Re: [Qemu-devel] [PATCH v3 1/1] virtio: serial: expose a 'guest_writable' callback for users

["Marc-André Lureau" <mlureau@redhat.com>]

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>

I have a somewhat related question, that perhaps someone may help me with:
Why isn't the qemu char driver/device interface based (for most devices)
on socketpair (and equivalent bi-directional pipe on other OS).

It looks to me like it could simplify API and event handling on both fe/be
sides, although it may cost a few more open fds and additional copy.

----- Mail original -----
> 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.shah@redhat.com>
> 
> ---
> v3: document the semantics of the callback (Peter Maydell, Markus)
> v2: check for port != NULL (Peter Maydell)
> ---
>  hw/char/virtio-serial-bus.c       | 31 +++++++++++++++++++++++++++++++
>  include/hw/virtio/virtio-serial.h | 11 +++++++++++
>  2 files changed, 42 insertions(+)
> 
> diff --git a/hw/char/virtio-serial-bus.c b/hw/char/virtio-serial-bus.c
> index c6870f1..bea7a17 100644
> --- a/hw/char/virtio-serial-bus.c
> +++ b/hw/char/virtio-serial-bus.c
> @@ -465,6 +465,37 @@ 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.  However, this call is
> +     * made for each buffer addition by the guest -- even though free
> +     * buffers existed prior to the current buffer addition.  This is
> +     * done so as not to maintain previous state, which will need
> +     * additional live-migration-related changes.
> +     */
> +    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..fe6e696 100644
> --- a/include/hw/virtio/virtio-serial.h
> +++ b/include/hw/virtio/virtio-serial.h
> @@ -98,6 +98,17 @@ typedef struct VirtIOSerialPortClass {
>          /* Guest is now ready to accept data (virtqueues set up). */
>      void (*guest_ready)(VirtIOSerialPort *port);
>  
> +        /*
> +	 * Guest has enqueued a buffer for the host to write into.
> +	 * Called each time a buffer is enqueued by the guest;
> +	 * irrespective of whether there already were free buffers the
> +	 * host could have consumed.
> +	 *
> +	 * This is dependent on both, the guest and host ends being
> +	 * connected.
> +	 */
> +    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
> --
> 1.9.3
> 
>