Re: [dpdk-dev] [PATCH v3 1/2] virtio: one way barrier for split vring used idx

[Joyce Kong <Joyce.Kong@arm.com>]

> -----Original Message-----
> From: Ye Xiaolong <xiaolong.ye@intel.com>
> Sent: Friday, April 17, 2020 2:52 PM
> To: Joyce Kong <Joyce.Kong@arm.com>
> Cc: maxime.coquelin@redhat.com; stephen@networkplumber.org;
> tiwei.bie@intel.com; zhihong.wang@intel.com; thomas@monjalon.net;
> jerinj@marvell.com; yinan.wang@intel.com; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; Gavin Hu <Gavin.Hu@arm.com>; nd
> <nd@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.kong@arm.com>
> >Reviewed-by: Gavin Hu <gavin.hu@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