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

["Joyce Kong (Arm Technology China)" <Joyce.Kong@arm.com>]

Hi Maxime,

> -----Original Message-----
> From: Maxime Coquelin <maxime.coquelin@redhat.com>
> Sent: Saturday, September 7, 2019 12:02 AM
> To: Joyce Kong (Arm Technology China) <Joyce.Kong@arm.com>;
> dev@dpdk.org
> Cc: nd <nd@arm.com>; tiwei.bie@intel.com; zhihong.wang@intel.com;
> amorenoz@redhat.com; xiao.w.wang@intel.com; yong.liu@intel.com;
> jfreimann@redhat.com; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; Gavin Hu (Arm Technology China)
> <Gavin.Hu@arm.com>
> Subject: Re: [PATCH v2 1/2] virtio: one way barrier for packed vring desc avail
> flags
> 
> 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.

Have wrapped the store/fetch operation in inline functions in v3.

Best Regards,
Joyce