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| author | Greg Kurz <groug@kaod.org> | 2021-05-17 15:26:37 +0200 |
|---|---|---|
| committer | Michael S. Tsirkin <mst@redhat.com> | 2021-07-02 11:13:39 -0400 |
| commit | 9cf4fd872d14ae109ce1ee430bf67499c2682fa5 (patch) | |
| tree | 5ed8f586d9d3bf606211a0fe5ccbcb48033d6710 /hw/block | |
| parent | 3909c079454a49b113c60a5cd91d749f78ca1c7f (diff) | |
| download | qemu-9cf4fd872d14ae109ce1ee430bf67499c2682fa5.zip qemu-9cf4fd872d14ae109ce1ee430bf67499c2682fa5.tar.gz qemu-9cf4fd872d14ae109ce1ee430bf67499c2682fa5.tar.bz2 | |
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>
Diffstat (limited to 'hw/block')
| -rw-r--r-- | hw/block/dataplane/virtio-blk.c | 16 |
1 files changed, 16 insertions, 0 deletions
diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c index cd81893..252c3a7 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++) { |