Re: [dpdk-dev] [PATCH v5 1/2] virtio: one way barrier for split vring used idx

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 4/30/2020 10:14 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 used
> index in split ring.
> 
> Signed-off-by: Joyce Kong <joyce.kong@arm.com>
> Reviewed-by: Gavin Hu <gavin.hu@arm.com>
> Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>

<...>

> @@ -464,8 +464,33 @@ virtio_get_queue_type(struct virtio_hw *hw, uint16_t vtpci_queue_idx)
>  		return VTNET_TQ;
>  }
>  
> -#define VIRTQUEUE_NUSED(vq) ((uint16_t)((vq)->vq_split.ring.used->idx - \
> -					(vq)->vq_used_cons_idx))
> +/* virtqueue_nused has load-acquire or rte_cio_rmb insed */
> +static inline uint16_t
> +virtqueue_nused(const struct virtqueue *vq)
> +{
> +	uint16_t idx;
> +
> +	if (vq->hw->weak_barriers) {
> +	/**
> +	 * x86 prefers to using rte_smp_rmb over __atomic_load_n as it
> +	 * reports a slightly better perf, which comes from the saved
> +	 * branch by the compiler.
> +	 * The if and else branches are identical with the smp and cio
> +	 * barriers both defined as compiler barriers on x86.
> +	 */
> +#ifdef RTE_ARCH_X86_64
> +		idx = vq->vq_split.ring.used->idx;
> +		rte_smp_rmb();
> +#else
> +		idx = __atomic_load_n(&(vq)->vq_split.ring.used->idx,
> +				__ATOMIC_ACQUIRE);
> +#endif
> +	} else {
> +		idx = vq->vq_split.ring.used->idx;
> +		rte_cio_rmb();
> +	}
> +	return idx - vq->vq_used_cons_idx;
> +}

AltiVec implementation (virtio_rxtx_simple_altivec.c) is also using
'VIRTQUEUE_NUSED' macro, it also needs to be updated with this change.