial.h
> @@ -98,6 +98,9 @@ typedef struct VirtIOSerialPortClass {
> /* Guest is now ready to accept data (virtqueues set up). */
> void (*guest_ready)(VirtIOSerialPort *port);
>
> + /* Guest vq became writable again */
> + void (*guest_writable)(VirtIOSerialPort *port);
> +
> /*
> * Guest wrote some data to the port. This data is handed over to
> * the app via this callback. The app can return a size less than
The code should work, but whether it makes sense is hard to judge for
virtio noobs like me without a user of guest_writable. The conditional
guarding vsc->guest_writable(port) in particular.
virtio_add_queue()'s callback being undocumented doesn't exactly help,
either. Fun: the parameter is called handle_output, the argument is
handle_input. Clear as mud!
