Re: [PATCH v2] vhost: Only access header if offloading is supported in dequeue path

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Tue, Oct 11, 2016 at 09:45:27AM +0200, Maxime Coquelin wrote:
> @@ -684,12 +699,12 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vring_desc *descs,
>  		  struct rte_mempool *mbuf_pool)
>  {
>  	struct vring_desc *desc;
> -	uint64_t desc_addr;
> +	uint64_t desc_addr = 0;
>  	uint32_t desc_avail, desc_offset;
>  	uint32_t mbuf_avail, mbuf_offset;
>  	uint32_t cpy_len;
>  	struct rte_mbuf *cur = m, *prev = m;
> -	struct virtio_net_hdr *hdr;
> +	struct virtio_net_hdr *hdr = NULL;
>  	/* A counter to avoid desc dead loop chain */
>  	uint32_t nr_desc = 1;
>  
> @@ -698,12 +713,14 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vring_desc *descs,
>  			(desc->flags & VRING_DESC_F_INDIRECT))
>  		return -1;
>  
> -	desc_addr = gpa_to_vva(dev, desc->addr);
> -	if (unlikely(!desc_addr))
> -		return -1;
> +	if (virtio_net_with_host_offload(dev)) {
> +		desc_addr = gpa_to_vva(dev, desc->addr);
> +		if (unlikely(!desc_addr))
> +			return -1;
>  
> -	hdr = (struct virtio_net_hdr *)((uintptr_t)desc_addr);
> -	rte_prefetch0(hdr);
> +		hdr = (struct virtio_net_hdr *)((uintptr_t)desc_addr);
> +		rte_prefetch0(hdr);
> +	}
>  
>  	/*
>  	 * A virtio driver normally uses at least 2 desc buffers
> @@ -720,18 +737,24 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vring_desc *descs,
>  		if (unlikely(!desc_addr))
>  			return -1;
>  
> -		rte_prefetch0((void *)(uintptr_t)desc_addr);
> -
>  		desc_offset = 0;
>  		desc_avail  = desc->len;
>  		nr_desc    += 1;
> -
> -		PRINT_PACKET(dev, (uintptr_t)desc_addr, desc->len, 0);
>  	} else {
> +		if (!desc_addr) {
> +			desc_addr = gpa_to_vva(dev, desc->addr);
> +			if (unlikely(!desc_addr))
> +				return -1;
> +		}
> +

I think this piece of code make things a bit complex. I think what you
want to achieve is, besides saving hdr prefetch, to save one call to
gpa_to_vva() for the non-ANY_LAYOUT case. Does that matter too much?

How about just saving the hdr prefetch?

	if (virtio_net_with_host_offload(dev)) {
		hdr = (struct virtio_net_hdr *)((uintptr_t)desc_addr);
		rte_prefetch0(hdr);
	}

	--yliu

>  		desc_avail  = desc->len - dev->vhost_hlen;
>  		desc_offset = dev->vhost_hlen;
>  	}
>  
> +	rte_prefetch0((void *)(uintptr_t)(desc_addr + desc_offset));
> +
> +	PRINT_PACKET(dev, (uintptr_t)(desc_addr + desc_offset), desc_avail, 0);
> +
>  	mbuf_offset = 0;
>  	mbuf_avail  = m->buf_len - RTE_PKTMBUF_HEADROOM;
>  	while (1) {
> @@ -795,7 +818,7 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vring_desc *descs,
>  	prev->data_len = mbuf_offset;
>  	m->pkt_len    += mbuf_offset;
>  
> -	if (hdr->flags != 0 || hdr->gso_type != VIRTIO_NET_HDR_GSO_NONE)
> +	if (virtio_net_with_host_offload(dev))
>  		vhost_dequeue_offload(hdr, m);
>  
>  	return 0;
> -- 
> 2.7.4