In case VIRTIO_F_ORDER_PLATFORM(36) is not negotiated, then the frontend and backend are assumed to be implemented in software, that is they can run on identical CPUs in an SMP configuration. Thus a weak form of memory barriers like rte_smp_r/wmb, other than rte_cio_r/wmb, is sufficient for this case(vq->hw->weak_barriers == 1) and yields better performance. For the above case, this patch helps yielding even better performance by replacing the two-way barriers with C11 one-way barriers.
Meanwhile, a read barrier is required to ensure ordering between descriptor's flags and content reads[1]. With C11, load-acquire can enforce the ordering instead of rmb barrier. [1]https://patchwork.dpdk.org/patch/49109/ Signed-off-by: Joyce Kong <joyce.k...@arm.com> Reviewed-by: Gavin Hu <gavin...@arm.com> Reviewed-by: Phil Yang <phil.y...@arm.com> --- drivers/net/virtio/virtio_rxtx.c | 26 ++++++++++++++++++------ drivers/net/virtio/virtio_user/virtio_user_dev.c | 6 +++++- lib/librte_vhost/vhost.h | 2 +- lib/librte_vhost/virtio_net.c | 11 +++++----- 4 files changed, 31 insertions(+), 14 deletions(-) diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c index 27ead19..2a2153c 100644 --- a/drivers/net/virtio/virtio_rxtx.c +++ b/drivers/net/virtio/virtio_rxtx.c @@ -456,8 +456,14 @@ virtqueue_enqueue_recv_refill_packed(struct virtqueue *vq, vq->vq_desc_head_idx = dxp->next; if (vq->vq_desc_head_idx == VQ_RING_DESC_CHAIN_END) vq->vq_desc_tail_idx = vq->vq_desc_head_idx; - virtio_wmb(hw->weak_barriers); - start_dp[idx].flags = flags; + + if (hw->weak_barriers) + __atomic_store_n(&start_dp[idx].flags, flags, + __ATOMIC_RELEASE); + else { + rte_cio_wmb(); + start_dp[idx].flags = flags; + } if (++vq->vq_avail_idx >= vq->vq_nentries) { vq->vq_avail_idx -= vq->vq_nentries; vq->vq_packed.cached_flags ^= @@ -671,8 +677,12 @@ virtqueue_enqueue_xmit_packed_fast(struct virtnet_tx *txvq, vq->vq_desc_tail_idx = VQ_RING_DESC_CHAIN_END; } - virtio_wmb(vq->hw->weak_barriers); - dp->flags = flags; + if (vq->hw->weak_barriers) + __atomic_store_n(&dp->flags, flags, __ATOMIC_RELEASE); + else { + rte_cio_wmb(); + dp->flags = flags; + } } static inline void @@ -763,8 +773,12 @@ virtqueue_enqueue_xmit_packed(struct virtnet_tx *txvq, struct rte_mbuf *cookie, vq->vq_desc_tail_idx = VQ_RING_DESC_CHAIN_END; } - virtio_wmb(vq->hw->weak_barriers); - head_dp->flags = head_flags; + if (vq->hw->weak_barriers) + __atomic_store_n(&head_dp->flags, head_flags, __ATOMIC_RELEASE); + else { + rte_cio_wmb(); + head_dp->flags = head_flags; + } } static inline void diff --git a/drivers/net/virtio/virtio_user/virtio_user_dev.c b/drivers/net/virtio/virtio_user/virtio_user_dev.c index fab87eb..7911c39 100644 --- a/drivers/net/virtio/virtio_user/virtio_user_dev.c +++ b/drivers/net/virtio/virtio_user/virtio_user_dev.c @@ -624,7 +624,7 @@ virtio_user_handle_ctrl_msg(struct virtio_user_dev *dev, struct vring *vring, static inline int desc_is_avail(struct vring_packed_desc *desc, bool wrap_counter) { - uint16_t flags = desc->flags; + uint16_t flags = __atomic_load_n(&desc->flags, __ATOMIC_ACQUIRE); return wrap_counter == !!(flags & VRING_PACKED_DESC_F_AVAIL) && wrap_counter != !!(flags & VRING_PACKED_DESC_F_USED); @@ -684,6 +684,10 @@ virtio_user_handle_cq_packed(struct virtio_user_dev *dev, uint16_t queue_idx) struct vring_packed *vring = &dev->packed_vrings[queue_idx]; uint16_t n_descs, flags; + /* Perform a load-acquire barrier in desc_is_avail to + * enforce the ordering between desc flags and desc + * content. + */ while (desc_is_avail(&vring->desc[vq->used_idx], vq->used_wrap_counter)) { diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h index 884befa..d294ed1 100644 --- a/lib/librte_vhost/vhost.h +++ b/lib/librte_vhost/vhost.h @@ -344,7 +344,7 @@ vq_is_packed(struct virtio_net *dev) static inline bool desc_is_avail(struct vring_packed_desc *desc, bool wrap_counter) { - uint16_t flags = *((volatile uint16_t *) &desc->flags); + uint16_t flags = __atomic_load_n(&desc->flags, __ATOMIC_ACQUIRE); return wrap_counter == !!(flags & VRING_DESC_F_AVAIL) && wrap_counter != !!(flags & VRING_DESC_F_USED); diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c index 5b85b83..e7463ff 100644 --- a/lib/librte_vhost/virtio_net.c +++ b/lib/librte_vhost/virtio_net.c @@ -503,14 +503,13 @@ fill_vec_buf_packed(struct virtio_net *dev, struct vhost_virtqueue *vq, if (avail_idx < vq->last_avail_idx) wrap_counter ^= 1; - if (unlikely(!desc_is_avail(&descs[avail_idx], wrap_counter))) - return -1; - /* - * The ordering between desc flags and desc - * content reads need to be enforced. + * Perform a load-acquire barrier in desc_is_avail to + * enforce the ordering between desc flags and desc + * content. */ - rte_smp_rmb(); + if (unlikely(!desc_is_avail(&descs[avail_idx], wrap_counter))) + return -1; *desc_count = 0; *len = 0; -- 2.7.4
