[Qemu-devel] [PATCH v1 2/2] virtio-blk: dataplane: notify guest as a batch

[Ming Lei <ming.lei@canonical.com>]

Now requests are submitted as a batch, so it is natural
to notify guest as a batch too.

This may suppress interrupt notification to VM a lot:

        - in my test, decreased by ~13K/sec

Signed-off-by: Ming Lei <ming.lei@canonical.com>
---
 hw/block/dataplane/virtio-blk.c |   22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index e88862d..d65e022 100644
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -34,6 +34,7 @@ struct VirtIOBlockDataPlane {
     VirtIODevice *vdev;
     Vring vring;                    /* virtqueue vring */
     EventNotifier *guest_notifier;  /* irq */
+    QEMUBH *bh;                     /* bh for guest notification */
 
     /* Note that these EventNotifiers are assigned by value.  This is
      * fine as long as you do not call event_notifier_cleanup on them
@@ -61,13 +62,28 @@ static void notify_guest(VirtIOBlockDataPlane *s)
     event_notifier_set(s->guest_notifier);
 }
 
+static void notify_guest_bh(void *opaque)
+{
+    VirtIOBlockDataPlane *s = opaque;
+
+    notify_guest(s);
+}
+
 static void complete_request_vring(VirtIOBlockReq *req, unsigned char status)
 {
+    VirtIOBlockDataPlane *s = req->dev->dataplane;
     stb_p(&req->in->status, status);
 
-    vring_push(&req->dev->dataplane->vring, &req->elem,
+    vring_push(&s->vring, &req->elem,
                req->qiov.size + sizeof(*req->in));
-    notify_guest(req->dev->dataplane);
+
+    /* Suppress notification to guest by BH and its scheduled
+     * flag because requests are completed as a batch after io
+     * plug & unplug is introduced, and the BH can still be
+     * executed in dataplane aio context even after it is
+     * stopped, so needn't worry about notification loss with BH.
+     */
+    qemu_bh_schedule(s->bh);
 }
 
 static void handle_notify(EventNotifier *e)
@@ -172,6 +188,7 @@ void virtio_blk_data_plane_create(VirtIODevice *vdev, VirtIOBlkConf *blk,
         s->iothread = &s->internal_iothread_obj;
     }
     s->ctx = iothread_get_aio_context(s->iothread);
+    s->bh = aio_bh_new(s->ctx, notify_guest_bh, s);
 
     error_setg(&s->blocker, "block device is in use by data plane");
     bdrv_op_block_all(blk->conf.bs, s->blocker);
@@ -190,6 +207,7 @@ void virtio_blk_data_plane_destroy(VirtIOBlockDataPlane *s)
     bdrv_op_unblock_all(s->blk->conf.bs, s->blocker);
     error_free(s->blocker);
     object_unref(OBJECT(s->iothread));
+    qemu_bh_delete(s->bh);
     g_free(s);
 }
 
-- 
1.7.9.5