A virtio driver does virtqueue_add_buf() multiple times before finally calling virtqueue_kick(); previously we only exposed the added buffers in the virtqueue_kick() call. This means we don't need a memory barrier in virtqueue_add_buf(), but it reduces concurrency as the device (ie. host) can't see the buffers until the kick.
Signed-off-by: Rusty Russell <[email protected]> --- drivers/virtio/virtio_ring.c | 37 ++++++++++++++++++++++--------------- 1 file changed, 22 insertions(+), 15 deletions(-) diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c --- a/drivers/virtio/virtio_ring.c +++ b/drivers/virtio/virtio_ring.c @@ -227,9 +227,15 @@ add_head: /* Put entry in available array (but don't update avail->idx until they * do sync). */ - avail = ((vq->vring.avail->idx + vq->num_added++) & (vq->vring.num-1)); + avail = (vq->vring.avail->idx & (vq->vring.num-1)); vq->vring.avail->ring[avail] = head; + /* Descriptors and available array need to be set before we expose the + * new available array entries. */ + virtio_wmb(); + vq->vring.avail->idx++; + vq->num_added++; + pr_debug("Added buffer head %i to %p\n", head, vq); END_USE(vq); @@ -248,13 +254,10 @@ bool virtqueue_kick_prepare(struct virtq * new available array entries. */ virtio_wmb(); - old = vq->vring.avail->idx; - new = vq->vring.avail->idx = old + vq->num_added; + old = vq->vring.avail->idx - vq->num_added; + new = vq->vring.avail->idx; vq->num_added = 0; - /* Need to update avail index before checking if we should notify */ - virtio_mb(); - if (vq->event) { needs_kick = vring_need_event(vring_avail_event(&vq->vring), new, old); _______________________________________________ Virtualization mailing list [email protected] https://lists.linuxfoundation.org/mailman/listinfo/virtualization
