virtio-blk: remove batch notification BH

There is a batching mechanism for virtio-blk Used Buffer Notifications
that is no longer needed because the previous commit added batching to
virtio_notify_irqfd().

Note that this mechanism was rarely used in practice because it is only
enabled when EVENT_IDX is not negotiated by the driver. Modern drivers
enable EVENT_IDX.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230913200045.1024233-5-stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

hw/block/dataplane/virtio-blk.c

@@ -31,9 +31,6 @@ struct VirtIOBlockDataPlane {
 
     VirtIOBlkConf *conf;
     VirtIODevice *vdev;
-    QEMUBH *bh;                     /* bh for guest notification */
-    unsigned long *batch_notify_vqs;
-    bool batch_notifications;
 
     /* Note that these EventNotifiers are assigned by value.  This is
      * fine as long as you do not call event_notifier_cleanup on them
@@ -47,36 +44,7 @@ struct VirtIOBlockDataPlane {
 /* Raise an interrupt to signal guest, if necessary */
 void virtio_blk_data_plane_notify(VirtIOBlockDataPlane *s, VirtQueue *vq)
 {
-    if (s->batch_notifications) {
-        set_bit(virtio_get_queue_index(vq), s->batch_notify_vqs);
-        qemu_bh_schedule(s->bh);
-    } else {
-        virtio_notify_irqfd(s->vdev, vq);
-    }
-}
-
-static void notify_guest_bh(void *opaque)
-{
-    VirtIOBlockDataPlane *s = opaque;
-    unsigned nvqs = s->conf->num_queues;
-    unsigned long bitmap[BITS_TO_LONGS(nvqs)];
-    unsigned j;
-
-    memcpy(bitmap, s->batch_notify_vqs, sizeof(bitmap));
-    memset(s->batch_notify_vqs, 0, sizeof(bitmap));
-
-    for (j = 0; j < nvqs; j += BITS_PER_LONG) {
-        unsigned long bits = bitmap[j / BITS_PER_LONG];
-
-        while (bits != 0) {
-            unsigned i = j + ctzl(bits);
-            VirtQueue *vq = virtio_get_queue(s->vdev, i);
-
-            virtio_notify_irqfd(s->vdev, vq);
-
-            bits &= bits - 1; /* clear right-most bit */
-        }
-    }
+    virtio_notify_irqfd(s->vdev, vq);
 }
 
 /* Context: QEMU global mutex held */
@@ -126,9 +94,6 @@ bool virtio_blk_data_plane_create(VirtIODevice *vdev, VirtIOBlkConf *conf,
     } else {
         s->ctx = qemu_get_aio_context();
     }
-    s->bh = aio_bh_new_guarded(s->ctx, notify_guest_bh, s,
-                               &DEVICE(vdev)->mem_reentrancy_guard);
-    s->batch_notify_vqs = bitmap_new(conf->num_queues);
 
     *dataplane = s;
 
@@ -146,8 +111,6 @@ void virtio_blk_data_plane_destroy(VirtIOBlockDataPlane *s)
 
     vblk = VIRTIO_BLK(s->vdev);
     assert(!vblk->dataplane_started);
-    g_free(s->batch_notify_vqs);
-    qemu_bh_delete(s->bh);
     if (s->iothread) {
         object_unref(OBJECT(s->iothread));
     }
@@ -173,12 +136,6 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
 
     s->starting = true;
 
-    if (!virtio_vdev_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX)) {
-        s->batch_notifications = true;
-    } else {
-        s->batch_notifications = false;
-    }
-
     /* Set up guest notifier (irq) */
     r = k->set_guest_notifiers(qbus->parent, nvqs, true);
     if (r != 0) {
@@ -370,9 +327,6 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
 
     aio_context_release(s->ctx);
 
-    qemu_bh_cancel(s->bh);
-    notify_guest_bh(s); /* final chance to notify guest */
-
     /* Clean up guest notifier (irq) */
     k->set_guest_notifiers(qbus->parent, nvqs, false);