hw/virtio: add config support to vhost-user-device

To use the generic device the user will need to provide the config
region size via the command line. We also add a notifier so the guest
can be pinged if the remote daemon updates the config.

With these changes:

  -device vhost-user-device-pci,virtio-id=41,num_vqs=2,config_size=8

is equivalent to:

  -device vhost-user-gpio-pci

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230710153522.3469097-11-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/vhost-user-device.c

@@ -117,6 +117,42 @@ static uint64_t vub_get_features(VirtIODevice *vdev,
     return vub->vhost_dev.features & ~(1ULL << VHOST_USER_F_PROTOCOL_FEATURES);
 }
 
+/*
+ * To handle VirtIO config we need to know the size of the config
+ * space. We don't cache the config but re-fetch it from the guest
+ * every time in case something has changed.
+ */
+static void vub_get_config(VirtIODevice *vdev, uint8_t *config)
+{
+    VHostUserBase *vub = VHOST_USER_BASE(vdev);
+    Error *local_err = NULL;
+
+    /*
+     * There will have been a warning during vhost_dev_init, but lets
+     * assert here as nothing will go right now.
+     */
+    g_assert(vub->config_size && vub->vhost_user.supports_config == true);
+
+    if (vhost_dev_get_config(&vub->vhost_dev, config,
+                             vub->config_size, &local_err)) {
+        error_report_err(local_err);
+    }
+}
+
+/*
+ * When the daemon signals an update to the config we just need to
+ * signal the guest as we re-read the config on demand above.
+ */
+static int vub_config_notifier(struct vhost_dev *dev)
+{
+    virtio_notify_config(dev->vdev);
+    return 0;
+}
+
+const VhostDevConfigOps vub_config_ops = {
+    .vhost_dev_config_notifier = vub_config_notifier,
+};
+
 static void vub_handle_output(VirtIODevice *vdev, VirtQueue *vq)
 {
     /*
@@ -141,12 +177,21 @@ static int vub_connect(DeviceState *dev)
 {
     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
     VHostUserBase *vub = VHOST_USER_BASE(vdev);
+    struct vhost_dev *vhost_dev = &vub->vhost_dev;
 
     if (vub->connected) {
         return 0;
     }
     vub->connected = true;
 
+    /*
+     * If we support VHOST_USER_GET_CONFIG we must enable the notifier
+     * so we can ping the guest when it updates.
+     */
+    if (vub->vhost_user.supports_config) {
+        vhost_dev_set_config_notifier(vhost_dev, &vub_config_ops);
+    }
+
     /* restore vhost state */
     if (virtio_device_started(vdev, vdev->status)) {
         vub_start(vdev);
@@ -214,11 +259,20 @@ static void vub_device_realize(DeviceState *dev, Error **errp)
         vub->num_vqs = 1; /* reasonable default? */
     }
 
+    /*
+     * We can't handle config requests unless we know the size of the
+     * config region, specialisations of the vhost-user-device will be
+     * able to set this.
+     */
+    if (vub->config_size) {
+        vub->vhost_user.supports_config = true;
+    }
+
     if (!vhost_user_init(&vub->vhost_user, &vub->chardev, errp)) {
         return;
     }
 
-    virtio_init(vdev, vub->virtio_id, 0);
+    virtio_init(vdev, vub->virtio_id, vub->config_size);
 
     /*
      * Disable guest notifiers, by default all notifications will be via the
@@ -268,6 +322,7 @@ static void vub_class_init(ObjectClass *klass, void *data)
     vdc->realize = vub_device_realize;
     vdc->unrealize = vub_device_unrealize;
     vdc->get_features = vub_get_features;
+    vdc->get_config = vub_get_config;
     vdc->set_status = vub_set_status;
 }
 
@@ -295,6 +350,7 @@ static Property vud_properties[] = {
     DEFINE_PROP_CHR("chardev", VHostUserBase, chardev),
     DEFINE_PROP_UINT16("virtio-id", VHostUserBase, virtio_id, 0),
     DEFINE_PROP_UINT32("num_vqs", VHostUserBase, num_vqs, 1),
+    DEFINE_PROP_UINT32("config_size", VHostUserBase, config_size, 0),
     DEFINE_PROP_END_OF_LIST(),
 };
 

include/hw/virtio/vhost-user-device.h

@@ -22,6 +22,7 @@ struct VHostUserBase {
     CharBackend chardev;
     uint16_t virtio_id;
     uint32_t num_vqs;
+    uint32_t config_size;
     /* State tracking */
     VhostUserState vhost_user;
     struct vhost_virtqueue *vhost_vq;