From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [PATCH v2 4/4] vhost: remove useless prefetch for
 packed ring descriptor
Date: Wed, 19 Dec 2018 10:49:38 -0500
Message-ID: <20181219104840-mutt-send-email-mst@kernel.org>
References: <20181219082113.24455-1-maxime.coquelin@redhat.com>
 <20181219082113.24455-5-maxime.coquelin@redhat.com>
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To: Maxime Coquelin <maxime.coquelin@redhat.com>
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On Wed, Dec 19, 2018 at 09:21:13AM +0100, Maxime Coquelin wrote:
> This prefetch does not show any performance improvement.
> 
> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>

Likely because of the RMB.
Try prefetching the *next* descriptor maybe?

> ---
>  lib/librte_vhost/virtio_net.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
> index a9be633de..4d201a3fd 100644
> --- a/lib/librte_vhost/virtio_net.c
> +++ b/lib/librte_vhost/virtio_net.c
> @@ -1437,8 +1437,6 @@ virtio_dev_tx_packed(struct virtio_net *dev, struct vhost_virtqueue *vq,
>  {
>  	uint16_t i;
>  
> -	rte_prefetch0(&vq->desc_packed[vq->last_avail_idx]);
> -
>  	if (unlikely(dev->dequeue_zero_copy)) {
>  		struct zcopy_mbuf *zmbuf, *next;
>  
> -- 
> 2.17.2