virtio-blk: implement BlockDevOps->drained_begin()

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@redhat.com>
Message-Id: <20230516190238.8401-18-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 37 insertions, 1 deletions

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 8f65ea4659..4ca66b5860 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -1506,8 +1506,44 @@ 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)