[PULL 02/13] virtio: Clarify MR transaction optimization

[Michael S. Tsirkin]

From: Greg Kurz <groug@kaod.org>

The device model batching its ioeventfds in a single MR transaction is
an optimization. Clarify this in virtio-scsi, virtio-blk and generic
virtio code. Also clarify that the transaction must commit before
closing ioeventfds so that no one is tempted to merge the loops
in the start functions error path and in the stop functions.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <162125799728.1394228.339855768563326832.stgit@bahia.lan>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 hw/block/dataplane/virtio-blk.c | 16 ++++++++++++++++
 hw/scsi/virtio-scsi-dataplane.c | 16 ++++++++++++++++
 hw/virtio/virtio.c              | 16 ++++++++++++++++
 3 files changed, 48 insertions(+)

diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index cd81893d1d..252c3a7a23 100644
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -198,6 +198,10 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
         goto fail_guest_notifiers;
     }
 
+    /*
+     * Batch all the host notifiers in a single transaction to avoid
+     * quadratic time complexity in address_space_update_ioeventfds().
+     */
     memory_region_transaction_begin();
 
     /* Set up virtqueue notify */
@@ -211,6 +215,10 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
                 virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), i, false);
             }
 
+            /*
+             * The transaction expects the ioeventfds to be open when it
+             * commits. Do it now, before the cleanup loop.
+             */
             memory_region_transaction_commit();
 
             while (j--) {
@@ -330,12 +338,20 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
 
     aio_context_release(s->ctx);
 
+    /*
+     * Batch all the host notifiers in a single transaction to avoid
+     * quadratic time complexity in address_space_update_ioeventfds().
+     */
     memory_region_transaction_begin();
 
     for (i = 0; i < nvqs; i++) {
         virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), i, false);
     }
 
+    /*
+     * The transaction expects the ioeventfds to be open when it
+     * commits. Do it now, before the cleanup loop.
+     */
     memory_region_transaction_commit();
 
     for (i = 0; i < nvqs; i++) {
diff --git a/hw/scsi/virtio-scsi-dataplane.c b/hw/scsi/virtio-scsi-dataplane.c
index 28e003250a..18eb824c97 100644
--- a/hw/scsi/virtio-scsi-dataplane.c
+++ b/hw/scsi/virtio-scsi-dataplane.c
@@ -152,6 +152,10 @@ int virtio_scsi_dataplane_start(VirtIODevice *vdev)
         goto fail_guest_notifiers;
     }
 
+    /*
+     * Batch all the host notifiers in a single transaction to avoid
+     * quadratic time complexity in address_space_update_ioeventfds().
+     */
     memory_region_transaction_begin();
 
     rc = virtio_scsi_set_host_notifier(s, vs->ctrl_vq, 0);
@@ -198,6 +202,10 @@ fail_host_notifiers:
         virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), i, false);
     }
 
+    /*
+     * The transaction expects the ioeventfds to be open when it
+     * commits. Do it now, before the cleanup loop.
+     */
     memory_region_transaction_commit();
 
     for (i = 0; i < vq_init_count; i++) {
@@ -238,12 +246,20 @@ void virtio_scsi_dataplane_stop(VirtIODevice *vdev)
 
     blk_drain_all(); /* ensure there are no in-flight requests */
 
+    /*
+     * Batch all the host notifiers in a single transaction to avoid
+     * quadratic time complexity in address_space_update_ioeventfds().
+     */
     memory_region_transaction_begin();
 
     for (i = 0; i < vs->conf.num_queues + 2; i++) {
         virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), i, false);
     }
 
+    /*
+     * The transaction expects the ioeventfds to be open when it
+     * commits. Do it now, before the cleanup loop.
+     */
     memory_region_transaction_commit();
 
     for (i = 0; i < vs->conf.num_queues + 2; i++) {
diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index ab516ac614..6dcf3baf56 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -3728,6 +3728,10 @@ static int 
virtio_device_start_ioeventfd_impl(VirtIODevice *vdev)
     VirtioBusState *qbus = VIRTIO_BUS(qdev_get_parent_bus(DEVICE(vdev)));
     int i, n, r, err;
 
+    /*
+     * Batch all the host notifiers in a single transaction to avoid
+     * quadratic time complexity in address_space_update_ioeventfds().
+     */
     memory_region_transaction_begin();
     for (n = 0; n < VIRTIO_QUEUE_MAX; n++) {
         VirtQueue *vq = &vdev->vq[n];
@@ -3766,6 +3770,10 @@ assign_error:
         r = virtio_bus_set_host_notifier(qbus, n, false);
         assert(r >= 0);
     }
+    /*
+     * The transaction expects the ioeventfds to be open when it
+     * commits. Do it now, before the cleanup loop.
+     */
     memory_region_transaction_commit();
 
     while (--i >= 0) {
@@ -3790,6 +3798,10 @@ static void 
virtio_device_stop_ioeventfd_impl(VirtIODevice *vdev)
     VirtioBusState *qbus = VIRTIO_BUS(qdev_get_parent_bus(DEVICE(vdev)));
     int n, r;
 
+    /*
+     * Batch all the host notifiers in a single transaction to avoid
+     * quadratic time complexity in address_space_update_ioeventfds().
+     */
     memory_region_transaction_begin();
     for (n = 0; n < VIRTIO_QUEUE_MAX; n++) {
         VirtQueue *vq = &vdev->vq[n];
@@ -3801,6 +3813,10 @@ static void 
virtio_device_stop_ioeventfd_impl(VirtIODevice *vdev)
         r = virtio_bus_set_host_notifier(qbus, n, false);
         assert(r >= 0);
     }
+    /*
+     * The transaction expects the ioeventfds to be open when it
+     * commits. Do it now, before the cleanup loop.
+     */
     memory_region_transaction_commit();
 
     for (n = 0; n < VIRTIO_QUEUE_MAX; n++) {
-- 
MST