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

[Amit Shah]

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 <address@hidden>

---

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.

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.
+         */
+    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
-- 
2.3.2