[PATCH v4 17/20] virtio-blk: implement BlockDevOps->drained_begin()

[Stefan Hajnoczi <stefanha@xxxxxxxxxx>]

Detach ioeventfds during drained sections to stop I/O submission from
the guest. virtio-blk is no longer reliant on aio_disable_external()
after this patch. This will allow us to remove the
aio_disable_external() API once all other code that relies on it is
converted.

Take extra care to avoid attaching/detaching ioeventfds if the data
plane is started/stopped during a drained section. This should be rare,
but maybe the mirror block job can trigger it.

Signed-off-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
---
 hw/block/dataplane/virtio-blk.c | 17 +++++++++------
 hw/block/virtio-blk.c           | 38 ++++++++++++++++++++++++++++++++-
 2 files changed, 48 insertions(+), 7 deletions(-)

diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index bd7cc6e76b..d77fc6028c 100644
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -245,13 +245,15 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
     }
 
     /* Get this show started by hooking up our callbacks */
-    aio_context_acquire(s->ctx);
-    for (i = 0; i < nvqs; i++) {
-        VirtQueue *vq = virtio_get_queue(s->vdev, i);
+    if (!blk_in_drain(s->conf->conf.blk)) {
+        aio_context_acquire(s->ctx);
+        for (i = 0; i < nvqs; i++) {
+            VirtQueue *vq = virtio_get_queue(s->vdev, i);
 
-        virtio_queue_aio_attach_host_notifier(vq, s->ctx);
+            virtio_queue_aio_attach_host_notifier(vq, s->ctx);
+        }
+        aio_context_release(s->ctx);
     }
-    aio_context_release(s->ctx);
     return 0;
 
   fail_aio_context:
@@ -317,7 +319,10 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
     trace_virtio_blk_data_plane_stop(s);
 
     aio_context_acquire(s->ctx);
-    aio_wait_bh_oneshot(s->ctx, virtio_blk_data_plane_stop_bh, s);
+
+    if (!blk_in_drain(s->conf->conf.blk)) {
+        aio_wait_bh_oneshot(s->ctx, virtio_blk_data_plane_stop_bh, s);
+    }
 
     /* Wait for virtio_blk_dma_restart_bh() and in flight I/O to complete */
     blk_drain(s->conf->conf.blk);
diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index cefca93b31..d8dedc575c 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -1109,8 +1109,44 @@ static void virtio_blk_resize(void *opaque)
     aio_bh_schedule_oneshot(qemu_get_aio_context(), virtio_resize_cb, vdev);
 }
 
+/* Suspend virtqueue ioeventfd processing during drain */
+static void virtio_blk_drained_begin(void *opaque)
+{
+    VirtIOBlock *s = opaque;
+    VirtIODevice *vdev = VIRTIO_DEVICE(opaque);
+    AioContext *ctx = blk_get_aio_context(s->conf.conf.blk);
+
+    if (!s->dataplane || !s->dataplane_started) {
+        return;
+    }
+
+    for (uint16_t i = 0; i < s->conf.num_queues; i++) {
+        VirtQueue *vq = virtio_get_queue(vdev, i);
+        virtio_queue_aio_detach_host_notifier(vq, ctx);
+    }
+}
+
+/* Resume virtqueue ioeventfd processing after drain */
+static void virtio_blk_drained_end(void *opaque)
+{
+    VirtIOBlock *s = opaque;
+    VirtIODevice *vdev = VIRTIO_DEVICE(opaque);
+    AioContext *ctx = blk_get_aio_context(s->conf.conf.blk);
+
+    if (!s->dataplane || !s->dataplane_started) {
+        return;
+    }
+
+    for (uint16_t i = 0; i < s->conf.num_queues; i++) {
+        VirtQueue *vq = virtio_get_queue(vdev, i);
+        virtio_queue_aio_attach_host_notifier(vq, ctx);
+    }
+}
+
 static const BlockDevOps virtio_block_ops = {
-    .resize_cb = virtio_blk_resize,
+    .resize_cb     = virtio_blk_resize,
+    .drained_begin = virtio_blk_drained_begin,
+    .drained_end   = virtio_blk_drained_end,
 };
 
 static void virtio_blk_device_realize(DeviceState *dev, Error **errp)
-- 
2.39.2