vhost: introduce spdk_vhost_dev_foreach_session

When device is changed, e.g. the underlying bdev is
hotremoved, all sessions need to be notified. For
instance, Vhost-SCSI would send an additional hotremove
eventq message. That's why we introduce a helper
function to iterate through all active sessions.

Eventually, we may want to poll different sessions
from different lcores, so there will be some kind of
internal cross-lcore message management required
- just like there is one for spdk_vhost_call_external_event_foreach().
For now, though, we can get away with a dumbest
implementation.

We still want to keep this API internal for the time
being. The end-user (RPC) should only modify the
device, and the whole concept of sessions should be
completely encapsulated.

Change-Id: I2e142632c07a23daeac15cabea4cffecf984e455
Signed-off-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/c/418736
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Pawel Wodkowski <pawelx.wodkowski@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>

lib/vhost/vhost.c

@@ -1349,6 +1349,21 @@ spdk_vhost_external_event_foreach_continue(struct spdk_vhost_dev *vdev,
 	spdk_vhost_event_async_send(vdev, fn, arg, true);
 }
 
+void
+spdk_vhost_dev_foreach_session(struct spdk_vhost_dev *vdev,
+			       spdk_vhost_session_fn fn, void *arg)
+{
+	int rc = 0;
+
+	if (vdev->session != NULL) {
+		rc = fn(vdev, vdev->session, arg);
+	}
+
+	if (rc >= 0) {
+		fn(vdev, NULL, arg);
+	}
+}
+
 void
 spdk_vhost_call_external_event_foreach(spdk_vhost_event_fn fn, void *arg)
 {

lib/vhost/vhost_internal.h

@@ -191,6 +191,25 @@ struct spdk_vhost_dev_destroy_ctx {
 	void *event_ctx;
 };
 
+/**
+ * Synchronized vhost session event used for backend callbacks.
+ *
+ * \param vdev vhost device. If the device has been deleted
+ * in the meantime, this function will be called one last
+ * time with vdev == NULL.
+ * \param vsession vhost session. If all sessions have been
+ * iterated through, this function will be called one last
+ * time with vsession == NULL.
+ * \param arg user-provided parameter.
+ *
+ * \return negative values will break the foreach call, meaning
+ * the function won't be called again. Return codes zero and
+ * positive don't have any effect.
+ */
+typedef int (*spdk_vhost_session_fn)(struct spdk_vhost_dev *vdev,
+				     struct spdk_vhost_session *vsession,
+				     void *arg);
+
 struct spdk_vhost_dev *spdk_vhost_dev_find(const char *ctrlr_name);
 
 void *spdk_vhost_gpa_to_vva(struct spdk_vhost_session *vsession, uint64_t addr, uint64_t len);
@@ -272,6 +291,18 @@ int spdk_vhost_dev_unregister(struct spdk_vhost_dev *vdev);
 int spdk_vhost_scsi_controller_construct(void);
 int spdk_vhost_blk_controller_construct(void);
 void spdk_vhost_dump_info_json(struct spdk_vhost_dev *vdev, struct spdk_json_write_ctx *w);
+
+/*
+ * Call function for each active session on the provided
+ * vhost device. The function will be called one-by-one
+ * on each session's thread.
+ *
+ * \param vdev vhost device
+ * \param fn function to call
+ * \param arg additional argument to \c fn
+ */
+void spdk_vhost_dev_foreach_session(struct spdk_vhost_dev *dev,
+				    spdk_vhost_session_fn fn, void *arg);
 void spdk_vhost_dev_backend_event_done(void *event_ctx, int response);
 void spdk_vhost_lock(void);
 void spdk_vhost_unlock(void);