Re: [dpdk-dev] [PATCH v4 1/2] virtio: one way barrier for packed vring desc avail flags

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

On 9/17/19 7:28 AM, 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 for avail
> flags in packed ring.
> 
> 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>
> Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> ---
>  drivers/net/virtio/virtio_rxtx.c                 | 13 +++++++------
>  drivers/net/virtio/virtio_user/virtio_user_dev.c |  6 +++++-
>  drivers/net/virtio/virtqueue.h                   | 21 +++++++++++++++++++++
>  lib/librte_vhost/vhost.h                         |  2 +-
>  lib/librte_vhost/virtio_net.c                    | 11 +++++------
>  5 files changed, 39 insertions(+), 14 deletions(-)
> 

Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>

Thanks,
Maxime