Re: [PATCH v2 09/13] vu_common: Pass iov_tail to vu_set_vnethdr()

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Mar 09, 2026 at 10:47:40AM +0100, Laurent Vivier wrote:
> Refactor vu_set_vnethdr() to take an iov_tail pointer instead of a
> direct pointer to the virtio_net_hdr_mrg_rxbuf structure.
> This makes the function use IOV_PEEK_HEADER() and IOV_PUT_HEADER()
> to read and write the virtio-net header through the iov_tail abstraction.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
>  tcp_vu.c    |  8 +++++---
>  udp_vu.c    |  3 +--
>  vu_common.c | 21 +++++++++++++++------
>  vu_common.h |  2 +-
>  4 files changed, 22 insertions(+), 12 deletions(-)
> 
> diff --git a/tcp_vu.c b/tcp_vu.c
> index 8da2dcfe78d0..688f48905d46 100644
> --- a/tcp_vu.c
> +++ b/tcp_vu.c
> @@ -94,10 +94,11 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
>  	if (elem_cnt != 1)
>  		return -1;
>  
> -	ASSERT(flags_elem[0].in_sg[0].iov_len >=
> +	payload = IOV_TAIL(&flags_elem[0].in_sg[0], elem_cnt, 0);
> +	ASSERT(iov_tail_size(&payload) >=
>  	       MAX(hdrlen + sizeof(*opts), ETH_ZLEN + VNET_HLEN));
>  
> -	vu_set_vnethdr(flags_elem[0].in_sg[0].iov_base, 1);
> +	vu_set_vnethdr(&payload, 1);
>  
>  	eh = vu_eth(flags_elem[0].in_sg[0].iov_base);
>  
> @@ -454,6 +455,7 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
>  	for (i = 0, previous_dlen = -1, check = NULL; i < head_cnt; i++) {
>  		struct iovec *iov = &elem[head[i]].in_sg[0];
>  		int buf_cnt = head[i + 1] - head[i];
> +		struct iov_tail data = IOV_TAIL(iov, buf_cnt, 0);
>  		size_t frame_size = iov_size(iov, buf_cnt);
>  		bool push = i == head_cnt - 1;
>  		ssize_t dlen;
> @@ -461,7 +463,7 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
>  		ASSERT(frame_size >= hdrlen);
>  
>  		dlen = frame_size - hdrlen;
> -		vu_set_vnethdr(iov->iov_base, buf_cnt);
> +		vu_set_vnethdr(&data, buf_cnt);
>  
>  		/* The IPv4 header checksum varies only with dlen */
>  		if (previous_dlen != dlen)
> diff --git a/udp_vu.c b/udp_vu.c
> index a21a03dbf23e..414750ff742a 100644
> --- a/udp_vu.c
> +++ b/udp_vu.c
> @@ -233,8 +233,7 @@ void udp_vu_sock_to_tap(const struct ctx *c, int s, int n, flow_sidx_t tosidx)
>  		vu_queue_rewind(vq, elem_cnt - elem_used);
>  		if (iov_cnt > 0) {
>  			struct iov_tail data = IOV_TAIL(iov_vu, iov_cnt, 0);
> -			vu_set_vnethdr(iov_vu[0].iov_base, elem_used);
> -			iov_drop_header(&data, VNET_HLEN);
> +			vu_set_vnethdr(&data, elem_used);
>  			udp_vu_prepare(c, &data, toside);
>  			if (*c->pcap) {
>  				udp_vu_csum(toside, &data);
> diff --git a/vu_common.c b/vu_common.c
> index 8afa5199908f..3538e59581b7 100644
> --- a/vu_common.c
> +++ b/vu_common.c
> @@ -120,17 +120,24 @@ int vu_collect(const struct vu_dev *vdev, struct vu_virtq *vq,
>  }
>  
>  /**
> - * vu_set_vnethdr() - set virtio-net headers
> - * @vnethdr:		Address of the header to set
> + * vu_set_vnethdr() - set virtio-net header
> + * @data:		IOV tail to write header to, updated to
> + * 			point after the virtio-net header
>   * @num_buffers:	Number of guest buffers of the frame
>   */
> -void vu_set_vnethdr(struct virtio_net_hdr_mrg_rxbuf *vnethdr, int num_buffers)
> +void vu_set_vnethdr(struct iov_tail *data, int num_buffers)
>  {
> +	struct virtio_net_hdr_mrg_rxbuf vnethdr_storage, *vnethdr;
> +
> +	vnethdr = IOV_PEEK_HEADER(data, vnethdr_storage);
> +
>  	vnethdr->hdr = VU_HEADER;
>  	/* Note: if VIRTIO_NET_F_MRG_RXBUF is not negotiated,
>  	 * num_buffers must be 1
>  	 */
>  	vnethdr->num_buffers = htole16(num_buffers);
> +
> +	IOV_PUT_HEADER(data, vnethdr);
>  }
>  
>  /**
> @@ -267,6 +274,7 @@ int vu_send_single(const struct ctx *c, const void *buf, size_t size)
>  	struct vu_virtq *vq = &vdev->vq[VHOST_USER_RX_QUEUE];
>  	struct vu_virtq_element elem[VIRTQUEUE_MAX_SIZE];
>  	struct iovec in_sg[VIRTQUEUE_MAX_SIZE];
> +	struct iov_tail data;
>  	size_t total;
>  	int elem_cnt;
>  	int i;
> @@ -295,15 +303,16 @@ int vu_send_single(const struct ctx *c, const void *buf, size_t size)
>  	}
>  	elem_cnt = iov_truncate(in_sg, elem_cnt, size);
>  
> -	vu_set_vnethdr(in_sg[0].iov_base, elem_cnt);
> +	data = IOV_TAIL(&in_sg[0], elem_cnt, 0);
> +	vu_set_vnethdr(&data, elem_cnt);
>  
>  	size -= VNET_HLEN;
>  
>  	/* copy data from the buffer to the iovec */
> -	iov_from_buf(in_sg, elem_cnt, VNET_HLEN, buf, size);
> +	iov_from_buf(in_sg, elem_cnt, data.off, buf, size);
>  
>  	if (*c->pcap)
> -		pcap_iov(in_sg, elem_cnt, VNET_HLEN);
> +		pcap_iov(data.iov, data.cnt, data.off);
>  
>  	vu_flush(vdev, vq, elem, elem_cnt);
>  
> diff --git a/vu_common.h b/vu_common.h
> index 5de0c987b936..d068c61695f8 100644
> --- a/vu_common.h
> +++ b/vu_common.h
> @@ -55,7 +55,7 @@ void vu_init_elem(struct vu_virtq_element *elem, struct iovec *iov,
>  int vu_collect(const struct vu_dev *vdev, struct vu_virtq *vq,
>  	       struct vu_virtq_element *elem, int max_elem, size_t size,
>  	       size_t *collected);
> -void vu_set_vnethdr(struct virtio_net_hdr_mrg_rxbuf *vnethdr, int num_buffers);
> +void vu_set_vnethdr(struct iov_tail *data, int num_buffers);
>  void vu_flush(const struct vu_dev *vdev, struct vu_virtq *vq,
>  	      struct vu_virtq_element *elem, int elem_cnt);
>  void vu_kick_cb(struct vu_dev *vdev, union epoll_ref ref,
> -- 
> 2.53.0
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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