[PATCH] virtio: Clarify MR transaction optimization

[Greg Kurz]

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>
---

I'm posting this because the wrong version of my "virtio: Improve boot
time of virtio-scsi-pci and virtio-blk-pci" series as explained here:

https://lists.nongnu.org/archive/html/qemu-devel/2021-05/msg04744.html

While here, I'm also adding the comments to the core virtio code, which
also does batching since commit 710fccf80d78 ("virtio: improve virtio
devices initialization time").
---
 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 cd81893d1d01..252c3a7a230c 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 28e003250a11..18eb824c97f5 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 e02544b2df76..260ad9226667 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -3730,6 +3730,10 @@ 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];
@@ -3768,6 +3772,10 @@ 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) {
@@ -3792,6 +3800,10 @@ 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];
@@ -3803,6 +3815,10 @@ 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++) {