[RFC PATCH v3 10/19] vhost: Add custom used buffer callback

["Eugenio Pérez" <eperezma@redhat.com>]

The callback allows SVQ users to know the VirtQueue requests and
responses. QEMU can use this to synchronize virtio device model state,
allowing to migrate it with minimum changes to the migration code.

In the case of networking, this will be used to inspect control
virtqueue messages.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
 hw/virtio/vhost-shadow-virtqueue.h | 16 +++++++++++++++-
 include/hw/virtio/vhost-vdpa.h     |  2 ++
 hw/virtio/vhost-shadow-virtqueue.c |  9 ++++++++-
 hw/virtio/vhost-vdpa.c             |  3 ++-
 4 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/hw/virtio/vhost-shadow-virtqueue.h b/hw/virtio/vhost-shadow-virtqueue.h
index 72aadb0aec..4ff6a0cda0 100644
--- a/hw/virtio/vhost-shadow-virtqueue.h
+++ b/hw/virtio/vhost-shadow-virtqueue.h
@@ -19,6 +19,13 @@ typedef struct SVQElement {
     VirtQueueElement elem;
 } SVQElement;
 
+typedef void (*VirtQueueElementCallback)(VirtIODevice *vdev,
+                                         const VirtQueueElement *elem);
+
+typedef struct VhostShadowVirtqueueOps {
+    VirtQueueElementCallback used_elem_handler;
+} VhostShadowVirtqueueOps;
+
 /* Shadow virtqueue to relay notifications */
 typedef struct VhostShadowVirtqueue {
     /* Shadow vring */
@@ -57,6 +64,12 @@ typedef struct VhostShadowVirtqueue {
     /* Next VirtQueue element that guest made available */
     SVQElement *next_guest_avail_elem;
 
+    /* Optional callbacks */
+    const VhostShadowVirtqueueOps *ops;
+
+    /* Optional custom used virtqueue element handler */
+    VirtQueueElementCallback used_elem_cb;
+
     /* Next head to expose to the device */
     uint16_t shadow_avail_idx;
 
@@ -83,7 +96,8 @@ void vhost_svq_start(VhostShadowVirtqueue *svq, VirtIODevice *vdev,
                      VirtQueue *vq);
 void vhost_svq_stop(VhostShadowVirtqueue *svq);
 
-VhostShadowVirtqueue *vhost_svq_new(VhostIOVATree *iova_tree);
+VhostShadowVirtqueue *vhost_svq_new(VhostIOVATree *iova_tree,
+                                    const VhostShadowVirtqueueOps *ops);
 
 void vhost_svq_free(gpointer vq);
 G_DEFINE_AUTOPTR_CLEANUP_FUNC(VhostShadowVirtqueue, vhost_svq_free);
diff --git a/include/hw/virtio/vhost-vdpa.h b/include/hw/virtio/vhost-vdpa.h
index a29dbb3f53..f1ba46a860 100644
--- a/include/hw/virtio/vhost-vdpa.h
+++ b/include/hw/virtio/vhost-vdpa.h
@@ -17,6 +17,7 @@
 #include "hw/virtio/vhost-iova-tree.h"
 #include "hw/virtio/virtio.h"
 #include "standard-headers/linux/vhost_types.h"
+#include "hw/virtio/vhost-shadow-virtqueue.h"
 
 typedef struct VhostVDPAHostNotifier {
     MemoryRegion mr;
@@ -35,6 +36,7 @@ typedef struct vhost_vdpa {
     /* IOVA mapping used by the Shadow Virtqueue */
     VhostIOVATree *iova_tree;
     GPtrArray *shadow_vqs;
+    const VhostShadowVirtqueueOps *shadow_vq_ops;
     struct vhost_dev *dev;
     VhostVDPAHostNotifier notifier[VIRTIO_QUEUE_MAX];
 } VhostVDPA;
diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
index 37e80c5ee0..112d0daf20 100644
--- a/hw/virtio/vhost-shadow-virtqueue.c
+++ b/hw/virtio/vhost-shadow-virtqueue.c
@@ -406,6 +406,10 @@ static void vhost_svq_flush(VhostShadowVirtqueue *svq,
                 return;
             }
             virtqueue_fill(vq, elem, len, i++);
+
+            if (svq->ops && svq->ops->used_elem_handler) {
+                svq->ops->used_elem_handler(svq->vdev, elem);
+            }
         }
 
         virtqueue_flush(vq, i);
@@ -592,12 +596,14 @@ void vhost_svq_stop(VhostShadowVirtqueue *svq)
  * shadow methods and file descriptors.
  *
  * @iova_tree: Tree to perform descriptors translations
+ * @ops: SVQ operations hooks
  *
  * Returns the new virtqueue or NULL.
  *
  * In case of error, reason is reported through error_report.
  */
-VhostShadowVirtqueue *vhost_svq_new(VhostIOVATree *iova_tree)
+VhostShadowVirtqueue *vhost_svq_new(VhostIOVATree *iova_tree,
+                                    const VhostShadowVirtqueueOps *ops)
 {
     g_autofree VhostShadowVirtqueue *svq = g_new0(VhostShadowVirtqueue, 1);
     int r;
@@ -619,6 +625,7 @@ VhostShadowVirtqueue *vhost_svq_new(VhostIOVATree *iova_tree)
     event_notifier_init_fd(&svq->svq_kick, VHOST_FILE_UNBIND);
     event_notifier_set_handler(&svq->hdev_call, vhost_svq_handle_call);
     svq->iova_tree = iova_tree;
+    svq->ops = ops;
     return g_steal_pointer(&svq);
 
 err_init_hdev_call:
diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
index 9eeac8fa8e..ebd17b6185 100644
--- a/hw/virtio/vhost-vdpa.c
+++ b/hw/virtio/vhost-vdpa.c
@@ -409,7 +409,8 @@ static int vhost_vdpa_init_svq(struct vhost_dev *hdev, struct vhost_vdpa *v,
 
     shadow_vqs = g_ptr_array_new_full(hdev->nvqs, vhost_svq_free);
     for (unsigned n = 0; n < hdev->nvqs; ++n) {
-        g_autoptr(VhostShadowVirtqueue) svq = vhost_svq_new(v->iova_tree);
+        g_autoptr(VhostShadowVirtqueue) svq = vhost_svq_new(v->iova_tree,
+                                                            v->shadow_vq_ops);
 
         if (unlikely(!svq)) {
             error_setg(errp, "Cannot create svq %u", n);
-- 
2.27.0