Re: [dpdk-dev] [PATCH v2 1/2] virtio: one way barrier for packed vring desc avail flags

[Maxime Coquelin <maxime.coquelin@redhat.com>]

Hi Joyce,

On 9/6/19 1:34 PM, Joyce Kong wrote:
> 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.kong@arm.com>
> Reviewed-by: Gavin Hu <gavin.hu@arm.com>
> Reviewed-by: Phil Yang <phil.yang@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;
> +		}
It looks good to me.
I just wonder whether it would be cleaner to put that in an inline
function:

static inline void
virtqueue_store_flags_packed()

Same for the fetch.