vhost-vdpa: Do not send empty IOTLB update batches

With the introduction of the batch hinting, meaningless batches can be
created with no IOTLB updates if the memory region was skipped by
vhost_vdpa_listener_skipped_section. This is the case of host notifiers
memory regions, device un/realize, and others. This causes the vdpa
device to receive dma mapping settings with no changes, a possibly
expensive operation for nothing.

To avoid that, VHOST_IOTLB_BATCH_BEGIN hint is delayed until we have a
meaningful (not skipped section) mapping or unmapping operation, and
VHOST_IOTLB_BATCH_END is not written unless at least one of _UPDATE /
_INVALIDATE has been issued.

v3:
  * Use a bool instead of a counter avoiding potential number wrapping
  * Fix bad check on _commit
  * Move VHOST_BACKEND_F_IOTLB_BATCH check to
    vhost_vdpa_iotlb_batch_begin_once

v2 (from RFC):
  * Rename misleading name
  * Abstract start batching function for listener_add/del

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <20210812140933.226288-1-eperezma@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/vhost-vdpa.c

@@ -89,19 +89,13 @@ static int vhost_vdpa_dma_unmap(struct vhost_vdpa *v, hwaddr iova,
     return ret;
 }
 
-static void vhost_vdpa_listener_begin(MemoryListener *listener)
+static void vhost_vdpa_listener_begin_batch(struct vhost_vdpa *v)
 {
-    struct vhost_vdpa *v = container_of(listener, struct vhost_vdpa, listener);
-    struct vhost_dev *dev = v->dev;
-    struct vhost_msg_v2 msg = {};
     int fd = v->device_fd;
-
+    struct vhost_msg_v2 msg = {
-    if (!(dev->backend_cap & (0x1ULL << VHOST_BACKEND_F_IOTLB_BATCH))) {
+        .type = v->msg_type,
-        return;
+        .iotlb.type = VHOST_IOTLB_BATCH_BEGIN,
-    }
+    };
-
-    msg.type = v->msg_type;
-    msg.iotlb.type = VHOST_IOTLB_BATCH_BEGIN;
 
     if (write(fd, &msg, sizeof(msg)) != sizeof(msg)) {
         error_report("failed to write, fd=%d, errno=%d (%s)",
@@ -109,6 +103,16 @@ static void vhost_vdpa_listener_begin(MemoryListener *listener)
     }
 }
 
+static void vhost_vdpa_iotlb_batch_begin_once(struct vhost_vdpa *v)
+{
+    if (v->dev->backend_cap & (0x1ULL << VHOST_BACKEND_F_IOTLB_BATCH) &&
+        !v->iotlb_batch_begin_sent) {
+        vhost_vdpa_listener_begin_batch(v);
+    }
+
+    v->iotlb_batch_begin_sent = true;
+}
+
 static void vhost_vdpa_listener_commit(MemoryListener *listener)
 {
     struct vhost_vdpa *v = container_of(listener, struct vhost_vdpa, listener);
@@ -120,6 +124,10 @@ static void vhost_vdpa_listener_commit(MemoryListener *listener)
         return;
     }
 
+    if (!v->iotlb_batch_begin_sent) {
+        return;
+    }
+
     msg.type = v->msg_type;
     msg.iotlb.type = VHOST_IOTLB_BATCH_END;
 
@@ -127,6 +135,8 @@ static void vhost_vdpa_listener_commit(MemoryListener *listener)
         error_report("failed to write, fd=%d, errno=%d (%s)",
                      fd, errno, strerror(errno));
     }
+
+    v->iotlb_batch_begin_sent = false;
 }
 
 static void vhost_vdpa_listener_region_add(MemoryListener *listener,
@@ -170,6 +180,7 @@ static void vhost_vdpa_listener_region_add(MemoryListener *listener,
 
     llsize = int128_sub(llend, int128_make64(iova));
 
+    vhost_vdpa_iotlb_batch_begin_once(v);
     ret = vhost_vdpa_dma_map(v, iova, int128_get64(llsize),
                              vaddr, section->readonly);
     if (ret) {
@@ -221,6 +232,7 @@ static void vhost_vdpa_listener_region_del(MemoryListener *listener,
 
     llsize = int128_sub(llend, int128_make64(iova));
 
+    vhost_vdpa_iotlb_batch_begin_once(v);
     ret = vhost_vdpa_dma_unmap(v, iova, int128_get64(llsize));
     if (ret) {
         error_report("vhost_vdpa dma unmap error!");
@@ -234,7 +246,6 @@ static void vhost_vdpa_listener_region_del(MemoryListener *listener,
  * depends on the addnop().
  */
 static const MemoryListener vhost_vdpa_memory_listener = {
-    .begin = vhost_vdpa_listener_begin,
     .commit = vhost_vdpa_listener_commit,
     .region_add = vhost_vdpa_listener_region_add,
     .region_del = vhost_vdpa_listener_region_del,

include/hw/virtio/vhost-vdpa.h

@@ -22,6 +22,7 @@ typedef struct VhostVDPAHostNotifier {
 typedef struct vhost_vdpa {
     int device_fd;
     uint32_t msg_type;
+    bool iotlb_batch_begin_sent;
     MemoryListener listener;
     struct vhost_dev *dev;
     VhostVDPAHostNotifier notifier[VIRTIO_QUEUE_MAX];