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

[Amit Shah <amit.shah@redhat.com>]

On (Thu) 19 Mar 2015 [11:21:04], Markus Armbruster wrote:
> Amit Shah <amit.shah@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.shah@redhat.com>
> >
> > ---
> >
> > Well I forgot to send this out for 2.3; I was waiting for the window to
> > open, and totally forgot about it when it was open.  Since this adds an
> > internal API, and there's no chance of regressions, I propose we include
> > this in 2.3.
> 
> Yes, it's just an internal interface, and the risk is very low (new code
> runs, but it looks harmless enough).  However, I don't see a pressing
> need for 2.3 as long as there are no users of the new interface.

Marc-Andre has written a spice consumer for this; and that has been on
hold because this patch hadn't gone in.  It's just my fault for
dropping this one..

> 
> > v4: fixed tabs in indentation (kraxel)
> > 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 c86814f..d14e872 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 ccf8459..a275199 100644
> > --- a/include/hw/virtio/virtio-serial.h
> > +++ b/include/hw/virtio/virtio-serial.h
> > @@ -60,6 +60,17 @@ 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.
> 
> Sounds awkward.  Perhaps: This is dependent on both the guest and
> thehost end being connected.

or an added comma?  I think it can go in later as a cleanup if this
patch goes in; I already sent pull req before seeing this mail..

> > +         */
> > +    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 looks good to me, but the interface is a bit hard to judge for
> virtio noobs like me without an actual user.  But since it's not an
> external interface:
> 
> Reviewed-by: Markus Armbruster <armbru@redhat.com>

Thanks, and Marc-Andre has confirmed it works for the spice use-case
(they were the ones who asked for this).

		Amit