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virtio: Clarify MR transaction optimization 

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>
This commit is contained in:
Greg Kurz 2021-05-17 15:26:37 +02:00 committed by Michael S. Tsirkin
parent 3909c07945
commit 9cf4fd872d
3 changed files with 48 additions and 0 deletions
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16
hw/block/dataplane/virtio-blk.c
View File

@ -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++) {

16
hw/scsi/virtio-scsi-dataplane.c
View File

@ -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++) {

16
hw/virtio/virtio.c
View File

@ -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++) {