Re: [PATCH v6 3/4] tcp_vu: Support multibuffer frames in tcp_vu_sock_recv()

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

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On Thu, Apr 16, 2026 at 06:16:17PM +0200, Laurent Vivier wrote:
> Previously, tcp_vu_sock_recv() assumed a 1:1 mapping between virtqueue
> elements and iovecs (one iovec per element), enforced by an ASSERT.
> This prevented the use of virtqueue elements with multiple buffers
> (e.g. when mergeable rx buffers are not negotiated and headers are
> provided in a separate buffer).
> 
> Introduce a struct vu_frame to track per-frame metadata: the range of
> elements and iovecs that make up each frame, and the frame's total size.
> This replaces the head[] array which only tracked element indices.
> 
> A separate iov_msg[] array is built for recvmsg() by cloning the data
> portions (after stripping headers) using iov_tail helpers.
> 
> Then a frame truncation after recvmsg() properly walks the frame and
> element arrays to adjust iovec counts and element counts.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

LGTM apart from a couple of minor points noted below.

> ---
>  tcp_vu.c | 174 ++++++++++++++++++++++++++++++++++++-------------------
>  1 file changed, 113 insertions(+), 61 deletions(-)
> 
> diff --git a/tcp_vu.c b/tcp_vu.c
> index 2017aec90342..96b16007701d 100644
> --- a/tcp_vu.c
> +++ b/tcp_vu.c
> @@ -35,9 +35,24 @@
>  #include "vu_common.h"
>  #include <time.h>
>  
> -static struct iovec iov_vu[VIRTQUEUE_MAX_SIZE + DISCARD_IOV_NUM];
> +static struct iovec iov_vu[VIRTQUEUE_MAX_SIZE];
>  static struct vu_virtq_element elem[VIRTQUEUE_MAX_SIZE];
> -static int head[VIRTQUEUE_MAX_SIZE + 1];
> +
> +/**
> + * struct vu_frame - Descriptor for a TCP frame mapped to virtqueue elements
> + * @idx_element:	Index of first element in elem[] for this frame
> + * @num_element:	Number of virtqueue elements used by this frame
> + * @idx_iovec:		Index of first iovec in iov_vu[] for this frame
> + * @num_iovec:		Number of iovecs covering this frame's buffers
> + * @size:		Total frame size including all headers
> + */
> +static struct vu_frame {
> +	int idx_element;
> +	int num_element;
> +	int idx_iovec;
> +	int num_iovec;
> +	size_t size;
> +} frame[VIRTQUEUE_MAX_SIZE];
>  
>  /**
>   * tcp_vu_hdrlen() - Sum size of all headers, from TCP to virtio-net
> @@ -174,8 +189,8 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
>   * @v6:			Set for IPv6 connections
>   * @already_sent:	Number of bytes already sent
>   * @fillsize:		Maximum bytes to fill in guest-side receiving window
> - * @iov_cnt:		number of iov (output)
> - * @head_cnt:		Pointer to store the count of head iov entries (output)
> + * @elem_used:		number of element (output)
> + * @frame_cnt:		Pointer to store the number of frames (output)
>   *
>   * Return: number of bytes received from the socket, or a negative error code
>   * on failure.
> @@ -183,57 +198,77 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
>  static ssize_t tcp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq,
>  				const struct tcp_tap_conn *conn, bool v6,
>  				uint32_t already_sent, size_t fillsize,
> -				int *iov_cnt, int *head_cnt)
> +				int *elem_used, int *frame_cnt)
>  {
> +	static struct iovec iov_msg[VIRTQUEUE_MAX_SIZE + DISCARD_IOV_NUM];
>  	const struct vu_dev *vdev = c->vdev;
>  	struct msghdr mh_sock = { 0 };
>  	uint16_t mss = MSS_GET(conn);
>  	size_t hdrlen, iov_used;
>  	int s = conn->sock;
> +	ssize_t ret, dlen;
>  	int elem_cnt;
> -	ssize_t ret;
> -	int i;
> -
> -	*iov_cnt = 0;
> +	int i, j;
>  
>  	hdrlen = tcp_vu_hdrlen(v6);
>  
> +	*elem_used = 0;
> +
>  	iov_used = 0;
>  	elem_cnt = 0;
> -	*head_cnt = 0;
> +	*frame_cnt = 0;
>  	while (fillsize > 0 && elem_cnt < ARRAY_SIZE(elem) &&
> -	       iov_used < VIRTQUEUE_MAX_SIZE) {
> -		size_t frame_size, dlen, in_total;
> -		struct iovec *iov;
> +	       iov_used < ARRAY_SIZE(iov_vu) &&
> +	       *frame_cnt < ARRAY_SIZE(frame)) {
> +		size_t frame_size, in_total;
>  		int cnt;
>  
>  		cnt = vu_collect(vdev, vq, &elem[elem_cnt],
>  				 ARRAY_SIZE(elem) - elem_cnt,
> -				 &iov_vu[DISCARD_IOV_NUM + iov_used],
> -				 VIRTQUEUE_MAX_SIZE - iov_used, &in_total,
> +				 &iov_vu[iov_used],
> +				 ARRAY_SIZE(iov_vu) - iov_used, &in_total,
>  				 MIN(mss, fillsize) + hdrlen,
>  				 &frame_size);
>  		if (cnt == 0)
>  			break;
> -		assert((size_t)cnt == in_total); /* one iovec per element */
> +
> +		frame[*frame_cnt].idx_element = elem_cnt;
> +		frame[*frame_cnt].num_element = cnt;
> +		frame[*frame_cnt].idx_iovec = iov_used;
> +		frame[*frame_cnt].num_iovec = in_total;
> +		frame[*frame_cnt].size = frame_size;
> +		(*frame_cnt)++;
>  
>  		iov_used += in_total;
> -		dlen = frame_size - hdrlen;
> +		elem_cnt += cnt;
>  
> -		/* reserve space for headers in iov */
> -		iov = &elem[elem_cnt].in_sg[0];
> -		assert(iov->iov_len >= hdrlen);
> -		iov->iov_base = (char *)iov->iov_base + hdrlen;
> -		iov->iov_len -= hdrlen;
> -		head[(*head_cnt)++] = elem_cnt;
> +		fillsize -= frame_size - hdrlen;
> +	}
>  
> -		fillsize -= dlen;
> -		elem_cnt += cnt;
> +	/* build an iov array without headers */
> +	for (i = 0, j = DISCARD_IOV_NUM; i < *frame_cnt &&
> +	     j < ARRAY_SIZE(iov_msg); i++) {
> +		struct iov_tail data;
> +		ssize_t cnt;
> +
> +		data = IOV_TAIL(&iov_vu[frame[i].idx_iovec],
> +				frame[i].num_iovec, 0);
> +		iov_drop_header(&data, hdrlen);
> +
> +		cnt = iov_tail_clone(&iov_msg[j], ARRAY_SIZE(iov_msg) - j,
> +				     &data);
> +		if (cnt == -1)
> +			die("Missing entries in iov_msg");

Is a fatal error really what we want here?

> +
> +		j += cnt;
>  	}
>  
> -	if (tcp_prepare_iov(&mh_sock, iov_vu, already_sent, elem_cnt))
> +	if (tcp_prepare_iov(&mh_sock, iov_msg, already_sent,
> +			    j - DISCARD_IOV_NUM)) {
>  		/* Expect caller to do a TCP reset */
> +		vu_queue_rewind(vq, elem_cnt);
>  		return -1;
> +	}
>  
>  	do
>  		ret = recvmsg(s, &mh_sock, MSG_PEEK);
> @@ -247,32 +282,50 @@ static ssize_t tcp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq,
>  	if (!peek_offset_cap)
>  		ret -= already_sent;
>  
> -	i = iov_skip_bytes(&iov_vu[DISCARD_IOV_NUM], iov_used,
> -			   MAX(hdrlen + ret, VNET_HLEN + ETH_ZLEN),
> -			   NULL);
> -	if ((size_t)i < iov_used)
> -		i++;
> +	dlen = ret;
>  
> -	/* adjust head count */
> -	while (*head_cnt > 0 && head[*head_cnt - 1] >= i)
> -		(*head_cnt)--;
> +	/* truncate frame */
> +	*elem_used = 0;

Is this redundant with the *elem_used = 0 right at the top of the function?

> +	for (i = 0; i < *frame_cnt; i++) {
> +		struct vu_frame *f = &frame[i];
>  
> -	/* mark end of array */
> -	head[*head_cnt] = i;
> -	*iov_cnt = i;
> +		if ((size_t)ret <= f->size - hdrlen) {
> +			unsigned cnt;
>  
> -	/* release unused buffers */
> -	vu_queue_rewind(vq, elem_cnt - i);
> +			cnt = iov_skip_bytes(&iov_vu[f->idx_iovec], f->num_iovec,
> +					     MAX(hdrlen + ret, VNET_HLEN + ETH_ZLEN),
> +					     NULL);
> +			if (cnt < (unsigned)f->num_iovec)
> +				cnt++;
> +
> +			f->size = ret + hdrlen;
> +			f->num_iovec = cnt;
>  
> -	/* restore space for headers in iov */
> -	for (i = 0; i < *head_cnt; i++) {
> -		struct iovec *iov = &elem[head[i]].in_sg[0];
> +			for (j = 0; j < f->num_element; j++) {
> +				struct vu_virtq_element *e;
>  
> -		iov->iov_base = (char *)iov->iov_base - hdrlen;
> -		iov->iov_len += hdrlen;
> +				e = &elem[f->idx_element + j];
> +				if (cnt <= e->in_num) {
> +					e->in_num = cnt;
> +					j++;
> +					break;
> +				}
> +				cnt -= e->in_num;
> +			}
> +			f->num_element = j;
> +			*elem_used += j;
> +			i++;
> +			break;
> +		}
> +		*elem_used += f->num_element;
> +		ret -= f->size - hdrlen;
>  	}
> +	*frame_cnt = i;
>  
> -	return ret;
> +	/* release unused buffers */
> +	vu_queue_rewind(vq, elem_cnt - *elem_used);
> +
> +	return dlen;
>  }
>  
>  /**
> @@ -348,7 +401,7 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
>  	struct vu_virtq *vq = &vdev->vq[VHOST_USER_RX_QUEUE];
>  	uint32_t already_sent, check;
>  	ssize_t len, previous_dlen;
> -	int i, iov_cnt, head_cnt;
> +	int i, elem_cnt, frame_cnt;
>  	size_t hdrlen, fillsize;
>  	int v6 = CONN_V6(conn);
>  
> @@ -386,7 +439,7 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
>  	 * data from the socket
>  	 */
>  	len = tcp_vu_sock_recv(c, vq, conn, v6, already_sent, fillsize,
> -			       &iov_cnt, &head_cnt);
> +			       &elem_cnt, &frame_cnt);
>  	if (len < 0) {
>  		if (len != -EAGAIN && len != -EWOULDBLOCK) {
>  			tcp_rst(c, conn);
> @@ -400,6 +453,7 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
>  	}
>  
>  	if (!len) {
> +		vu_queue_rewind(vq, elem_cnt);
>  		if (already_sent) {
>  			conn_flag(c, conn, STALLED);
>  		} else if ((conn->events & (SOCK_FIN_RCVD | TAP_FIN_SENT)) ==
> @@ -440,34 +494,32 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
>  	check = IP4_CSUM;
>  	if (*c->pcap)
>  		check |= TCP_CSUM;
> -	for (i = 0, previous_dlen = -1; i < head_cnt; i++) {
> -		struct iovec *iov = &elem[head[i]].in_sg[0];
> -		int buf_cnt = head[i + 1] - head[i];
> -		size_t frame_size = iov_size(iov, buf_cnt);
> -		bool push = i == head_cnt - 1;
> +	for (i = 0, previous_dlen = -1; i < frame_cnt; i++) {
> +		struct iovec *iov = &iov_vu[frame[i].idx_iovec];
> +		int iov_cnt = frame[i].num_iovec;
> +		bool push = i == frame_cnt - 1;
>  		ssize_t dlen;
>  
> -		assert(frame_size >= hdrlen);
> +		assert(frame[i].size >= hdrlen);
>  
> -		dlen = frame_size - hdrlen;
> -		if (dlen > len)
> -			dlen = len;
> -		len -= dlen;
> +		dlen = frame[i].size - hdrlen;
>  
>  		/* The IPv4 header checksum varies only with dlen */
>  		if (previous_dlen != dlen)
>  			check |= IP4_CSUM;
>  		previous_dlen = dlen;
>  
> -		tcp_vu_prepare(c, conn, iov, buf_cnt, dlen, &check, push);
> +		tcp_vu_prepare(c, conn, iov, iov_cnt, dlen, &check, push);
>  
> -		vu_pad(elem[head[i]].in_sg, buf_cnt, dlen + hdrlen);
> -		vu_flush(vdev, vq, &elem[head[i]], buf_cnt, dlen + hdrlen);
> +		vu_pad(&iov[frame[i].idx_iovec], frame[i].num_iovec,
> +		       dlen + hdrlen);
>  
>  		if (*c->pcap) {
> -			pcap_iov(iov, buf_cnt, VNET_HLEN,
> +			pcap_iov(iov, iov_cnt, VNET_HLEN,
>  				 dlen + hdrlen - VNET_HLEN);
>  		}
> +		vu_flush(vdev, vq, &elem[frame[i].idx_element],
> +			 frame[i].num_element, dlen + hdrlen);
>  
>  		conn->seq_to_tap += dlen;
>  	}
> -- 
> 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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