> -----Original Message-----
> From: Ye Xiaolong <xiaolong...@intel.com>
> Sent: Friday, April 17, 2020 2:52 PM
> To: Joyce Kong <joyce.k...@arm.com>
> Cc: maxime.coque...@redhat.com; step...@networkplumber.org;
> tiwei....@intel.com; zhihong.w...@intel.com; tho...@monjalon.net;
> jer...@marvell.com; yinan.w...@intel.com; Honnappa Nagarahalli
> <honnappa.nagaraha...@arm.com>; Gavin Hu <gavin...@arm.com>; nd
> <n...@arm.com>; dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH v3 1/2] virtio: one way barrier for split vring
> used idx
>
> On 04/06, 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 used
> >index in split ring.
> >
> >Signed-off-by: Joyce Kong <joyce.k...@arm.com>
> >Reviewed-by: Gavin Hu <gavin...@arm.com>
> >---
> > drivers/net/virtio/virtio_ethdev.c | 9 ++--
> > drivers/net/virtio/virtio_ring.h | 2 +-
> > drivers/net/virtio/virtio_rxtx.c | 46 +++++++++----------
> > drivers/net/virtio/virtio_rxtx_simple_neon.c | 5 +-
> > drivers/net/virtio/virtio_rxtx_simple_sse.c | 5 +-
> > .../net/virtio/virtio_user/virtio_user_dev.c | 8 ++--
> > drivers/net/virtio/virtqueue.c | 2 +-
> > drivers/net/virtio/virtqueue.h | 37 ++++++++++++---
> > lib/librte_vhost/virtio_net.c | 5 +-
> > 9 files changed, 71 insertions(+), 48 deletions(-)
> >
> >diff --git a/drivers/net/virtio/virtio_ethdev.c
> >b/drivers/net/virtio/virtio_ethdev.c
> >index f9d0ea70d..a4a865bfa 100644
> >--- a/drivers/net/virtio/virtio_ethdev.c
> >+++ b/drivers/net/virtio/virtio_ethdev.c
> >@@ -285,13 +285,12 @@ virtio_send_command_split(struct virtnet_ctl
> >*cvq,
> >
> > virtqueue_notify(vq);
> >
> >- rte_rmb();
> >- while (VIRTQUEUE_NUSED(vq) == 0) {
> >- rte_rmb();
> >+ /* virtqueue_nused has a load-acquire or rte_cio_rmb inside */
> >+ while (virtqueue_nused(vq) == 0)
> > usleep(100);
> >- }
> >
> >- while (VIRTQUEUE_NUSED(vq)) {
> >+ /* virtqueue_nused has a load-acquire or rte_cio_rmb inside */
> >+ while (virtqueue_nused(vq)) {
> > uint32_t idx, desc_idx, used_idx;
> > struct vring_used_elem *uep;
> >
> >diff --git a/drivers/net/virtio/virtio_ring.h
> >b/drivers/net/virtio/virtio_ring.h
> >index 7ba34662e..0f6574f68 100644
> >--- a/drivers/net/virtio/virtio_ring.h
> >+++ b/drivers/net/virtio/virtio_ring.h
> >@@ -59,7 +59,7 @@ struct vring_used_elem {
> >
> > struct vring_used {
> > uint16_t flags;
> >- volatile uint16_t idx;
> >+ uint16_t idx;
> > struct vring_used_elem ring[0];
> > };
> >
> >diff --git a/drivers/net/virtio/virtio_rxtx.c
> >b/drivers/net/virtio/virtio_rxtx.c
> >index 752faa0f6..9ba26fd95 100644
> >--- a/drivers/net/virtio/virtio_rxtx.c
> >+++ b/drivers/net/virtio/virtio_rxtx.c
> >@@ -45,7 +45,7 @@ virtio_dev_rx_queue_done(void *rxq, uint16_t offset)
> > struct virtnet_rx *rxvq = rxq;
> > struct virtqueue *vq = rxvq->vq;
> >
> >- return VIRTQUEUE_NUSED(vq) >= offset;
> >+ return virtqueue_nused(vq) >= offset;
> > }
> >
> > void
> >@@ -1243,9 +1243,8 @@ virtio_recv_pkts(void *rx_queue, struct rte_mbuf
> **rx_pkts, uint16_t nb_pkts)
> > if (unlikely(hw->started == 0))
> > return nb_rx;
> >
> >- nb_used = VIRTQUEUE_NUSED(vq);
> >-
> >- virtio_rmb(hw->weak_barriers);
> >+ /* virtqueue_nused has a load-acquire or rte_cio_rmb inside */
>
> Small nit, I don't think we need to add this comment to every occurrence of
> virtqueue_nused, what about moving it to the definition of this function?
>
> Thanks,
> Xiaolong
Will modify as this in v4.
Thanks,
Joyce
