[RFC PATCH] vhost-vdpa: Do not send empty IOTLB update batches

[Eugenio Pérez]

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, but others could fall on this category. This caused 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.

Signed-off-by: Eugenio Pérez <epere...@redhat.com>
---
 include/hw/virtio/vhost-vdpa.h |  1 +
 hw/virtio/vhost-vdpa.c         | 38 +++++++++++++++++++++++-----------
 2 files changed, 27 insertions(+), 12 deletions(-)

diff --git a/include/hw/virtio/vhost-vdpa.h b/include/hw/virtio/vhost-vdpa.h
index e98e327f12..0a7edbe4eb 100644
--- a/include/hw/virtio/vhost-vdpa.h
+++ b/include/hw/virtio/vhost-vdpa.h
@@ -23,6 +23,7 @@ typedef struct vhost_vdpa {
     int device_fd;
     int index;
     uint32_t msg_type;
+    size_t n_iotlb_sent_batch;
     MemoryListener listener;
     struct vhost_dev *dev;
     VhostVDPAHostNotifier notifier[VIRTIO_QUEUE_MAX];
diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
index 6ce94a1f4d..2517fc6103 100644
--- a/hw/virtio/vhost-vdpa.c
+++ b/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;
-
-    if (!(dev->backend_cap & (0x1ULL << VHOST_BACKEND_F_IOTLB_BATCH))) {
-        return;
-    }
-
-    msg.type = v->msg_type;
-    msg.iotlb.type = VHOST_IOTLB_BATCH_BEGIN;
+    struct vhost_msg_v2 msg = {
+        .type = v->msg_type,
+        .iotlb.type = VHOST_IOTLB_BATCH_BEGIN,
+    };
 
     if (write(fd, &msg, sizeof(msg)) != sizeof(msg)) {
         error_report("failed to write, fd=%d, errno=%d (%s)",
@@ -120,6 +114,11 @@ static void vhost_vdpa_listener_commit(MemoryListener 
*listener)
         return;
     }
 
+    if (v->n_iotlb_sent_batch == 0) {
+        return;
+    }
+
+    v->n_iotlb_sent_batch = 0;
     msg.type = v->msg_type;
     msg.iotlb.type = VHOST_IOTLB_BATCH_END;
 
@@ -170,6 +169,14 @@ static void vhost_vdpa_listener_region_add(MemoryListener 
*listener,
 
     llsize = int128_sub(llend, int128_make64(iova));
 
+    if (v->dev->backend_cap & (0x1ULL << VHOST_BACKEND_F_IOTLB_BATCH)) {
+        if (v->n_iotlb_sent_batch == 0) {
+            vhost_vdpa_listener_begin_batch(v);
+        }
+
+        v->n_iotlb_sent_batch++;
+    }
+
     ret = vhost_vdpa_dma_map(v, iova, int128_get64(llsize),
                              vaddr, section->readonly);
     if (ret) {
@@ -221,6 +228,14 @@ static void vhost_vdpa_listener_region_del(MemoryListener 
*listener,
 
     llsize = int128_sub(llend, int128_make64(iova));
 
+    if (v->dev->backend_cap & (0x1ULL << VHOST_BACKEND_F_IOTLB_BATCH)) {
+        if (v->n_iotlb_sent_batch == 0) {
+            vhost_vdpa_listener_begin_batch(v);
+        }
+
+        v->n_iotlb_sent_batch++;
+    }
+
     ret = vhost_vdpa_dma_unmap(v, iova, int128_get64(llsize));
     if (ret) {
         error_report("vhost_vdpa dma unmap error!");
@@ -234,7 +249,6 @@ 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,
-- 
2.27.0